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fuchsia / third_party / rust / 7bade6ef730cff83f3591479a98916920f66decd / . / compiler / rustc_mir_build / src / thir / pattern / _match.rs
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//! Note: most of the tests relevant to this file can be found (at the time of writing) in
//! src/tests/ui/pattern/usefulness.
//!
//! This file includes the logic for exhaustiveness and usefulness checking for
//! pattern-matching. Specifically, given a list of patterns for a type, we can
//! tell whether:
//! (a) the patterns cover every possible constructor for the type (exhaustiveness)
//! (b) each pattern is necessary (usefulness)
//!
//! The algorithm implemented here is a modified version of the one described in:
//! http://moscova.inria.fr/~maranget/papers/warn/index.html
//! However, to save future implementors from reading the original paper, we
//! summarise the algorithm here to hopefully save time and be a little clearer
//! (without being so rigorous).
//!
//! # Premise
//!
//! The core of the algorithm revolves about a "usefulness" check. In particular, we
//! are trying to compute a predicate `U(P, p)` where `P` is a list of patterns (we refer to this as
//! a matrix). `U(P, p)` represents whether, given an existing list of patterns
//! `P_1 ..= P_m`, adding a new pattern `p` will be "useful" (that is, cover previously-
//! uncovered values of the type).
//!
//! If we have this predicate, then we can easily compute both exhaustiveness of an
//! entire set of patterns and the individual usefulness of each one.
//! (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e., adding a wildcard
//! match doesn't increase the number of values we're matching)
//! (b) a pattern `P_i` is not useful if `U(P[0..=(i-1), P_i)` is false (i.e., adding a
//! pattern to those that have come before it doesn't increase the number of values
//! we're matching).
//!
//! # Core concept
//!
//! The idea that powers everything that is done in this file is the following: a value is made
//! from a constructor applied to some fields. Examples of constructors are `Some`, `None`, `(,)`
//! (the 2-tuple constructor), `Foo {..}` (the constructor for a struct `Foo`), and `2` (the
//! constructor for the number `2`). Fields are just a (possibly empty) list of values.
//!
//! Some of the constructors listed above might feel weird: `None` and `2` don't take any
//! arguments. This is part of what makes constructors so general: we will consider plain values
//! like numbers and string literals to be constructors that take no arguments, also called "0-ary
//! constructors"; they are the simplest case of constructors. This allows us to see any value as
//! made up from a tree of constructors, each having a given number of children. For example:
//! `(None, Ok(0))` is made from 4 different constructors.
//!
//! This idea can be extended to patterns: a pattern captures a set of possible values, and we can
//! describe this set using constructors. For example, `Err(_)` captures all values of the type
//! `Result<T, E>` that start with the `Err` constructor (for some choice of `T` and `E`). The
//! wildcard `_` captures all values of the given type starting with any of the constructors for
//! that type.
//!
//! We use this to compute whether different patterns might capture a same value. Do the patterns
//! `Ok("foo")` and `Err(_)` capture a common value? The answer is no, because the first pattern
//! captures only values starting with the `Ok` constructor and the second only values starting
//! with the `Err` constructor. Do the patterns `Some(42)` and `Some(1..10)` intersect? They might,
//! since they both capture values starting with `Some`. To be certain, we need to dig under the
//! `Some` constructor and continue asking the question. This is the main idea behind the
//! exhaustiveness algorithm: by looking at patterns constructor-by-constructor, we can efficiently
//! figure out if some new pattern might capture a value that hadn't been captured by previous
//! patterns.
//!
//! Constructors are represented by the `Constructor` enum, and its fields by the `Fields` enum.
//! Most of the complexity of this file resides in transforming between patterns and
//! (`Constructor`, `Fields`) pairs, handling all the special cases correctly.
//!
//! Caveat: this constructors/fields distinction doesn't quite cover every Rust value. For example
//! a value of type `Rc<u64>` doesn't fit this idea very well, nor do various other things.
//! However, this idea covers most of the cases that are relevant to exhaustiveness checking.
//!
//!
//! # Algorithm
//!
//! Recall that `U(P, p)` represents whether, given an existing list of patterns (aka matrix) `P`,
//! adding a new pattern `p` will cover previously-uncovered values of the type.
//! During the course of the algorithm, the rows of the matrix won't just be individual patterns,
//! but rather partially-deconstructed patterns in the form of a list of fields. The paper
//! calls those pattern-vectors, and we will call them pattern-stacks. The same holds for the
//! new pattern `p`.
//!
//! For example, say we have the following:
//!
//! ```
//! // x: (Option<bool>, Result<()>)
//! match x {
//! (Some(true), _) => {}
//! (None, Err(())) => {}
//! (None, Err(_)) => {}
//! }
//! ```
//!
//! Here, the matrix `P` starts as:
//!
//! ```
//! [
//! [(Some(true), _)],
//! [(None, Err(()))],
//! [(None, Err(_))],
//! ]
//! ```
//!
//! We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering
//! `[(Some(false), _)]`, for instance). In addition, row 3 is not useful, because
//! all the values it covers are already covered by row 2.
//!
//! A list of patterns can be thought of as a stack, because we are mainly interested in the top of
//! the stack at any given point, and we can pop or apply constructors to get new pattern-stacks.
//! To match the paper, the top of the stack is at the beginning / on the left.
//!
//! There are two important operations on pattern-stacks necessary to understand the algorithm:
//!
//! 1. We can pop a given constructor off the top of a stack. This operation is called
//! `specialize`, and is denoted `S(c, p)` where `c` is a constructor (like `Some` or
//! `None`) and `p` a pattern-stack.
//! If the pattern on top of the stack can cover `c`, this removes the constructor and
//! pushes its arguments onto the stack. It also expands OR-patterns into distinct patterns.
//! Otherwise the pattern-stack is discarded.
//! This essentially filters those pattern-stacks whose top covers the constructor `c` and
//! discards the others.
//!
//! For example, the first pattern above initially gives a stack `[(Some(true), _)]`. If we
//! pop the tuple constructor, we are left with `[Some(true), _]`, and if we then pop the
//! `Some` constructor we get `[true, _]`. If we had popped `None` instead, we would get
//! nothing back.
//!
//! This returns zero or more new pattern-stacks, as follows. We look at the pattern `p_1`
//! on top of the stack, and we have four cases:
//! 1.1. `p_1 = c(r_1, .., r_a)`, i.e. the top of the stack has constructor `c`. We
//! push onto the stack the arguments of this constructor, and return the result:
//! r_1, .., r_a, p_2, .., p_n
//! 1.2. `p_1 = c'(r_1, .., r_a')` where `c ≠ c'`. We discard the current stack and
//! return nothing.
//! 1.3. `p_1 = _`. We push onto the stack as many wildcards as the constructor `c` has
//! arguments (its arity), and return the resulting stack:
//! _, .., _, p_2, .., p_n
//! 1.4. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting
//! stack:
//! S(c, (r_1, p_2, .., p_n))
//! S(c, (r_2, p_2, .., p_n))
//!
//! 2. We can pop a wildcard off the top of the stack. This is called `D(p)`, where `p` is
//! a pattern-stack.
//! This is used when we know there are missing constructor cases, but there might be
//! existing wildcard patterns, so to check the usefulness of the matrix, we have to check
//! all its *other* components.
//!
//! It is computed as follows. We look at the pattern `p_1` on top of the stack,
//! and we have three cases:
//! 2.1. `p_1 = c(r_1, .., r_a)`. We discard the current stack and return nothing.
//! 2.2. `p_1 = _`. We return the rest of the stack:
//! p_2, .., p_n
//! 2.3. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting
//! stack.
//! D((r_1, p_2, .., p_n))
//! D((r_2, p_2, .., p_n))
//!
//! Note that the OR-patterns are not always used directly in Rust, but are used to derive the
//! exhaustive integer matching rules, so they're written here for posterity.
//!
//! Both those operations extend straightforwardly to a list or pattern-stacks, i.e. a matrix, by
//! working row-by-row. Popping a constructor ends up keeping only the matrix rows that start with
//! the given constructor, and popping a wildcard keeps those rows that start with a wildcard.
//!
//!
//! The algorithm for computing `U`
//! -------------------------------
//! The algorithm is inductive (on the number of columns: i.e., components of tuple patterns).
//! That means we're going to check the components from left-to-right, so the algorithm
//! operates principally on the first component of the matrix and new pattern-stack `p`.
//! This algorithm is realised in the `is_useful` function.
//!
//! Base case. (`n = 0`, i.e., an empty tuple pattern)
//! - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`),
//! then `U(P, p)` is false.
//! - Otherwise, `P` must be empty, so `U(P, p)` is true.
//!
//! Inductive step. (`n > 0`, i.e., whether there's at least one column
//! [which may then be expanded into further columns later])
//! We're going to match on the top of the new pattern-stack, `p_1`.
//! - If `p_1 == c(r_1, .., r_a)`, i.e. we have a constructor pattern.
//! Then, the usefulness of `p_1` can be reduced to whether it is useful when
//! we ignore all the patterns in the first column of `P` that involve other constructors.
//! This is where `S(c, P)` comes in:
//! `U(P, p) := U(S(c, P), S(c, p))`
//! This special case is handled in `is_useful_specialized`.
//!
//! For example, if `P` is:
//!
//! ```
//! [
//! [Some(true), _],
//! [None, 0],
//! ]
//! ```
//!
//! and `p` is [Some(false), 0], then we don't care about row 2 since we know `p` only
//! matches values that row 2 doesn't. For row 1 however, we need to dig into the
//! arguments of `Some` to know whether some new value is covered. So we compute
//! `U([[true, _]], [false, 0])`.
//!
//! - If `p_1 == _`, then we look at the list of constructors that appear in the first
//! component of the rows of `P`:
//! + If there are some constructors that aren't present, then we might think that the
//! wildcard `_` is useful, since it covers those constructors that weren't covered
//! before.
//! That's almost correct, but only works if there were no wildcards in those first
//! components. So we need to check that `p` is useful with respect to the rows that
//! start with a wildcard, if there are any. This is where `D` comes in:
//! `U(P, p) := U(D(P), D(p))`
//!
//! For example, if `P` is:
//!
//! ```
//! [
//! [_, true, _],
//! [None, false, 1],
//! ]
//! ```
//!
//! and `p` is [_, false, _], the `Some` constructor doesn't appear in `P`. So if we
//! only had row 2, we'd know that `p` is useful. However row 1 starts with a
//! wildcard, so we need to check whether `U([[true, _]], [false, 1])`.
//!
//! + Otherwise, all possible constructors (for the relevant type) are present. In this
//! case we must check whether the wildcard pattern covers any unmatched value. For
//! that, we can think of the `_` pattern as a big OR-pattern that covers all
//! possible constructors. For `Option`, that would mean `_ = None | Some(_)` for
//! example. The wildcard pattern is useful in this case if it is useful when
//! specialized to one of the possible constructors. So we compute:
//! `U(P, p) := ∃(k ϵ constructors) U(S(k, P), S(k, p))`
//!
//! For example, if `P` is:
//!
//! ```
//! [
//! [Some(true), _],
//! [None, false],
//! ]
//! ```
//!
//! and `p` is [_, false], both `None` and `Some` constructors appear in the first
//! components of `P`. We will therefore try popping both constructors in turn: we
//! compute `U([[true, _]], [_, false])` for the `Some` constructor, and `U([[false]],
//! [false])` for the `None` constructor. The first case returns true, so we know that
//! `p` is useful for `P`. Indeed, it matches `[Some(false), _]` that wasn't matched
//! before.
//!
//! - If `p_1 == r_1 | r_2`, then the usefulness depends on each `r_i` separately:
//! `U(P, p) := U(P, (r_1, p_2, .., p_n))
//! || U(P, (r_2, p_2, .., p_n))`
//!
//! Modifications to the algorithm
//! ------------------------------
//! The algorithm in the paper doesn't cover some of the special cases that arise in Rust, for
//! example uninhabited types and variable-length slice patterns. These are drawn attention to
//! throughout the code below. I'll make a quick note here about how exhaustive integer matching is
//! accounted for, though.
//!
//! Exhaustive integer matching
//! ---------------------------
//! An integer type can be thought of as a (huge) sum type: 1 | 2 | 3 | ...
//! So to support exhaustive integer matching, we can make use of the logic in the paper for
//! OR-patterns. However, we obviously can't just treat ranges x..=y as individual sums, because
//! they are likely gigantic. So we instead treat ranges as constructors of the integers. This means
//! that we have a constructor *of* constructors (the integers themselves). We then need to work
//! through all the inductive step rules above, deriving how the ranges would be treated as
//! OR-patterns, and making sure that they're treated in the same way even when they're ranges.
//! There are really only four special cases here:
//! - When we match on a constructor that's actually a range, we have to treat it as if we would
//! an OR-pattern.
//! + It turns out that we can simply extend the case for single-value patterns in
//! `specialize` to either be *equal* to a value constructor, or *contained within* a range
//! constructor.
//! + When the pattern itself is a range, you just want to tell whether any of the values in
//! the pattern range coincide with values in the constructor range, which is precisely
//! intersection.
//! Since when encountering a range pattern for a value constructor, we also use inclusion, it
//! means that whenever the constructor is a value/range and the pattern is also a value/range,
//! we can simply use intersection to test usefulness.
//! - When we're testing for usefulness of a pattern and the pattern's first component is a
//! wildcard.
//! + If all the constructors appear in the matrix, we have a slight complication. By default,
//! the behaviour (i.e., a disjunction over specialised matrices for each constructor) is
//! invalid, because we want a disjunction over every *integer* in each range, not just a
//! disjunction over every range. This is a bit more tricky to deal with: essentially we need
//! to form equivalence classes of subranges of the constructor range for which the behaviour
//! of the matrix `P` and new pattern `p` are the same. This is described in more
//! detail in `split_grouped_constructors`.
//! + If some constructors are missing from the matrix, it turns out we don't need to do
//! anything special (because we know none of the integers are actually wildcards: i.e., we
//! can't span wildcards using ranges).
use self::Constructor::*;
use self::SliceKind::*;
use self::Usefulness::*;
use self::WitnessPreference::*;
use rustc_data_structures::captures::Captures;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_index::vec::Idx;
use super::{compare_const_vals, PatternFoldable, PatternFolder};
use super::{FieldPat, Pat, PatKind, PatRange};
use rustc_arena::TypedArena;
use rustc_attr::{SignedInt, UnsignedInt};
use rustc_errors::ErrorReported;
use rustc_hir::def_id::DefId;
use rustc_hir::{HirId, RangeEnd};
use rustc_middle::mir::interpret::{truncate, AllocId, ConstValue, Pointer, Scalar};
use rustc_middle::mir::Field;
use rustc_middle::ty::layout::IntegerExt;
use rustc_middle::ty::{self, Const, Ty, TyCtxt};
use rustc_session::lint;
use rustc_span::{Span, DUMMY_SP};
use rustc_target::abi::{Integer, Size, VariantIdx};
use smallvec::{smallvec, SmallVec};
use std::borrow::Cow;
use std::cmp::{self, max, min, Ordering};
use std::convert::TryInto;
use std::fmt;
use std::iter::{FromIterator, IntoIterator};
use std::ops::RangeInclusive;
crate fn expand_pattern<'a, 'tcx>(cx: &MatchCheckCtxt<'a, 'tcx>, pat: Pat<'tcx>) -> Pat<'tcx> {
LiteralExpander { tcx: cx.tcx, param_env: cx.param_env }.fold_pattern(&pat)
}
struct LiteralExpander<'tcx> {
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
}
impl<'tcx> LiteralExpander<'tcx> {
/// Derefs `val` and potentially unsizes the value if `crty` is an array and `rty` a slice.
///
/// `crty` and `rty` can differ because you can use array constants in the presence of slice
/// patterns. So the pattern may end up being a slice, but the constant is an array. We convert
/// the array to a slice in that case.
fn fold_const_value_deref(
&mut self,
val: ConstValue<'tcx>,
// the pattern's pointee type
rty: Ty<'tcx>,
// the constant's pointee type
crty: Ty<'tcx>,
) -> ConstValue<'tcx> {
debug!("fold_const_value_deref {:?} {:?} {:?}", val, rty, crty);
match (val, &crty.kind(), &rty.kind()) {
// the easy case, deref a reference
(ConstValue::Scalar(p), x, y) if x == y => {
match p {
Scalar::Ptr(p) => {
let alloc = self.tcx.global_alloc(p.alloc_id).unwrap_memory();
ConstValue::ByRef { alloc, offset: p.offset }
}
Scalar::Raw { .. } => {
let layout = self.tcx.layout_of(self.param_env.and(rty)).unwrap();
if layout.is_zst() {
// Deref of a reference to a ZST is a nop.
ConstValue::Scalar(Scalar::zst())
} else {
// FIXME(oli-obk): this is reachable for `const FOO: &&&u32 = &&&42;`
bug!("cannot deref {:#?}, {} -> {}", val, crty, rty);
}
}
}
}
// unsize array to slice if pattern is array but match value or other patterns are slice
(ConstValue::Scalar(Scalar::Ptr(p)), ty::Array(t, n), ty::Slice(u)) => {
assert_eq!(t, u);
ConstValue::Slice {
data: self.tcx.global_alloc(p.alloc_id).unwrap_memory(),
start: p.offset.bytes().try_into().unwrap(),
end: n.eval_usize(self.tcx, ty::ParamEnv::empty()).try_into().unwrap(),
}
}
// fat pointers stay the same
(ConstValue::Slice { .. }, _, _)
| (_, ty::Slice(_), ty::Slice(_))
| (_, ty::Str, ty::Str) => val,
// FIXME(oli-obk): this is reachable for `const FOO: &&&u32 = &&&42;` being used
_ => bug!("cannot deref {:#?}, {} -> {}", val, crty, rty),
}
}
}
impl<'tcx> PatternFolder<'tcx> for LiteralExpander<'tcx> {
fn fold_pattern(&mut self, pat: &Pat<'tcx>) -> Pat<'tcx> {
debug!("fold_pattern {:?} {:?} {:?}", pat, pat.ty.kind(), pat.kind);
match (pat.ty.kind(), &*pat.kind) {
(&ty::Ref(_, rty, _), &PatKind::Constant { value: Const { val, ty: const_ty } })
if const_ty.is_ref() =>
{
let crty =
if let ty::Ref(_, crty, _) = const_ty.kind() { crty } else { unreachable!() };
if let ty::ConstKind::Value(val) = val {
Pat {
ty: pat.ty,
span: pat.span,
kind: box PatKind::Deref {
subpattern: Pat {
ty: rty,
span: pat.span,
kind: box PatKind::Constant {
value: Const::from_value(
self.tcx,
self.fold_const_value_deref(*val, rty, crty),
rty,
),
},
},
},
}
} else {
bug!("cannot deref {:#?}, {} -> {}", val, crty, rty)
}
}
(_, &PatKind::Binding { subpattern: Some(ref s), .. }) => s.fold_with(self),
(_, &PatKind::AscribeUserType { subpattern: ref s, .. }) => s.fold_with(self),
_ => pat.super_fold_with(self),
}
}
}
impl<'tcx> Pat<'tcx> {
pub(super) fn is_wildcard(&self) -> bool {
match *self.kind {
PatKind::Binding { subpattern: None, .. } | PatKind::Wild => true,
_ => false,
}
}
}
/// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
/// works well.
#[derive(Debug, Clone, PartialEq)]
crate struct PatStack<'p, 'tcx>(SmallVec<[&'p Pat<'tcx>; 2]>);
impl<'p, 'tcx> PatStack<'p, 'tcx> {
crate fn from_pattern(pat: &'p Pat<'tcx>) -> Self {
PatStack(smallvec![pat])
}
fn from_vec(vec: SmallVec<[&'p Pat<'tcx>; 2]>) -> Self {
PatStack(vec)
}
fn from_slice(s: &[&'p Pat<'tcx>]) -> Self {
PatStack(SmallVec::from_slice(s))
}
fn is_empty(&self) -> bool {
self.0.is_empty()
}
fn len(&self) -> usize {
self.0.len()
}
fn head(&self) -> &'p Pat<'tcx> {
self.0[0]
}
fn to_tail(&self) -> Self {
PatStack::from_slice(&self.0[1..])
}
fn iter(&self) -> impl Iterator<Item = &Pat<'tcx>> {
self.0.iter().copied()
}
// If the first pattern is an or-pattern, expand this pattern. Otherwise, return `None`.
fn expand_or_pat(&self) -> Option<Vec<Self>> {
if self.is_empty() {
None
} else if let PatKind::Or { pats } = &*self.head().kind {
Some(
pats.iter()
.map(|pat| {
let mut new_patstack = PatStack::from_pattern(pat);
new_patstack.0.extend_from_slice(&self.0[1..]);
new_patstack
})
.collect(),
)
} else {
None
}
}
/// This computes `D(self)`. See top of the file for explanations.
fn specialize_wildcard(&self) -> Option<Self> {
if self.head().is_wildcard() { Some(self.to_tail()) } else { None }
}
/// This computes `S(constructor, self)`. See top of the file for explanations.
fn specialize_constructor(
&self,
cx: &mut MatchCheckCtxt<'p, 'tcx>,
constructor: &Constructor<'tcx>,
ctor_wild_subpatterns: &Fields<'p, 'tcx>,
) -> Option<PatStack<'p, 'tcx>> {
let new_fields =
specialize_one_pattern(cx, self.head(), constructor, ctor_wild_subpatterns)?;
Some(new_fields.push_on_patstack(&self.0[1..]))
}
}
impl<'p, 'tcx> Default for PatStack<'p, 'tcx> {
fn default() -> Self {
PatStack(smallvec![])
}
}
impl<'p, 'tcx> FromIterator<&'p Pat<'tcx>> for PatStack<'p, 'tcx> {
fn from_iter<T>(iter: T) -> Self
where
T: IntoIterator<Item = &'p Pat<'tcx>>,
{
PatStack(iter.into_iter().collect())
}
}
/// Depending on the match patterns, the specialization process might be able to use a fast path.
/// Tracks whether we can use the fast path and the lookup table needed in those cases.
#[derive(Clone, Debug, PartialEq)]
enum SpecializationCache {
/// Patterns consist of only enum variants.
/// Variant patterns does not intersect with each other (in contrast to range patterns),
/// so it is possible to precompute the result of `Matrix::specialize_constructor` at a
/// lower computational complexity.
/// `lookup` is responsible for holding the precomputed result of
/// `Matrix::specialize_constructor`, while `wilds` is used for two purposes: the first one is
/// the precomputed result of `Matrix::specialize_wildcard`, and the second is to be used as a
/// fallback for `Matrix::specialize_constructor` when it tries to apply a constructor that
/// has not been seen in the `Matrix`. See `update_cache` for further explanations.
Variants { lookup: FxHashMap<DefId, SmallVec<[usize; 1]>>, wilds: SmallVec<[usize; 1]> },
/// Does not belong to the cases above, use the slow path.
Incompatible,
}
/// A 2D matrix.
#[derive(Clone, PartialEq)]
crate struct Matrix<'p, 'tcx> {
patterns: Vec<PatStack<'p, 'tcx>>,
cache: SpecializationCache,
}
impl<'p, 'tcx> Matrix<'p, 'tcx> {
crate fn empty() -> Self {
// Use `SpecializationCache::Incompatible` as a placeholder; we will initialize it on the
// first call to `push`. See the first half of `update_cache`.
Matrix { patterns: vec![], cache: SpecializationCache::Incompatible }
}
/// Pushes a new row to the matrix. If the row starts with an or-pattern, this expands it.
crate fn push(&mut self, row: PatStack<'p, 'tcx>) {
if let Some(rows) = row.expand_or_pat() {
for row in rows {
// We recursively expand the or-patterns of the new rows.
// This is necessary as we might have `0 | (1 | 2)` or e.g., `x @ 0 | x @ (1 | 2)`.
self.push(row)
}
} else {
self.patterns.push(row);
self.update_cache(self.patterns.len() - 1);
}
}
fn update_cache(&mut self, idx: usize) {
let row = &self.patterns[idx];
// We don't know which kind of cache could be used until we see the first row; therefore an
// empty `Matrix` is initialized with `SpecializationCache::Empty`, then the cache is
// assigned the appropriate variant below on the first call to `push`.
if self.patterns.is_empty() {
self.cache = if row.is_empty() {
SpecializationCache::Incompatible
} else {
match *row.head().kind {
PatKind::Variant { .. } => SpecializationCache::Variants {
lookup: FxHashMap::default(),
wilds: SmallVec::new(),
},
// Note: If the first pattern is a wildcard, then all patterns after that is not
// useful. The check is simple enough so we treat it as the same as unsupported
// patterns.
_ => SpecializationCache::Incompatible,
}
};
}
// Update the cache.
match &mut self.cache {
SpecializationCache::Variants { ref mut lookup, ref mut wilds } => {
let head = row.head();
match *head.kind {
_ if head.is_wildcard() => {
// Per rule 1.3 in the top-level comments, a wildcard pattern is included in
// the result of `specialize_constructor` for *any* `Constructor`.
// We push the wildcard pattern to the precomputed result for constructors
// that we have seen before; results for constructors we have not yet seen
// defaults to `wilds`, which is updated right below.
for (_, v) in lookup.iter_mut() {
v.push(idx);
}
// Per rule 2.1 and 2.2 in the top-level comments, only wildcard patterns
// are included in the result of `specialize_wildcard`.
// What we do here is to track the wildcards we have seen; so in addition to
// acting as the precomputed result of `specialize_wildcard`, `wilds` also
// serves as the default value of `specialize_constructor` for constructors
// that are not in `lookup`.
wilds.push(idx);
}
PatKind::Variant { adt_def, variant_index, .. } => {
// Handle the cases of rule 1.1 and 1.2 in the top-level comments.
// A variant pattern can only be included in the results of
// `specialize_constructor` for a particular constructor, therefore we are
// using a HashMap to track that.
lookup
.entry(adt_def.variants[variant_index].def_id)
// Default to `wilds` for absent keys. See above for an explanation.
.or_insert_with(|| wilds.clone())
.push(idx);
}
_ => {
self.cache = SpecializationCache::Incompatible;
}
}
}
SpecializationCache::Incompatible => {}
}
}
/// Iterate over the first component of each row
fn heads<'a>(&'a self) -> impl Iterator<Item = &'a Pat<'tcx>> + Captures<'p> {
self.patterns.iter().map(|r| r.head())
}
/// This computes `D(self)`. See top of the file for explanations.
fn specialize_wildcard(&self) -> Self {
match &self.cache {
SpecializationCache::Variants { wilds, .. } => {
let result =
wilds.iter().filter_map(|&i| self.patterns[i].specialize_wildcard()).collect();
// When debug assertions are enabled, check the results against the "slow path"
// result.
debug_assert_eq!(
result,
Self {
patterns: self.patterns.clone(),
cache: SpecializationCache::Incompatible
}
.specialize_wildcard()
);
result
}
SpecializationCache::Incompatible => {
self.patterns.iter().filter_map(|r| r.specialize_wildcard()).collect()
}
}
}
/// This computes `S(constructor, self)`. See top of the file for explanations.
fn specialize_constructor(
&self,
cx: &mut MatchCheckCtxt<'p, 'tcx>,
constructor: &Constructor<'tcx>,
ctor_wild_subpatterns: &Fields<'p, 'tcx>,
) -> Matrix<'p, 'tcx> {
match &self.cache {
SpecializationCache::Variants { lookup, wilds } => {
let result: Self = if let Constructor::Variant(id) = constructor {
lookup
.get(id)
// Default to `wilds` for absent keys. See `update_cache` for an explanation.
.unwrap_or(&wilds)
.iter()
.filter_map(|&i| {
self.patterns[i].specialize_constructor(
cx,
constructor,
ctor_wild_subpatterns,
)
})
.collect()
} else {
unreachable!()
};
// When debug assertions are enabled, check the results against the "slow path"
// result.
debug_assert_eq!(
result,
Matrix {
patterns: self.patterns.clone(),
cache: SpecializationCache::Incompatible
}
.specialize_constructor(
cx,
constructor,
ctor_wild_subpatterns
)
);
result
}
SpecializationCache::Incompatible => self
.patterns
.iter()
.filter_map(|r| r.specialize_constructor(cx, constructor, ctor_wild_subpatterns))
.collect(),
}
}
}
/// Pretty-printer for matrices of patterns, example:
///
/// ```text
/// +++++++++++++++++++++++++++++
/// + _ + [] +
/// +++++++++++++++++++++++++++++
/// + true + [First] +
/// +++++++++++++++++++++++++++++
/// + true + [Second(true)] +
/// +++++++++++++++++++++++++++++
/// + false + [_] +
/// +++++++++++++++++++++++++++++
/// + _ + [_, _, tail @ ..] +
/// +++++++++++++++++++++++++++++
impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "\n")?;
let Matrix { patterns: m, .. } = self;
let pretty_printed_matrix: Vec<Vec<String>> =
m.iter().map(|row| row.iter().map(|pat| format!("{:?}", pat)).collect()).collect();
let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
assert!(m.iter().all(|row| row.len() == column_count));
let column_widths: Vec<usize> = (0..column_count)
.map(|col| pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0))
.collect();
let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
let br = "+".repeat(total_width);
write!(f, "{}\n", br)?;
for row in pretty_printed_matrix {
write!(f, "+")?;
for (column, pat_str) in row.into_iter().enumerate() {
write!(f, " ")?;
write!(f, "{:1$}", pat_str, column_widths[column])?;
write!(f, " +")?;
}
write!(f, "\n")?;
write!(f, "{}\n", br)?;
}
Ok(())
}
}
impl<'p, 'tcx> FromIterator<PatStack<'p, 'tcx>> for Matrix<'p, 'tcx> {
fn from_iter<T>(iter: T) -> Self
where
T: IntoIterator<Item = PatStack<'p, 'tcx>>,
{
let mut matrix = Matrix::empty();
for x in iter {
// Using `push` ensures we correctly expand or-patterns.
matrix.push(x);
}
matrix
}
}
crate struct MatchCheckCtxt<'a, 'tcx> {
crate tcx: TyCtxt<'tcx>,
/// The module in which the match occurs. This is necessary for
/// checking inhabited-ness of types because whether a type is (visibly)
/// inhabited can depend on whether it was defined in the current module or
/// not. E.g., `struct Foo { _private: ! }` cannot be seen to be empty
/// outside it's module and should not be matchable with an empty match
/// statement.
crate module: DefId,
crate param_env: ty::ParamEnv<'tcx>,
crate pattern_arena: &'a TypedArena<Pat<'tcx>>,
}
impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
if self.tcx.features().exhaustive_patterns {
self.tcx.is_ty_uninhabited_from(self.module, ty, self.param_env)
} else {
false
}
}
/// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`.
crate fn is_foreign_non_exhaustive_enum(&self, ty: Ty<'tcx>) -> bool {
match ty.kind() {
ty::Adt(def, ..) => {
def.is_enum() && def.is_variant_list_non_exhaustive() && !def.did.is_local()
}
_ => false,
}
}
}
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
enum SliceKind {
/// Patterns of length `n` (`[x, y]`).
FixedLen(u64),
/// Patterns using the `..` notation (`[x, .., y]`).
/// Captures any array constructor of `length >= i + j`.
/// In the case where `array_len` is `Some(_)`,
/// this indicates that we only care about the first `i` and the last `j` values of the array,
/// and everything in between is a wildcard `_`.
VarLen(u64, u64),
}
impl SliceKind {
fn arity(self) -> u64 {
match self {
FixedLen(length) => length,
VarLen(prefix, suffix) => prefix + suffix,
}
}
/// Whether this pattern includes patterns of length `other_len`.
fn covers_length(self, other_len: u64) -> bool {
match self {
FixedLen(len) => len == other_len,
VarLen(prefix, suffix) => prefix + suffix <= other_len,
}
}
/// Returns a collection of slices that spans the values covered by `self`, subtracted by the
/// values covered by `other`: i.e., `self \ other` (in set notation).
fn subtract(self, other: Self) -> SmallVec<[Self; 1]> {
// Remember, `VarLen(i, j)` covers the union of `FixedLen` from `i + j` to infinity.
// Naming: we remove the "neg" constructors from the "pos" ones.
match self {
FixedLen(pos_len) => {
if other.covers_length(pos_len) {
smallvec![]
} else {
smallvec![self]
}
}
VarLen(pos_prefix, pos_suffix) => {
let pos_len = pos_prefix + pos_suffix;
match other {
FixedLen(neg_len) => {
if neg_len < pos_len {
smallvec![self]
} else {
(pos_len..neg_len)
.map(FixedLen)
// We know that `neg_len + 1 >= pos_len >= pos_suffix`.
.chain(Some(VarLen(neg_len + 1 - pos_suffix, pos_suffix)))
.collect()
}
}
VarLen(neg_prefix, neg_suffix) => {
let neg_len = neg_prefix + neg_suffix;
if neg_len <= pos_len {
smallvec![]
} else {
(pos_len..neg_len).map(FixedLen).collect()
}
}
}
}
}
}
}
/// A constructor for array and slice patterns.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
struct Slice {
/// `None` if the matched value is a slice, `Some(n)` if it is an array of size `n`.
array_len: Option<u64>,
/// The kind of pattern it is: fixed-length `[x, y]` or variable length `[x, .., y]`.
kind: SliceKind,
}
impl Slice {
/// Returns what patterns this constructor covers: either fixed-length patterns or
/// variable-length patterns.
fn pattern_kind(self) -> SliceKind {
match self {
Slice { array_len: Some(len), kind: VarLen(prefix, suffix) }
if prefix + suffix == len =>
{
FixedLen(len)
}
_ => self.kind,
}
}
/// Returns what values this constructor covers: either values of only one given length, or
/// values of length above a given length.
/// This is different from `pattern_kind()` because in some cases the pattern only takes into
/// account a subset of the entries of the array, but still only captures values of a given
/// length.
fn value_kind(self) -> SliceKind {
match self {
Slice { array_len: Some(len), kind: VarLen(_, _) } => FixedLen(len),
_ => self.kind,
}
}
fn arity(self) -> u64 {
self.pattern_kind().arity()
}
}
/// A value can be decomposed into a constructor applied to some fields. This struct represents
/// the constructor. See also `Fields`.
///
/// `pat_constructor` retrieves the constructor corresponding to a pattern.
/// `specialize_one_pattern` returns the list of fields corresponding to a pattern, given a
/// constructor. `Constructor::apply` reconstructs the pattern from a pair of `Constructor` and
/// `Fields`.
#[derive(Clone, Debug, PartialEq)]
enum Constructor<'tcx> {
/// The constructor for patterns that have a single constructor, like tuples, struct patterns
/// and fixed-length arrays.
Single,
/// Enum variants.
Variant(DefId),
/// Literal values.
ConstantValue(&'tcx ty::Const<'tcx>),
/// Ranges of integer literal values (`2`, `2..=5` or `2..5`).
IntRange(IntRange<'tcx>),
/// Ranges of floating-point literal values (`2.0..=5.2`).
FloatRange(&'tcx ty::Const<'tcx>, &'tcx ty::Const<'tcx>, RangeEnd),
/// Array and slice patterns.
Slice(Slice),
/// Fake extra constructor for enums that aren't allowed to be matched exhaustively.
NonExhaustive,
}
impl<'tcx> Constructor<'tcx> {
fn is_slice(&self) -> bool {
match self {
Slice(_) => true,
_ => false,
}
}
fn variant_index_for_adt<'a>(
&self,
cx: &MatchCheckCtxt<'a, 'tcx>,
adt: &'tcx ty::AdtDef,
) -> VariantIdx {
match *self {
Variant(id) => adt.variant_index_with_id(id),
Single => {
assert!(!adt.is_enum());
VariantIdx::new(0)
}
ConstantValue(c) => cx
.tcx
.destructure_const(cx.param_env.and(c))
.variant
.expect("destructed const of adt without variant id"),
_ => bug!("bad constructor {:?} for adt {:?}", self, adt),
}
}
// Returns the set of constructors covered by `self` but not by
// anything in `other_ctors`.
fn subtract_ctors(&self, other_ctors: &Vec<Constructor<'tcx>>) -> Vec<Constructor<'tcx>> {
if other_ctors.is_empty() {
return vec![self.clone()];
}
match self {
// Those constructors can only match themselves.
Single | Variant(_) | ConstantValue(..) | FloatRange(..) => {
if other_ctors.iter().any(|c| c == self) { vec![] } else { vec![self.clone()] }
}
&Slice(slice) => {
let mut other_slices = other_ctors
.iter()
.filter_map(|c: &Constructor<'_>| match c {
Slice(slice) => Some(*slice),
// FIXME(oli-obk): implement `deref` for `ConstValue`
ConstantValue(..) => None,
_ => bug!("bad slice pattern constructor {:?}", c),
})
.map(Slice::value_kind);
match slice.value_kind() {
FixedLen(self_len) => {
if other_slices.any(|other_slice| other_slice.covers_length(self_len)) {
vec![]
} else {
vec![Slice(slice)]
}
}
kind @ VarLen(..) => {
let mut remaining_slices = vec![kind];
// For each used slice, subtract from the current set of slices.
for other_slice in other_slices {
remaining_slices = remaining_slices
.into_iter()
.flat_map(|remaining_slice| remaining_slice.subtract(other_slice))
.collect();
// If the constructors that have been considered so far already cover
// the entire range of `self`, no need to look at more constructors.
if remaining_slices.is_empty() {
break;
}
}
remaining_slices
.into_iter()
.map(|kind| Slice { array_len: slice.array_len, kind })
.map(Slice)
.collect()
}
}
}
IntRange(self_range) => {
let mut remaining_ranges = vec![self_range.clone()];
for other_ctor in other_ctors {
if let IntRange(other_range) = other_ctor {
if other_range == self_range {
// If the `self` range appears directly in a `match` arm, we can
// eliminate it straight away.
remaining_ranges = vec![];
} else {
// Otherwise explicitly compute the remaining ranges.
remaining_ranges = other_range.subtract_from(remaining_ranges);
}
// If the ranges that have been considered so far already cover the entire
// range of values, we can return early.
if remaining_ranges.is_empty() {
break;
}
}
}
// Convert the ranges back into constructors.
remaining_ranges.into_iter().map(IntRange).collect()
}
// This constructor is never covered by anything else
NonExhaustive => vec![NonExhaustive],
}
}
/// Apply a constructor to a list of patterns, yielding a new pattern. `pats`
/// must have as many elements as this constructor's arity.
///
/// This is roughly the inverse of `specialize_one_pattern`.
///
/// Examples:
/// `self`: `Constructor::Single`
/// `ty`: `(u32, u32, u32)`
/// `pats`: `[10, 20, _]`
/// returns `(10, 20, _)`
///
/// `self`: `Constructor::Variant(Option::Some)`
/// `ty`: `Option<bool>`
/// `pats`: `[false]`
/// returns `Some(false)`
fn apply<'p>(
&self,
cx: &MatchCheckCtxt<'p, 'tcx>,
ty: Ty<'tcx>,
fields: Fields<'p, 'tcx>,
) -> Pat<'tcx> {
let mut subpatterns = fields.all_patterns();
let pat = match self {
Single | Variant(_) => match ty.kind() {
ty::Adt(..) | ty::Tuple(..) => {
let subpatterns = subpatterns
.enumerate()
.map(|(i, p)| FieldPat { field: Field::new(i), pattern: p })
.collect();
if let ty::Adt(adt, substs) = ty.kind() {
if adt.is_enum() {
PatKind::Variant {
adt_def: adt,
substs,
variant_index: self.variant_index_for_adt(cx, adt),
subpatterns,
}
} else {
PatKind::Leaf { subpatterns }
}
} else {
PatKind::Leaf { subpatterns }
}
}
ty::Ref(..) => PatKind::Deref { subpattern: subpatterns.next().unwrap() },
ty::Slice(_) | ty::Array(..) => bug!("bad slice pattern {:?} {:?}", self, ty),
_ => PatKind::Wild,
},
Slice(slice) => match slice.pattern_kind() {
FixedLen(_) => {
PatKind::Slice { prefix: subpatterns.collect(), slice: None, suffix: vec![] }
}
VarLen(prefix, _) => {
let mut prefix: Vec<_> = subpatterns.by_ref().take(prefix as usize).collect();
if slice.array_len.is_some() {
// Improves diagnostics a bit: if the type is a known-size array, instead
// of reporting `[x, _, .., _, y]`, we prefer to report `[x, .., y]`.
// This is incorrect if the size is not known, since `[_, ..]` captures
// arrays of lengths `>= 1` whereas `[..]` captures any length.
while !prefix.is_empty() && prefix.last().unwrap().is_wildcard() {
prefix.pop();
}
}
let suffix: Vec<_> = if slice.array_len.is_some() {
// Same as above.
subpatterns.skip_while(Pat::is_wildcard).collect()
} else {
subpatterns.collect()
};
let wild = Pat::wildcard_from_ty(ty);
PatKind::Slice { prefix, slice: Some(wild), suffix }
}
},
&ConstantValue(value) => PatKind::Constant { value },
&FloatRange(lo, hi, end) => PatKind::Range(PatRange { lo, hi, end }),
IntRange(range) => return range.to_pat(cx.tcx),
NonExhaustive => PatKind::Wild,
};
Pat { ty, span: DUMMY_SP, kind: Box::new(pat) }
}
/// Like `apply`, but where all the subpatterns are wildcards `_`.
fn apply_wildcards<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Pat<'tcx> {
self.apply(cx, ty, Fields::wildcards(cx, self, ty))
}
}
/// Some fields need to be explicitly hidden away in certain cases; see the comment above the
/// `Fields` struct. This struct represents such a potentially-hidden field. When a field is hidden
/// we still keep its type around.
#[derive(Debug, Copy, Clone)]
enum FilteredField<'p, 'tcx> {
Kept(&'p Pat<'tcx>),
Hidden(Ty<'tcx>),
}
impl<'p, 'tcx> FilteredField<'p, 'tcx> {
fn kept(self) -> Option<&'p Pat<'tcx>> {
match self {
FilteredField::Kept(p) => Some(p),
FilteredField::Hidden(_) => None,
}
}
fn to_pattern(self) -> Pat<'tcx> {
match self {
FilteredField::Kept(p) => p.clone(),
FilteredField::Hidden(ty) => Pat::wildcard_from_ty(ty),
}
}
}
/// A value can be decomposed into a constructor applied to some fields. This struct represents
/// those fields, generalized to allow patterns in each field. See also `Constructor`.
///
/// If a private or `non_exhaustive` field is uninhabited, the code mustn't observe that it is
/// uninhabited. For that, we filter these fields out of the matrix. This is subtle because we
/// still need to have those fields back when going to/from a `Pat`. Most of this is handled
/// automatically in `Fields`, but when constructing or deconstructing `Fields` you need to be
/// careful. As a rule, when going to/from the matrix, use the filtered field list; when going
/// to/from `Pat`, use the full field list.
/// This filtering is uncommon in practice, because uninhabited fields are rarely used, so we avoid
/// it when possible to preserve performance.
#[derive(Debug, Clone)]
enum Fields<'p, 'tcx> {
/// Lists of patterns that don't contain any filtered fields.
/// `Slice` and `Vec` behave the same; the difference is only to avoid allocating and
/// triple-dereferences when possible. Frankly this is premature optimization, I (Nadrieril)
/// have not measured if it really made a difference.
Slice(&'p [Pat<'tcx>]),
Vec(SmallVec<[&'p Pat<'tcx>; 2]>),
/// Patterns where some of the fields need to be hidden. `kept_count` caches the number of
/// non-hidden fields.
Filtered {
fields: SmallVec<[FilteredField<'p, 'tcx>; 2]>,
kept_count: usize,
},
}
impl<'p, 'tcx> Fields<'p, 'tcx> {
fn empty() -> Self {
Fields::Slice(&[])
}
/// Construct a new `Fields` from the given pattern. Must not be used if the pattern is a field
/// of a struct/tuple/variant.
fn from_single_pattern(pat: &'p Pat<'tcx>) -> Self {
Fields::Slice(std::slice::from_ref(pat))
}
/// Construct a new `Fields` from the given patterns. You must be sure those patterns can't
/// contain fields that need to be filtered out. When in doubt, prefer `replace_fields`.
fn from_slice_unfiltered(pats: &'p [Pat<'tcx>]) -> Self {
Fields::Slice(pats)
}
/// Convenience; internal use.
fn wildcards_from_tys(
cx: &MatchCheckCtxt<'p, 'tcx>,
tys: impl IntoIterator<Item = Ty<'tcx>>,
) -> Self {
let wilds = tys.into_iter().map(Pat::wildcard_from_ty);
let pats = cx.pattern_arena.alloc_from_iter(wilds);
Fields::Slice(pats)
}
/// Creates a new list of wildcard fields for a given constructor.
fn wildcards(
cx: &MatchCheckCtxt<'p, 'tcx>,
constructor: &Constructor<'tcx>,
ty: Ty<'tcx>,
) -> Self {
let wildcard_from_ty = |ty| &*cx.pattern_arena.alloc(Pat::wildcard_from_ty(ty));
let ret = match constructor {
Single | Variant(_) => match ty.kind() {
ty::Tuple(ref fs) => {
Fields::wildcards_from_tys(cx, fs.into_iter().map(|ty| ty.expect_ty()))
}
ty::Ref(_, rty, _) => Fields::from_single_pattern(wildcard_from_ty(rty)),
ty::Adt(adt, substs) => {
if adt.is_box() {
// Use T as the sub pattern type of Box<T>.
Fields::from_single_pattern(wildcard_from_ty(substs.type_at(0)))
} else {
let variant = &adt.variants[constructor.variant_index_for_adt(cx, adt)];
// Whether we must not match the fields of this variant exhaustively.
let is_non_exhaustive =
variant.is_field_list_non_exhaustive() && !adt.did.is_local();
let field_tys = variant.fields.iter().map(|field| field.ty(cx.tcx, substs));
// In the following cases, we don't need to filter out any fields. This is
// the vast majority of real cases, since uninhabited fields are uncommon.
let has_no_hidden_fields = (adt.is_enum() && !is_non_exhaustive)
|| !field_tys.clone().any(|ty| cx.is_uninhabited(ty));
if has_no_hidden_fields {
Fields::wildcards_from_tys(cx, field_tys)
} else {
let mut kept_count = 0;
let fields = variant
.fields
.iter()
.map(|field| {
let ty = field.ty(cx.tcx, substs);
let is_visible = adt.is_enum()
|| field.vis.is_accessible_from(cx.module, cx.tcx);
let is_uninhabited = cx.is_uninhabited(ty);
// In the cases of either a `#[non_exhaustive]` field list
// or a non-public field, we hide uninhabited fields in
// order not to reveal the uninhabitedness of the whole
// variant.
if is_uninhabited && (!is_visible || is_non_exhaustive) {
FilteredField::Hidden(ty)
} else {
kept_count += 1;
FilteredField::Kept(wildcard_from_ty(ty))
}
})
.collect();
Fields::Filtered { fields, kept_count }
}
}
}
_ => Fields::empty(),
},
Slice(slice) => match *ty.kind() {
ty::Slice(ty) | ty::Array(ty, _) => {
let arity = slice.arity();
Fields::wildcards_from_tys(cx, (0..arity).map(|_| ty))
}
_ => bug!("bad slice pattern {:?} {:?}", constructor, ty),
},
ConstantValue(..) | FloatRange(..) | IntRange(..) | NonExhaustive => Fields::empty(),
};
debug!("Fields::wildcards({:?}, {:?}) = {:#?}", constructor, ty, ret);
ret
}
/// Returns the number of patterns from the viewpoint of match-checking, i.e. excluding hidden
/// fields. This is what we want in most cases in this file, the only exception being
/// conversion to/from `Pat`.
fn len(&self) -> usize {
match self {
Fields::Slice(pats) => pats.len(),
Fields::Vec(pats) => pats.len(),
Fields::Filtered { kept_count, .. } => *kept_count,
}
}
/// Returns the complete list of patterns, including hidden fields.
fn all_patterns(self) -> impl Iterator<Item = Pat<'tcx>> {
let pats: SmallVec<[_; 2]> = match self {
Fields::Slice(pats) => pats.iter().cloned().collect(),
Fields::Vec(pats) => pats.into_iter().cloned().collect(),
Fields::Filtered { fields, .. } => {
// We don't skip any fields here.
fields.into_iter().map(|p| p.to_pattern()).collect()
}
};
pats.into_iter()
}
/// Overrides some of the fields with the provided patterns. Exactly like
/// `replace_fields_indexed`, except that it takes `FieldPat`s as input.
fn replace_with_fieldpats(
&self,
new_pats: impl IntoIterator<Item = &'p FieldPat<'tcx>>,
) -> Self {
self.replace_fields_indexed(
new_pats.into_iter().map(|pat| (pat.field.index(), &pat.pattern)),
)
}
/// Overrides some of the fields with the provided patterns. This is used when a pattern
/// defines some fields but not all, for example `Foo { field1: Some(_), .. }`: here we start with a
/// `Fields` that is just one wildcard per field of the `Foo` struct, and override the entry
/// corresponding to `field1` with the pattern `Some(_)`. This is also used for slice patterns
/// for the same reason.
fn replace_fields_indexed(
&self,
new_pats: impl IntoIterator<Item = (usize, &'p Pat<'tcx>)>,
) -> Self {
let mut fields = self.clone();
if let Fields::Slice(pats) = fields {
fields = Fields::Vec(pats.iter().collect());
}
match &mut fields {
Fields::Vec(pats) => {
for (i, pat) in new_pats {
pats[i] = pat
}
}
Fields::Filtered { fields, .. } => {
for (i, pat) in new_pats {
if let FilteredField::Kept(p) = &mut fields[i] {
*p = pat
}
}
}
Fields::Slice(_) => unreachable!(),
}
fields
}
/// Replaces contained fields with the given filtered list of patterns, e.g. taken from the
/// matrix. There must be `len()` patterns in `pats`.
fn replace_fields(
&self,
cx: &MatchCheckCtxt<'p, 'tcx>,
pats: impl IntoIterator<Item = Pat<'tcx>>,
) -> Self {
let pats: &[_] = cx.pattern_arena.alloc_from_iter(pats);
match self {
Fields::Filtered { fields, kept_count } => {
let mut pats = pats.iter();
let mut fields = fields.clone();
for f in &mut fields {
if let FilteredField::Kept(p) = f {
// We take one input pattern for each `Kept` field, in order.
*p = pats.next().unwrap();
}
}
Fields::Filtered { fields, kept_count: *kept_count }
}
_ => Fields::Slice(pats),
}
}
fn push_on_patstack(self, stack: &[&'p Pat<'tcx>]) -> PatStack<'p, 'tcx> {
let pats: SmallVec<_> = match self {
Fields::Slice(pats) => pats.iter().chain(stack.iter().copied()).collect(),
Fields::Vec(mut pats) => {
pats.extend_from_slice(stack);
pats
}
Fields::Filtered { fields, .. } => {
// We skip hidden fields here
fields.into_iter().filter_map(|p| p.kept()).chain(stack.iter().copied()).collect()
}
};
PatStack::from_vec(pats)
}
}
#[derive(Clone, Debug)]
crate enum Usefulness<'tcx> {
/// Carries a list of unreachable subpatterns. Used only in the presence of or-patterns.
Useful(Vec<Span>),
/// Carries a list of witnesses of non-exhaustiveness.
UsefulWithWitness(Vec<Witness<'tcx>>),
NotUseful,
}
impl<'tcx> Usefulness<'tcx> {
fn new_useful(preference: WitnessPreference) -> Self {
match preference {
ConstructWitness => UsefulWithWitness(vec![Witness(vec![])]),
LeaveOutWitness => Useful(vec![]),
}
}
fn is_useful(&self) -> bool {
match *self {
NotUseful => false,
_ => true,
}
}
fn apply_constructor<'p>(
self,
cx: &MatchCheckCtxt<'p, 'tcx>,
ctor: &Constructor<'tcx>,
ty: Ty<'tcx>,
ctor_wild_subpatterns: &Fields<'p, 'tcx>,
) -> Self {
match self {
UsefulWithWitness(witnesses) => UsefulWithWitness(
witnesses
.into_iter()
.map(|witness| witness.apply_constructor(cx, &ctor, ty, ctor_wild_subpatterns))
.collect(),
),
x => x,
}
}
fn apply_wildcard(self, ty: Ty<'tcx>) -> Self {
match self {
UsefulWithWitness(witnesses) => {
let wild = Pat::wildcard_from_ty(ty);
UsefulWithWitness(
witnesses
.into_iter()
.map(|mut witness| {
witness.0.push(wild.clone());
witness
})
.collect(),
)
}
x => x,
}
}
fn apply_missing_ctors(
self,
cx: &MatchCheckCtxt<'_, 'tcx>,
ty: Ty<'tcx>,
missing_ctors: &MissingConstructors<'tcx>,
) -> Self {
match self {
UsefulWithWitness(witnesses) => {
let new_patterns: Vec<_> =
missing_ctors.iter().map(|ctor| ctor.apply_wildcards(cx, ty)).collect();
// Add the new patterns to each witness
UsefulWithWitness(
witnesses
.into_iter()
.flat_map(|witness| {
new_patterns.iter().map(move |pat| {
let mut witness = witness.clone();
witness.0.push(pat.clone());
witness
})
})
.collect(),
)
}
x => x,
}
}
}
#[derive(Copy, Clone, Debug)]
crate enum WitnessPreference {
ConstructWitness,
LeaveOutWitness,
}
#[derive(Copy, Clone, Debug)]
struct PatCtxt<'tcx> {
ty: Ty<'tcx>,
span: Span,
}
/// A witness of non-exhaustiveness for error reporting, represented
/// as a list of patterns (in reverse order of construction) with
/// wildcards inside to represent elements that can take any inhabitant
/// of the type as a value.
///
/// A witness against a list of patterns should have the same types
/// and length as the pattern matched against. Because Rust `match`
/// is always against a single pattern, at the end the witness will
/// have length 1, but in the middle of the algorithm, it can contain
/// multiple patterns.
///
/// For example, if we are constructing a witness for the match against
///
/// ```
/// struct Pair(Option<(u32, u32)>, bool);
///
/// match (p: Pair) {
/// Pair(None, _) => {}
/// Pair(_, false) => {}
/// }
/// ```
///
/// We'll perform the following steps:
/// 1. Start with an empty witness
/// `Witness(vec![])`
/// 2. Push a witness `Some(_)` against the `None`
/// `Witness(vec![Some(_)])`
/// 3. Push a witness `true` against the `false`
/// `Witness(vec![Some(_), true])`
/// 4. Apply the `Pair` constructor to the witnesses
/// `Witness(vec![Pair(Some(_), true)])`
///
/// The final `Pair(Some(_), true)` is then the resulting witness.
#[derive(Clone, Debug)]
crate struct Witness<'tcx>(Vec<Pat<'tcx>>);
impl<'tcx> Witness<'tcx> {
crate fn single_pattern(self) -> Pat<'tcx> {
assert_eq!(self.0.len(), 1);
self.0.into_iter().next().unwrap()
}
/// Constructs a partial witness for a pattern given a list of
/// patterns expanded by the specialization step.
///
/// When a pattern P is discovered to be useful, this function is used bottom-up
/// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
/// of values, V, where each value in that set is not covered by any previously
/// used patterns and is covered by the pattern P'. Examples:
///
/// left_ty: tuple of 3 elements
/// pats: [10, 20, _] => (10, 20, _)
///
/// left_ty: struct X { a: (bool, &'static str), b: usize}
/// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
fn apply_constructor<'p>(
mut self,
cx: &MatchCheckCtxt<'p, 'tcx>,
ctor: &Constructor<'tcx>,
ty: Ty<'tcx>,
ctor_wild_subpatterns: &Fields<'p, 'tcx>,
) -> Self {
let pat = {
let len = self.0.len();
let arity = ctor_wild_subpatterns.len();
let pats = self.0.drain((len - arity)..).rev();
let fields = ctor_wild_subpatterns.replace_fields(cx, pats);
ctor.apply(cx, ty, fields)
};
self.0.push(pat);
self
}
}
/// This determines the set of all possible constructors of a pattern matching
/// values of type `left_ty`. For vectors, this would normally be an infinite set
/// but is instead bounded by the maximum fixed length of slice patterns in
/// the column of patterns being analyzed.
///
/// We make sure to omit constructors that are statically impossible. E.g., for
/// `Option<!>`, we do not include `Some(_)` in the returned list of constructors.
/// Invariant: this returns an empty `Vec` if and only if the type is uninhabited (as determined by
/// `cx.is_uninhabited()`).
fn all_constructors<'a, 'tcx>(
cx: &mut MatchCheckCtxt<'a, 'tcx>,
pcx: PatCtxt<'tcx>,
) -> Vec<Constructor<'tcx>> {
debug!("all_constructors({:?})", pcx.ty);
let make_range = |start, end| {
IntRange(
// `unwrap()` is ok because we know the type is an integer.
IntRange::from_range(cx.tcx, start, end, pcx.ty, &RangeEnd::Included, pcx.span)
.unwrap(),
)
};
match *pcx.ty.kind() {
ty::Bool => {
[true, false].iter().map(|&b| ConstantValue(ty::Const::from_bool(cx.tcx, b))).collect()
}
ty::Array(ref sub_ty, len) if len.try_eval_usize(cx.tcx, cx.param_env).is_some() => {
let len = len.eval_usize(cx.tcx, cx.param_env);
if len != 0 && cx.is_uninhabited(sub_ty) {
vec![]
} else {
vec![Slice(Slice { array_len: Some(len), kind: VarLen(0, 0) })]
}
}
// Treat arrays of a constant but unknown length like slices.
ty::Array(ref sub_ty, _) | ty::Slice(ref sub_ty) => {
let kind = if cx.is_uninhabited(sub_ty) { FixedLen(0) } else { VarLen(0, 0) };
vec![Slice(Slice { array_len: None, kind })]
}
ty::Adt(def, substs) if def.is_enum() => {
let ctors: Vec<_> = if cx.tcx.features().exhaustive_patterns {
// If `exhaustive_patterns` is enabled, we exclude variants known to be
// uninhabited.
def.variants
.iter()
.filter(|v| {
!v.uninhabited_from(cx.tcx, substs, def.adt_kind(), cx.param_env)
.contains(cx.tcx, cx.module)
})
.map(|v| Variant(v.def_id))
.collect()
} else {
def.variants.iter().map(|v| Variant(v.def_id)).collect()
};
// If the enum is declared as `#[non_exhaustive]`, we treat it as if it had an
// additional "unknown" constructor.
// There is no point in enumerating all possible variants, because the user can't
// actually match against them all themselves. So we always return only the fictitious
// constructor.
// E.g., in an example like:
//
// ```
// let err: io::ErrorKind = ...;
// match err {
// io::ErrorKind::NotFound => {},
// }
// ```
//
// we don't want to show every possible IO error, but instead have only `_` as the
// witness.
let is_declared_nonexhaustive = cx.is_foreign_non_exhaustive_enum(pcx.ty);
// If `exhaustive_patterns` is disabled and our scrutinee is an empty enum, we treat it
// as though it had an "unknown" constructor to avoid exposing its emptyness. Note that
// an empty match will still be considered exhaustive because that case is handled
// separately in `check_match`.
let is_secretly_empty =
def.variants.is_empty() && !cx.tcx.features().exhaustive_patterns;
if is_secretly_empty || is_declared_nonexhaustive { vec![NonExhaustive] } else { ctors }
}
ty::Char => {
vec![
// The valid Unicode Scalar Value ranges.
make_range('\u{0000}' as u128, '\u{D7FF}' as u128),
make_range('\u{E000}' as u128, '\u{10FFFF}' as u128),
]
}
ty::Int(_) | ty::Uint(_)
if pcx.ty.is_ptr_sized_integral()
&& !cx.tcx.features().precise_pointer_size_matching =>
{
// `usize`/`isize` are not allowed to be matched exhaustively unless the
// `precise_pointer_size_matching` feature is enabled. So we treat those types like
// `#[non_exhaustive]` enums by returning a special unmatcheable constructor.
vec![NonExhaustive]
}
ty::Int(ity) => {
let bits = Integer::from_attr(&cx.tcx, SignedInt(ity)).size().bits() as u128;
let min = 1u128 << (bits - 1);
let max = min - 1;
vec![make_range(min, max)]
}
ty::Uint(uty) => {
let size = Integer::from_attr(&cx.tcx, UnsignedInt(uty)).size();
let max = truncate(u128::MAX, size);
vec![make_range(0, max)]
}
_ => {
if cx.is_uninhabited(pcx.ty) {
vec![]
} else {
vec![Single]
}
}
}
}
/// An inclusive interval, used for precise integer exhaustiveness checking.
/// `IntRange`s always store a contiguous range. This means that values are
/// encoded such that `0` encodes the minimum value for the integer,
/// regardless of the signedness.
/// For example, the pattern `-128..=127i8` is encoded as `0..=255`.
/// This makes comparisons and arithmetic on interval endpoints much more
/// straightforward. See `signed_bias` for details.
///
/// `IntRange` is never used to encode an empty range or a "range" that wraps
/// around the (offset) space: i.e., `range.lo <= range.hi`.
#[derive(Clone, Debug)]
struct IntRange<'tcx> {
range: RangeInclusive<u128>,
ty: Ty<'tcx>,
span: Span,
}
impl<'tcx> IntRange<'tcx> {
#[inline]
fn is_integral(ty: Ty<'_>) -> bool {
match ty.kind() {
ty::Char | ty::Int(_) | ty::Uint(_) => true,
_ => false,
}
}
fn is_singleton(&self) -> bool {
self.range.start() == self.range.end()
}
fn boundaries(&self) -> (u128, u128) {
(*self.range.start(), *self.range.end())
}
/// Don't treat `usize`/`isize` exhaustively unless the `precise_pointer_size_matching` feature
/// is enabled.
fn treat_exhaustively(&self, tcx: TyCtxt<'tcx>) -> bool {
!self.ty.is_ptr_sized_integral() || tcx.features().precise_pointer_size_matching
}
#[inline]
fn integral_size_and_signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'_>) -> Option<(Size, u128)> {
match *ty.kind() {
ty::Char => Some((Size::from_bytes(4), 0)),
ty::Int(ity) => {
let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
Some((size, 1u128 << (size.bits() as u128 - 1)))
}
ty::Uint(uty) => Some((Integer::from_attr(&tcx, UnsignedInt(uty)).size(), 0)),
_ => None,
}
}
#[inline]
fn from_const(
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
value: &Const<'tcx>,
span: Span,
) -> Option<IntRange<'tcx>> {
if let Some((target_size, bias)) = Self::integral_size_and_signed_bias(tcx, value.ty) {
let ty = value.ty;
let val = (|| {
if let ty::ConstKind::Value(ConstValue::Scalar(scalar)) = value.val {
// For this specific pattern we can skip a lot of effort and go
// straight to the result, after doing a bit of checking. (We
// could remove this branch and just fall through, which
// is more general but much slower.)
if let Ok(bits) = scalar.to_bits_or_ptr(target_size, &tcx) {
return Some(bits);
}
}
// This is a more general form of the previous case.
value.try_eval_bits(tcx, param_env, ty)
})()?;
let val = val ^ bias;
Some(IntRange { range: val..=val, ty, span })
} else {
None
}
}
#[inline]
fn from_range(
tcx: TyCtxt<'tcx>,
lo: u128,
hi: u128,
ty: Ty<'tcx>,
end: &RangeEnd,
span: Span,
) -> Option<IntRange<'tcx>> {
if Self::is_integral(ty) {
// Perform a shift if the underlying types are signed,
// which makes the interval arithmetic simpler.
let bias = IntRange::signed_bias(tcx, ty);
let (lo, hi) = (lo ^ bias, hi ^ bias);
let offset = (*end == RangeEnd::Excluded) as u128;
if lo > hi || (lo == hi && *end == RangeEnd::Excluded) {
// This should have been caught earlier by E0030.
bug!("malformed range pattern: {}..={}", lo, (hi - offset));
}
Some(IntRange { range: lo..=(hi - offset), ty, span })
} else {
None
}
}
fn from_pat(
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
pat: &Pat<'tcx>,
) -> Option<IntRange<'tcx>> {
// This MUST be kept in sync with `pat_constructor`.
match *pat.kind {
PatKind::AscribeUserType { .. } => bug!(), // Handled by `expand_pattern`
PatKind::Or { .. } => bug!("Or-pattern should have been expanded earlier on."),
PatKind::Binding { .. }
| PatKind::Wild
| PatKind::Leaf { .. }
| PatKind::Deref { .. }
| PatKind::Variant { .. }
| PatKind::Array { .. }
| PatKind::Slice { .. } => None,
PatKind::Constant { value } => Self::from_const(tcx, param_env, value, pat.span),
PatKind::Range(PatRange { lo, hi, end }) => {
let ty = lo.ty;
Self::from_range(
tcx,
lo.eval_bits(tcx, param_env, lo.ty),
hi.eval_bits(tcx, param_env, hi.ty),
ty,
&end,
pat.span,
)
}
}
}
// The return value of `signed_bias` should be XORed with an endpoint to encode/decode it.
fn signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> u128 {
match *ty.kind() {
ty::Int(ity) => {
let bits = Integer::from_attr(&tcx, SignedInt(ity)).size().bits() as u128;
1u128 << (bits - 1)
}
_ => 0,
}
}
/// Returns a collection of ranges that spans the values covered by `ranges`, subtracted
/// by the values covered by `self`: i.e., `ranges \ self` (in set notation).
fn subtract_from(&self, ranges: Vec<IntRange<'tcx>>) -> Vec<IntRange<'tcx>> {
let mut remaining_ranges = vec![];
let ty = self.ty;
let span = self.span;
let (lo, hi) = self.boundaries();
for subrange in ranges {
let (subrange_lo, subrange_hi) = subrange.range.into_inner();
if lo > subrange_hi || subrange_lo > hi {
// The pattern doesn't intersect with the subrange at all,
// so the subrange remains untouched.
remaining_ranges.push(IntRange { range: subrange_lo..=subrange_hi, ty, span });
} else {
if lo > subrange_lo {
// The pattern intersects an upper section of the
// subrange, so a lower section will remain.
remaining_ranges.push(IntRange { range: subrange_lo..=(lo - 1), ty, span });
}
if hi < subrange_hi {
// The pattern intersects a lower section of the
// subrange, so an upper section will remain.
remaining_ranges.push(IntRange { range: (hi + 1)..=subrange_hi, ty, span });
}
}
}
remaining_ranges
}
fn is_subrange(&self, other: &Self) -> bool {
other.range.start() <= self.range.start() && self.range.end() <= other.range.end()
}
fn intersection(&self, tcx: TyCtxt<'tcx>, other: &Self) -> Option<Self> {
let ty = self.ty;
let (lo, hi) = self.boundaries();
let (other_lo, other_hi) = other.boundaries();
if self.treat_exhaustively(tcx) {
if lo <= other_hi && other_lo <= hi {
let span = other.span;
Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi), ty, span })
} else {
None
}
} else {
// If the range should not be treated exhaustively, fallback to checking for inclusion.
if self.is_subrange(other) { Some(self.clone()) } else { None }
}
}
fn suspicious_intersection(&self, other: &Self) -> bool {
// `false` in the following cases:
// 1 ---- // 1 ---------- // 1 ---- // 1 ----
// 2 ---------- // 2 ---- // 2 ---- // 2 ----
//
// The following are currently `false`, but could be `true` in the future (#64007):
// 1 --------- // 1 ---------
// 2 ---------- // 2 ----------
//
// `true` in the following cases:
// 1 ------- // 1 -------
// 2 -------- // 2 -------
let (lo, hi) = self.boundaries();
let (other_lo, other_hi) = other.boundaries();
lo == other_hi || hi == other_lo
}
fn to_pat(&self, tcx: TyCtxt<'tcx>) -> Pat<'tcx> {
let (lo, hi) = self.boundaries();
let bias = IntRange::signed_bias(tcx, self.ty);
let (lo, hi) = (lo ^ bias, hi ^ bias);
let ty = ty::ParamEnv::empty().and(self.ty);
let lo_const = ty::Const::from_bits(tcx, lo, ty);
let hi_const = ty::Const::from_bits(tcx, hi, ty);
let kind = if lo == hi {
PatKind::Constant { value: lo_const }
} else {
PatKind::Range(PatRange { lo: lo_const, hi: hi_const, end: RangeEnd::Included })
};
// This is a brand new pattern, so we don't reuse `self.span`.
Pat { ty: self.ty, span: DUMMY_SP, kind: Box::new(kind) }
}
}
/// Ignore spans when comparing, they don't carry semantic information as they are only for lints.
impl<'tcx> std::cmp::PartialEq for IntRange<'tcx> {
fn eq(&self, other: &Self) -> bool {
self.range == other.range && self.ty == other.ty
}
}
// A struct to compute a set of constructors equivalent to `all_ctors \ used_ctors`.
struct MissingConstructors<'tcx> {
all_ctors: Vec<Constructor<'tcx>>,
used_ctors: Vec<Constructor<'tcx>>,
}
impl<'tcx> MissingConstructors<'tcx> {
fn new(all_ctors: Vec<Constructor<'tcx>>, used_ctors: Vec<Constructor<'tcx>>) -> Self {
MissingConstructors { all_ctors, used_ctors }
}
fn into_inner(self) -> (Vec<Constructor<'tcx>>, Vec<Constructor<'tcx>>) {
(self.all_ctors, self.used_ctors)
}
fn is_empty(&self) -> bool {
self.iter().next().is_none()
}
/// Whether this contains all the constructors for the given type or only a
/// subset.
fn all_ctors_are_missing(&self) -> bool {
self.used_ctors.is_empty()
}
/// Iterate over all_ctors \ used_ctors
fn iter<'a>(&'a self) -> impl Iterator<Item = Constructor<'tcx>> + Captures<'a> {
self.all_ctors.iter().flat_map(move |req_ctor| req_ctor.subtract_ctors(&self.used_ctors))
}
}
impl<'tcx> fmt::Debug for MissingConstructors<'tcx> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let ctors: Vec<_> = self.iter().collect();
write!(f, "{:?}", ctors)
}
}
/// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html.
/// The algorithm from the paper has been modified to correctly handle empty
/// types. The changes are:
/// (0) We don't exit early if the pattern matrix has zero rows. We just
/// continue to recurse over columns.
/// (1) all_constructors will only return constructors that are statically
/// possible. E.g., it will only return `Ok` for `Result<T, !>`.
///
/// This finds whether a (row) vector `v` of patterns is 'useful' in relation
/// to a set of such vectors `m` - this is defined as there being a set of
/// inputs that will match `v` but not any of the sets in `m`.
///
/// All the patterns at each column of the `matrix ++ v` matrix must have the same type.
///
/// This is used both for reachability checking (if a pattern isn't useful in
/// relation to preceding patterns, it is not reachable) and exhaustiveness
/// checking (if a wildcard pattern is useful in relation to a matrix, the
/// matrix isn't exhaustive).
///
/// `is_under_guard` is used to inform if the pattern has a guard. If it
/// has one it must not be inserted into the matrix. This shouldn't be
/// relied on for soundness.
crate fn is_useful<'p, 'tcx>(
cx: &mut MatchCheckCtxt<'p, 'tcx>,
matrix: &Matrix<'p, 'tcx>,
v: &PatStack<'p, 'tcx>,
witness_preference: WitnessPreference,
hir_id: HirId,
is_under_guard: bool,
is_top_level: bool,
) -> Usefulness<'tcx> {
let Matrix { patterns: rows, .. } = matrix;
debug!("is_useful({:#?}, {:#?})", matrix, v);
// The base case. We are pattern-matching on () and the return value is
// based on whether our matrix has a row or not.
// NOTE: This could potentially be optimized by checking rows.is_empty()
// first and then, if v is non-empty, the return value is based on whether
// the type of the tuple we're checking is inhabited or not.
if v.is_empty() {
return if rows.is_empty() {
Usefulness::new_useful(witness_preference)
} else {
NotUseful
};
};
assert!(rows.iter().all(|r| r.len() == v.len()));
// If the first pattern is an or-pattern, expand it.
if let Some(vs) = v.expand_or_pat() {
// We need to push the already-seen patterns into the matrix in order to detect redundant
// branches like `Some(_) | Some(0)`. We also keep track of the unreachable subpatterns.
let mut matrix = matrix.clone();
// `Vec` of all the unreachable branches of the current or-pattern.
let mut unreachable_branches = Vec::new();
// Subpatterns that are unreachable from all branches. E.g. in the following case, the last
// `true` is unreachable only from one branch, so it is overall reachable.
//
// ```
// match (true, true) {
// (true, true) => {}
// (false | true, false | true) => {}
// }
// ```
let mut unreachable_subpats = FxHashSet::default();
// Whether any branch at all is useful.
let mut any_is_useful = false;
for v in vs {
let res = is_useful(cx, &matrix, &v, witness_preference, hir_id, is_under_guard, false);
match res {
Useful(pats) => {
if !any_is_useful {
any_is_useful = true;
// Initialize with the first set of unreachable subpatterns encountered.
unreachable_subpats = pats.into_iter().collect();
} else {
// Keep the patterns unreachable from both this and previous branches.
unreachable_subpats =
pats.into_iter().filter(|p| unreachable_subpats.contains(p)).collect();
}
}
NotUseful => unreachable_branches.push(v.head().span),
UsefulWithWitness(_) => {
bug!("Encountered or-pat in `v` during exhaustiveness checking")
}
}
// If pattern has a guard don't add it to the matrix
if !is_under_guard {
matrix.push(v);
}
}
if any_is_useful {
// Collect all the unreachable patterns.
unreachable_branches.extend(unreachable_subpats);
return Useful(unreachable_branches);
} else {
return NotUseful;
}
}
// FIXME(Nadrieril): Hack to work around type normalization issues (see #72476).
let ty = matrix.heads().next().map(|r| r.ty).unwrap_or(v.head().ty);
let pcx = PatCtxt { ty, span: v.head().span };
debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v.head());
let ret = if let Some(constructor) = pat_constructor(cx.tcx, cx.param_env, v.head()) {
debug!("is_useful - expanding constructor: {:#?}", constructor);
split_grouped_constructors(
cx.tcx,
cx.param_env,
pcx,
vec![constructor],
matrix,
pcx.span,
Some(hir_id),
)
.into_iter()
.map(|c| {
is_useful_specialized(
cx,
matrix,
v,
c,
pcx.ty,
witness_preference,
hir_id,
is_under_guard,
)
})
.find(|result| result.is_useful())
.unwrap_or(NotUseful)
} else {
debug!("is_useful - expanding wildcard");
let used_ctors: Vec<Constructor<'_>> =
matrix.heads().filter_map(|p| pat_constructor(cx.tcx, cx.param_env, p)).collect();
debug!("is_useful_used_ctors = {:#?}", used_ctors);
// `all_ctors` are all the constructors for the given type, which
// should all be represented (or caught with the wild pattern `_`).
let all_ctors = all_constructors(cx, pcx);
debug!("is_useful_all_ctors = {:#?}", all_ctors);
// `missing_ctors` is the set of constructors from the same type as the
// first column of `matrix` that are matched only by wildcard patterns
// from the first column.
//
// Therefore, if there is some pattern that is unmatched by `matrix`,
// it will still be unmatched if the first constructor is replaced by
// any of the constructors in `missing_ctors`
// Missing constructors are those that are not matched by any non-wildcard patterns in the
// current column. We only fully construct them on-demand, because they're rarely used and
// can be big.
let missing_ctors = MissingConstructors::new(all_ctors, used_ctors);
debug!("is_useful_missing_ctors.empty()={:#?}", missing_ctors.is_empty(),);
if missing_ctors.is_empty() {
let (all_ctors, _) = missing_ctors.into_inner();
split_grouped_constructors(cx.tcx, cx.param_env, pcx, all_ctors, matrix, DUMMY_SP, None)
.into_iter()
.map(|c| {
is_useful_specialized(
cx,
matrix,
v,
c,
pcx.ty,
witness_preference,
hir_id,
is_under_guard,
)
})
.find(|result| result.is_useful())
.unwrap_or(NotUseful)
} else {
let matrix = matrix.specialize_wildcard();
let v = v.to_tail();
let usefulness =
is_useful(cx, &matrix, &v, witness_preference, hir_id, is_under_guard, false);
// In this case, there's at least one "free"
// constructor that is only matched against by
// wildcard patterns.
//
// There are 2 ways we can report a witness here.
// Commonly, we can report all the "free"
// constructors as witnesses, e.g., if we have:
//
// ```
// enum Direction { N, S, E, W }
// let Direction::N = ...;
// ```
//
// we can report 3 witnesses: `S`, `E`, and `W`.
//
// However, there is a case where we don't want
// to do this and instead report a single `_` witness:
// if the user didn't actually specify a constructor
// in this arm, e.g., in
//
// ```
// let x: (Direction, Direction, bool) = ...;
// let (_, _, false) = x;
// ```
//
// we don't want to show all 16 possible witnesses
// `(<direction-1>, <direction-2>, true)` - we are
// satisfied with `(_, _, true)`. In this case,
// `used_ctors` is empty.
// The exception is: if we are at the top-level, for example in an empty match, we
// sometimes prefer reporting the list of constructors instead of just `_`.
let report_ctors_rather_than_wildcard = is_top_level && !IntRange::is_integral(pcx.ty);
if missing_ctors.all_ctors_are_missing() && !report_ctors_rather_than_wildcard {
// All constructors are unused. Add a wild pattern
// rather than each individual constructor.
usefulness.apply_wildcard(pcx.ty)
} else {
// Construct for each missing constructor a "wild" version of this
// constructor, that matches everything that can be built with
// it. For example, if `ctor` is a `Constructor::Variant` for
// `Option::Some`, we get the pattern `Some(_)`.
usefulness.apply_missing_ctors(cx, pcx.ty, &missing_ctors)
}
}
};
debug!("is_useful::returns({:#?}, {:#?}) = {:?}", matrix, v, ret);
ret
}
/// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e., `is_useful` applied
/// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
fn is_useful_specialized<'p, 'tcx>(
cx: &mut MatchCheckCtxt<'p, 'tcx>,
matrix: &Matrix<'p, 'tcx>,
v: &PatStack<'p, 'tcx>,
ctor: Constructor<'tcx>,
ty: Ty<'tcx>,
witness_preference: WitnessPreference,
hir_id: HirId,
is_under_guard: bool,
) -> Usefulness<'tcx> {
debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, ty);
// We cache the result of `Fields::wildcards` because it is used a lot.
let ctor_wild_subpatterns = Fields::wildcards(cx, &ctor, ty);
let matrix = matrix.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns);
v.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns)
.map(|v| is_useful(cx, &matrix, &v, witness_preference, hir_id, is_under_guard, false))
.map(|u| u.apply_constructor(cx, &ctor, ty, &ctor_wild_subpatterns))
.unwrap_or(NotUseful)
}
/// Determines the constructor that the given pattern can be specialized to.
/// Returns `None` in case of a catch-all, which can't be specialized.
fn pat_constructor<'tcx>(
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
pat: &Pat<'tcx>,
) -> Option<Constructor<'tcx>> {
// This MUST be kept in sync with `IntRange::from_pat`.
match *pat.kind {
PatKind::AscribeUserType { .. } => bug!(), // Handled by `expand_pattern`
PatKind::Binding { .. } | PatKind::Wild => None,
PatKind::Leaf { .. } | PatKind::Deref { .. } => Some(Single),
PatKind::Variant { adt_def, variant_index, .. } => {
Some(Variant(adt_def.variants[variant_index].def_id))
}
PatKind::Constant { value } => {
if let Some(int_range) = IntRange::from_const(tcx, param_env, value, pat.span) {
Some(IntRange(int_range))
} else {
match (value.val, &value.ty.kind()) {
(_, ty::Array(_, n)) => {
let len = n.eval_usize(tcx, param_env);
Some(Slice(Slice { array_len: Some(len), kind: FixedLen(len) }))
}
(ty::ConstKind::Value(ConstValue::Slice { start, end, .. }), ty::Slice(_)) => {
let len = (end - start) as u64;
Some(Slice(Slice { array_len: None, kind: FixedLen(len) }))
}
// FIXME(oli-obk): implement `deref` for `ConstValue`
// (ty::ConstKind::Value(ConstValue::ByRef { .. }), ty::Slice(_)) => { ... }
_ => Some(ConstantValue(value)),
}
}
}
PatKind::Range(PatRange { lo, hi, end }) => {
let ty = lo.ty;
if let Some(int_range) = IntRange::from_range(
tcx,
lo.eval_bits(tcx, param_env, lo.ty),
hi.eval_bits(tcx, param_env, hi.ty),
ty,
&end,
pat.span,
) {
Some(IntRange(int_range))
} else {
Some(FloatRange(lo, hi, end))
}
}
PatKind::Array { ref prefix, ref slice, ref suffix }
| PatKind::Slice { ref prefix, ref slice, ref suffix } => {
let array_len = match pat.ty.kind() {
ty::Array(_, length) => Some(length.eval_usize(tcx, param_env)),
ty::Slice(_) => None,
_ => span_bug!(pat.span, "bad ty {:?} for slice pattern", pat.ty),
};
let prefix = prefix.len() as u64;
let suffix = suffix.len() as u64;
let kind =
if slice.is_some() { VarLen(prefix, suffix) } else { FixedLen(prefix + suffix) };
Some(Slice(Slice { array_len, kind }))
}
PatKind::Or { .. } => bug!("Or-pattern should have been expanded earlier on."),
}
}
// checks whether a constant is equal to a user-written slice pattern. Only supports byte slices,
// meaning all other types will compare unequal and thus equal patterns often do not cause the
// second pattern to lint about unreachable match arms.
fn slice_pat_covered_by_const<'tcx>(
tcx: TyCtxt<'tcx>,
_span: Span,
const_val: &'tcx ty::Const<'tcx>,
prefix: &[Pat<'tcx>],
slice: &Option<Pat<'tcx>>,
suffix: &[Pat<'tcx>],
param_env: ty::ParamEnv<'tcx>,
) -> Result<bool, ErrorReported> {
let const_val_val = if let ty::ConstKind::Value(val) = const_val.val {
val
} else {
bug!(
"slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
const_val,
prefix,
slice,
suffix,
)
};
let data: &[u8] = match (const_val_val, &const_val.ty.kind()) {
(ConstValue::ByRef { offset, alloc, .. }, ty::Array(t, n)) => {
assert_eq!(*t, tcx.types.u8);
let n = n.eval_usize(tcx, param_env);
let ptr = Pointer::new(AllocId(0), offset);
alloc.get_bytes(&tcx, ptr, Size::from_bytes(n)).unwrap()
}
(ConstValue::Slice { data, start, end }, ty::Slice(t)) => {
assert_eq!(*t, tcx.types.u8);
let ptr = Pointer::new(AllocId(0), Size::from_bytes(start));
data.get_bytes(&tcx, ptr, Size::from_bytes(end - start)).unwrap()
}
// FIXME(oli-obk): create a way to extract fat pointers from ByRef
(_, ty::Slice(_)) => return Ok(false),
_ => bug!(
"slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
const_val,
prefix,
slice,
suffix,
),
};
let pat_len = prefix.len() + suffix.len();
if data.len() < pat_len || (slice.is_none() && data.len() > pat_len) {
return Ok(false);
}
for (ch, pat) in data[..prefix.len()]
.iter()
.zip(prefix)
.chain(data[data.len() - suffix.len()..].iter().zip(suffix))
{
if let box PatKind::Constant { value } = pat.kind {
let b = value.eval_bits(tcx, param_env, pat.ty);
assert_eq!(b as u8 as u128, b);
if b as u8 != *ch {
return Ok(false);
}
}
}
Ok(true)
}
/// For exhaustive integer matching, some constructors are grouped within other constructors
/// (namely integer typed values are grouped within ranges). However, when specialising these
/// constructors, we want to be specialising for the underlying constructors (the integers), not
/// the groups (the ranges). Thus we need to split the groups up. Splitting them up naïvely would
/// mean creating a separate constructor for every single value in the range, which is clearly
/// impractical. However, observe that for some ranges of integers, the specialisation will be
/// identical across all values in that range (i.e., there are equivalence classes of ranges of
/// constructors based on their `is_useful_specialized` outcome). These classes are grouped by
/// the patterns that apply to them (in the matrix `P`). We can split the range whenever the
/// patterns that apply to that range (specifically: the patterns that *intersect* with that range)
/// change.
/// Our solution, therefore, is to split the range constructor into subranges at every single point
/// the group of intersecting patterns changes (using the method described below).
/// And voilà! We're testing precisely those ranges that we need to, without any exhaustive matching
/// on actual integers. The nice thing about this is that the number of subranges is linear in the
/// number of rows in the matrix (i.e., the number of cases in the `match` statement), so we don't
/// need to be worried about matching over gargantuan ranges.
///
/// Essentially, given the first column of a matrix representing ranges, looking like the following:
///
/// |------| |----------| |-------| ||
/// |-------| |-------| |----| ||
/// |---------|
///
/// We split the ranges up into equivalence classes so the ranges are no longer overlapping:
///
/// |--|--|||-||||--||---|||-------| |-|||| ||
///
/// The logic for determining how to split the ranges is fairly straightforward: we calculate
/// boundaries for each interval range, sort them, then create constructors for each new interval
/// between every pair of boundary points. (This essentially sums up to performing the intuitive
/// merging operation depicted above.)
///
/// `hir_id` is `None` when we're evaluating the wildcard pattern, do not lint for overlapping in
/// ranges that case.
///
/// This also splits variable-length slices into fixed-length slices.
fn split_grouped_constructors<'p, 'tcx>(
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
pcx: PatCtxt<'tcx>,
ctors: Vec<Constructor<'tcx>>,
matrix: &Matrix<'p, 'tcx>,
span: Span,
hir_id: Option<HirId>,
) -> Vec<Constructor<'tcx>> {
let ty = pcx.ty;
let mut split_ctors = Vec::with_capacity(ctors.len());
debug!("split_grouped_constructors({:#?}, {:#?})", matrix, ctors);
for ctor in ctors.into_iter() {
match ctor {
IntRange(ctor_range) if ctor_range.treat_exhaustively(tcx) => {
// Fast-track if the range is trivial. In particular, don't do the overlapping
// ranges check.
if ctor_range.is_singleton() {
split_ctors.push(IntRange(ctor_range));
continue;
}
/// Represents a border between 2 integers. Because the intervals spanning borders
/// must be able to cover every integer, we need to be able to represent
/// 2^128 + 1 such borders.
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug)]
enum Border {
JustBefore(u128),
AfterMax,
}
// A function for extracting the borders of an integer interval.
fn range_borders(r: IntRange<'_>) -> impl Iterator<Item = Border> {
let (lo, hi) = r.range.into_inner();
let from = Border::JustBefore(lo);
let to = match hi.checked_add(1) {
Some(m) => Border::JustBefore(m),
None => Border::AfterMax,
};
vec![from, to].into_iter()
}
// Collect the span and range of all the intersecting ranges to lint on likely
// incorrect range patterns. (#63987)
let mut overlaps = vec![];
// `borders` is the set of borders between equivalence classes: each equivalence
// class lies between 2 borders.
let row_borders = matrix
.patterns
.iter()
.flat_map(|row| {
IntRange::from_pat(tcx, param_env, row.head()).map(|r| (r, row.len()))
})
.flat_map(|(range, row_len)| {
let intersection = ctor_range.intersection(tcx, &range);
let should_lint = ctor_range.suspicious_intersection(&range);
if let (Some(range), 1, true) = (&intersection, row_len, should_lint) {
// FIXME: for now, only check for overlapping ranges on simple range
// patterns. Otherwise with the current logic the following is detected
// as overlapping:
// match (10u8, true) {
// (0 ..= 125, false) => {}
// (126 ..= 255, false) => {}
// (0 ..= 255, true) => {}
// }
overlaps.push(range.clone());
}
intersection
})
.flat_map(range_borders);
let ctor_borders = range_borders(ctor_range.clone());
let mut borders: Vec<_> = row_borders.chain(ctor_borders).collect();
borders.sort_unstable();
lint_overlapping_patterns(tcx, hir_id, ctor_range, ty, overlaps);
// We're going to iterate through every adjacent pair of borders, making sure that
// each represents an interval of nonnegative length, and convert each such
// interval into a constructor.
split_ctors.extend(
borders
.array_windows()
.filter_map(|&pair| match pair {
[Border::JustBefore(n), Border::JustBefore(m)] => {
if n < m {
Some(IntRange { range: n..=(m - 1), ty, span })
} else {
None
}
}
[Border::JustBefore(n), Border::AfterMax] => {
Some(IntRange { range: n..=u128::MAX, ty, span })
}
[Border::AfterMax, _] => None,
})
.map(IntRange),
);
}
Slice(Slice { array_len, kind: VarLen(self_prefix, self_suffix) }) => {
// The exhaustiveness-checking paper does not include any details on
// checking variable-length slice patterns. However, they are matched
// by an infinite collection of fixed-length array patterns.
//
// Checking the infinite set directly would take an infinite amount
// of time. However, it turns out that for each finite set of
// patterns `P`, all sufficiently large array lengths are equivalent:
//
// Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
// to exactly the subset `Pₜ` of `P` can be transformed to a slice
// `sₘ` for each sufficiently-large length `m` that applies to exactly
// the same subset of `P`.
//
// Because of that, each witness for reachability-checking from one
// of the sufficiently-large lengths can be transformed to an
// equally-valid witness from any other length, so we only have
// to check slice lengths from the "minimal sufficiently-large length"
// and below.
//
// Note that the fact that there is a *single* `sₘ` for each `m`
// not depending on the specific pattern in `P` is important: if
// you look at the pair of patterns
// `[true, ..]`
// `[.., false]`
// Then any slice of length ≥1 that matches one of these two
// patterns can be trivially turned to a slice of any
// other length ≥1 that matches them and vice-versa - for
// but the slice from length 2 `[false, true]` that matches neither
// of these patterns can't be turned to a slice from length 1 that
// matches neither of these patterns, so we have to consider
// slices from length 2 there.
//
// Now, to see that that length exists and find it, observe that slice
// patterns are either "fixed-length" patterns (`[_, _, _]`) or
// "variable-length" patterns (`[_, .., _]`).
//
// For fixed-length patterns, all slices with lengths *longer* than
// the pattern's length have the same outcome (of not matching), so
// as long as `L` is greater than the pattern's length we can pick
// any `sₘ` from that length and get the same result.
//
// For variable-length patterns, the situation is more complicated,
// because as seen above the precise value of `sₘ` matters.
//
// However, for each variable-length pattern `p` with a prefix of length
// `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
// `slₚ` elements are examined.
//
// Therefore, as long as `L` is positive (to avoid concerns about empty
// types), all elements after the maximum prefix length and before
// the maximum suffix length are not examined by any variable-length
// pattern, and therefore can be added/removed without affecting
// them - creating equivalent patterns from any sufficiently-large
// length.
//
// Of course, if fixed-length patterns exist, we must be sure
// that our length is large enough to miss them all, so
// we can pick `L = max(max(FIXED_LEN)+1, max(PREFIX_LEN) + max(SUFFIX_LEN))`
//
// for example, with the above pair of patterns, all elements
// but the first and last can be added/removed, so any
// witness of length ≥2 (say, `[false, false, true]`) can be
// turned to a witness from any other length ≥2.
let mut max_prefix_len = self_prefix;
let mut max_suffix_len = self_suffix;
let mut max_fixed_len = 0;
let head_ctors =
matrix.heads().filter_map(|pat| pat_constructor(tcx, param_env, pat));
for ctor in head_ctors {
if let Slice(slice) = ctor {
match slice.pattern_kind() {
FixedLen(len) => {
max_fixed_len = cmp::max(max_fixed_len, len);
}
VarLen(prefix, suffix) => {
max_prefix_len = cmp::max(max_prefix_len, prefix);
max_suffix_len = cmp::max(max_suffix_len, suffix);
}
}
}
}
// For diagnostics, we keep the prefix and suffix lengths separate, so in the case
// where `max_fixed_len + 1` is the largest, we adapt `max_prefix_len` accordingly,
// so that `L = max_prefix_len + max_suffix_len`.
if max_fixed_len + 1 >= max_prefix_len + max_suffix_len {
// The subtraction can't overflow thanks to the above check.
// The new `max_prefix_len` is also guaranteed to be larger than its previous
// value.
max_prefix_len = max_fixed_len + 1 - max_suffix_len;
}
match array_len {
Some(len) => {
let kind = if max_prefix_len + max_suffix_len < len {
VarLen(max_prefix_len, max_suffix_len)
} else {
FixedLen(len)
};
split_ctors.push(Slice(Slice { array_len, kind }));
}
None => {
// `ctor` originally covered the range `(self_prefix +
// self_suffix..infinity)`. We now split it into two: lengths smaller than
// `max_prefix_len + max_suffix_len` are treated independently as
// fixed-lengths slices, and lengths above are captured by a final VarLen
// constructor.
split_ctors.extend(
(self_prefix + self_suffix..max_prefix_len + max_suffix_len)
.map(|len| Slice(Slice { array_len, kind: FixedLen(len) })),
);
split_ctors.push(Slice(Slice {
array_len,
kind: VarLen(max_prefix_len, max_suffix_len),
}));
}
}
}
// Any other constructor can be used unchanged.
_ => split_ctors.push(ctor),
}
}
debug!("split_grouped_constructors(..)={:#?}", split_ctors);
split_ctors
}
fn lint_overlapping_patterns<'tcx>(
tcx: TyCtxt<'tcx>,
hir_id: Option<HirId>,
ctor_range: IntRange<'tcx>,
ty: Ty<'tcx>,
overlaps: Vec<IntRange<'tcx>>,
) {
if let (true, Some(hir_id)) = (!overlaps.is_empty(), hir_id) {
tcx.struct_span_lint_hir(
lint::builtin::OVERLAPPING_PATTERNS,
hir_id,
ctor_range.span,
|lint| {
let mut err = lint.build("multiple patterns covering the same range");
err.span_label(ctor_range.span, "overlapping patterns");
for int_range in overlaps {
// Use the real type for user display of the ranges:
err.span_label(
int_range.span,
&format!(
"this range overlaps on `{}`",
IntRange { range: int_range.range, ty, span: DUMMY_SP }.to_pat(tcx),
),
);
}
err.emit();
},
);
}
}
fn constructor_covered_by_range<'tcx>(
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
ctor: &Constructor<'tcx>,
pat: &Pat<'tcx>,
) -> Option<()> {
if let Single = ctor {
return Some(());
}
let (pat_from, pat_to, pat_end, ty) = match *pat.kind {
PatKind::Constant { value } => (value, value, RangeEnd::Included, value.ty),
PatKind::Range(PatRange { lo, hi, end }) => (lo, hi, end, lo.ty),
_ => bug!("`constructor_covered_by_range` called with {:?}", pat),
};
let (ctor_from, ctor_to, ctor_end) = match *ctor {
ConstantValue(value) => (value, value, RangeEnd::Included),
FloatRange(from, to, ctor_end) => (from, to, ctor_end),
_ => bug!("`constructor_covered_by_range` called with {:?}", ctor),
};
trace!("constructor_covered_by_range {:#?}, {:#?}, {:#?}, {}", ctor, pat_from, pat_to, ty);
let to = compare_const_vals(tcx, ctor_to, pat_to, param_env, ty)?;
let from = compare_const_vals(tcx, ctor_from, pat_from, param_env, ty)?;
let intersects = (from == Ordering::Greater || from == Ordering::Equal)
&& (to == Ordering::Less || (pat_end == ctor_end && to == Ordering::Equal));
if intersects { Some(()) } else { None }
}
/// This is the main specialization step. It expands the pattern
/// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
/// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
/// Returns `None` if the pattern does not have the given constructor.
///
/// OTOH, slice patterns with a subslice pattern (tail @ ..) can be expanded into multiple
/// different patterns.
/// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
/// fields filled with wild patterns.
///
/// This is roughly the inverse of `Constructor::apply`.
fn specialize_one_pattern<'p, 'tcx>(
cx: &mut MatchCheckCtxt<'p, 'tcx>,
pat: &'p Pat<'tcx>,
constructor: &Constructor<'tcx>,
ctor_wild_subpatterns: &Fields<'p, 'tcx>,
) -> Option<Fields<'p, 'tcx>> {
if let NonExhaustive = constructor {
// Only a wildcard pattern can match the special extra constructor
if !pat.is_wildcard() {
return None;
}
return Some(Fields::empty());
}
let result = match *pat.kind {
PatKind::AscribeUserType { .. } => bug!(), // Handled by `expand_pattern`
PatKind::Binding { .. } | PatKind::Wild => Some(ctor_wild_subpatterns.clone()),
PatKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
let variant = &adt_def.variants[variant_index];
if constructor != &Variant(variant.def_id) {
return None;
}
Some(ctor_wild_subpatterns.replace_with_fieldpats(subpatterns))
}
PatKind::Leaf { ref subpatterns } => {
Some(ctor_wild_subpatterns.replace_with_fieldpats(subpatterns))
}
PatKind::Deref { ref subpattern } => Some(Fields::from_single_pattern(subpattern)),
PatKind::Constant { value } if constructor.is_slice() => {
// We extract an `Option` for the pointer because slices of zero
// elements don't necessarily point to memory, they are usually
// just integers. The only time they should be pointing to memory
// is when they are subslices of nonzero slices.
let (alloc, offset, n, ty) = match value.ty.kind() {
ty::Array(t, n) => {
let n = n.eval_usize(cx.tcx, cx.param_env);
// Shortcut for `n == 0` where no matter what `alloc` and `offset` we produce,
// the result would be exactly what we early return here.
if n == 0 {
if ctor_wild_subpatterns.len() as u64 != n {
return None;
}
return Some(Fields::empty());
}
match value.val {
ty::ConstKind::Value(ConstValue::ByRef { offset, alloc, .. }) => {
(Cow::Borrowed(alloc), offset, n, t)
}
_ => span_bug!(pat.span, "array pattern is {:?}", value,),
}
}
ty::Slice(t) => {
match value.val {
ty::ConstKind::Value(ConstValue::Slice { data, start, end }) => {
let offset = Size::from_bytes(start);
let n = (end - start) as u64;
(Cow::Borrowed(data), offset, n, t)
}
ty::ConstKind::Value(ConstValue::ByRef { .. }) => {
// FIXME(oli-obk): implement `deref` for `ConstValue`
return None;
}
_ => span_bug!(
pat.span,
"slice pattern constant must be scalar pair but is {:?}",
value,
),
}
}
_ => span_bug!(
pat.span,
"unexpected const-val {:?} with ctor {:?}",
value,
constructor,
),
};
if ctor_wild_subpatterns.len() as u64 != n {
return None;
}
// Convert a constant slice/array pattern to a list of patterns.
let layout = cx.tcx.layout_of(cx.param_env.and(ty)).ok()?;
let ptr = Pointer::new(AllocId(0), offset);
let pats = cx.pattern_arena.alloc_from_iter((0..n).filter_map(|i| {
let ptr = ptr.offset(layout.size * i, &cx.tcx).ok()?;
let scalar = alloc.read_scalar(&cx.tcx, ptr, layout.size).ok()?;
let scalar = scalar.check_init().ok()?;
let value = ty::Const::from_scalar(cx.tcx, scalar, ty);
let pattern = Pat { ty, span: pat.span, kind: box PatKind::Constant { value } };
Some(pattern)
}));
// Ensure none of the dereferences failed.
if pats.len() as u64 != n {
return None;
}
Some(Fields::from_slice_unfiltered(pats))
}
PatKind::Constant { .. } | PatKind::Range { .. } => {
// If the constructor is a:
// - Single value: add a row if the pattern contains the constructor.
// - Range: add a row if the constructor intersects the pattern.
if let IntRange(ctor) = constructor {
let pat = IntRange::from_pat(cx.tcx, cx.param_env, pat)?;
ctor.intersection(cx.tcx, &pat)?;
// Constructor splitting should ensure that all intersections we encounter
// are actually inclusions.
assert!(ctor.is_subrange(&pat));
} else {
// Fallback for non-ranges and ranges that involve
// floating-point numbers, which are not conveniently handled
// by `IntRange`. For these cases, the constructor may not be a
// range so intersection actually devolves into being covered
// by the pattern.
constructor_covered_by_range(cx.tcx, cx.param_env, constructor, pat)?;
}
Some(Fields::empty())
}
PatKind::Array { ref prefix, ref slice, ref suffix }
| PatKind::Slice { ref prefix, ref slice, ref suffix } => match *constructor {
Slice(_) => {
// Number of subpatterns for this pattern
let pat_len = prefix.len() + suffix.len();
// Number of subpatterns for this constructor
let arity = ctor_wild_subpatterns.len();
if (slice.is_none() && arity != pat_len) || pat_len > arity {
return None;
}
// Replace the prefix and the suffix with the given patterns, leaving wildcards in
// the middle if there was a subslice pattern `..`.
let prefix = prefix.iter().enumerate();
let suffix = suffix.iter().enumerate().map(|(i, p)| (arity - suffix.len() + i, p));
Some(ctor_wild_subpatterns.replace_fields_indexed(prefix.chain(suffix)))
}
ConstantValue(cv) => {
match slice_pat_covered_by_const(
cx.tcx,
pat.span,
cv,
prefix,
slice,
suffix,
cx.param_env,
) {
Ok(true) => Some(Fields::empty()),
Ok(false) => None,
Err(ErrorReported) => None,
}
}
_ => span_bug!(pat.span, "unexpected ctor {:?} for slice pat", constructor),
},
PatKind::Or { .. } => bug!("Or-pattern should have been expanded earlier on."),
};
debug!(
"specialize({:#?}, {:#?}, {:#?}) = {:#?}",
pat, constructor, ctor_wild_subpatterns, result
);
result
}