blob: b317bb7cff0e3c9a6e0b720f0dd0b2a63e1123f9 [file] [log] [blame]
// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! Code related to match expressions. These are sufficiently complex
//! to warrant their own module and submodules. :) This main module
//! includes the high-level algorithm, the submodules contain the
//! details.
use build::{BlockAnd, BlockAndExtension, Builder};
use build::{GuardFrame, GuardFrameLocal, LocalsForNode};
use build::ForGuard::{self, OutsideGuard, RefWithinGuard, ValWithinGuard};
use build::scope::{CachedBlock, DropKind};
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::bitvec::BitArray;
use rustc::ty::{self, Ty};
use rustc::mir::*;
use rustc::hir;
use hair::*;
use syntax::ast::{Name, NodeId};
use syntax_pos::Span;
// helper functions, broken out by category:
mod simplify;
mod test;
mod util;
/// ArmHasGuard is isomorphic to a boolean flag. It indicates whether
/// a match arm has a guard expression attached to it.
#[derive(Copy, Clone, Debug)]
pub(crate) struct ArmHasGuard(pub bool);
impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
pub fn match_expr(&mut self,
destination: &Place<'tcx>,
span: Span,
mut block: BasicBlock,
discriminant: ExprRef<'tcx>,
arms: Vec<Arm<'tcx>>)
-> BlockAnd<()> {
let tcx = self.hir.tcx();
let discriminant_span = discriminant.span();
let discriminant_place = unpack!(block = self.as_place(block, discriminant));
// Matching on a `discriminant_place` with an uninhabited type doesn't
// generate any memory reads by itself, and so if the place "expression"
// contains unsafe operations like raw pointer dereferences or union
// field projections, we wouldn't know to require an `unsafe` block
// around a `match` equivalent to `std::intrinsics::unreachable()`.
// See issue #47412 for this hole being discovered in the wild.
//
// HACK(eddyb) Work around the above issue by adding a dummy inspection
// of `discriminant_place`, specifically by applying `Rvalue::Discriminant`
// (which will work regardless of type) and storing the result in a temp.
//
// NOTE: Under NLL, the above issue should no longer occur because it
// injects a borrow of the matched input, which should have the same effect
// as eddyb's hack. Once NLL is the default, we can remove the hack.
let dummy_source_info = self.source_info(discriminant_span);
let dummy_access = Rvalue::Discriminant(discriminant_place.clone());
let dummy_ty = dummy_access.ty(&self.local_decls, tcx);
let dummy_temp = self.temp(dummy_ty, dummy_source_info.span);
self.cfg.push_assign(block, dummy_source_info, &dummy_temp, dummy_access);
let source_info = self.source_info(discriminant_span);
let borrowed_input_temp = if tcx.generate_borrow_of_any_match_input() {
// The region is unknown at this point; we rely on NLL
// inference to find an appropriate one. Therefore you can
// only use this when NLL is turned on.
assert!(tcx.use_mir_borrowck());
let borrowed_input =
Rvalue::Ref(tcx.types.re_empty, BorrowKind::Shared, discriminant_place.clone());
let borrowed_input_ty = borrowed_input.ty(&self.local_decls, tcx);
let borrowed_input_temp = self.temp(borrowed_input_ty, span);
self.cfg.push_assign(block, source_info, &borrowed_input_temp, borrowed_input);
Some(borrowed_input_temp)
} else {
None
};
let mut arm_blocks = ArmBlocks {
blocks: arms.iter()
.map(|_| self.cfg.start_new_block())
.collect(),
};
// Get the arm bodies and their scopes, while declaring bindings.
let arm_bodies: Vec<_> = arms.iter().map(|arm| {
// BUG: use arm lint level
let body = self.hir.mirror(arm.body.clone());
let scope = self.declare_bindings(None, body.span,
LintLevel::Inherited,
&arm.patterns[..],
ArmHasGuard(arm.guard.is_some()),
Some((Some(&discriminant_place), discriminant_span)));
(body, scope.unwrap_or(self.source_scope))
}).collect();
// create binding start block for link them by false edges
let candidate_count = arms.iter().fold(0, |ac, c| ac + c.patterns.len());
let pre_binding_blocks: Vec<_> = (0..candidate_count + 1)
.map(|_| self.cfg.start_new_block()).collect();
// assemble a list of candidates: there is one candidate per
// pattern, which means there may be more than one candidate
// *per arm*. These candidates are kept sorted such that the
// highest priority candidate comes first in the list.
// (i.e. same order as in source)
let candidates: Vec<_> =
arms.iter()
.enumerate()
.flat_map(|(arm_index, arm)| {
arm.patterns.iter().enumerate()
.map(move |(pat_index, pat)| {
(arm_index, pat_index, pat, arm.guard.clone())
})
})
.zip(pre_binding_blocks.iter().zip(pre_binding_blocks.iter().skip(1)))
.map(|((arm_index, pat_index, pattern, guard),
(pre_binding_block, next_candidate_pre_binding_block))| {
if let (true, Some(borrow_temp)) = (tcx.emit_read_for_match(),
borrowed_input_temp.clone()) {
// inject a fake read of the borrowed input at
// the start of each arm's pattern testing
// code.
//
// This should ensure that you cannot change
// the variant for an enum while you are in
// the midst of matching on it.
let pattern_source_info = self.source_info(pattern.span);
self.cfg.push(*pre_binding_block, Statement {
source_info: pattern_source_info,
kind: StatementKind::ReadForMatch(borrow_temp.clone()),
});
}
// One might ask: why not build up the match pair such that it
// matches via `borrowed_input_temp.deref()` instead of
// using the `discriminant_place` directly, as it is doing here?
//
// The basic answer is that if you do that, then you end up with
// accceses to a shared borrow of the input and that conflicts with
// any arms that look like e.g.
//
// match Some(&4) {
// ref mut foo => {
// ... /* mutate `foo` in arm body */ ...
// }
// }
//
// (Perhaps we could further revise the MIR
// construction here so that it only does a
// shared borrow at the outset and delays doing
// the mutable borrow until after the pattern is
// matched *and* the guard (if any) for the arm
// has been run.)
Candidate {
span: pattern.span,
match_pairs: vec![MatchPair::new(discriminant_place.clone(), pattern)],
bindings: vec![],
guard,
arm_index,
pat_index,
pre_binding_block: *pre_binding_block,
next_candidate_pre_binding_block: *next_candidate_pre_binding_block,
}
})
.collect();
let outer_source_info = self.source_info(span);
self.cfg.terminate(*pre_binding_blocks.last().unwrap(),
outer_source_info, TerminatorKind::Unreachable);
// this will generate code to test discriminant_place and
// branch to the appropriate arm block
let otherwise = self.match_candidates(span, &mut arm_blocks, candidates, block);
if !otherwise.is_empty() {
// All matches are exhaustive. However, because some matches
// only have exponentially-large exhaustive decision trees, we
// sometimes generate an inexhaustive decision tree.
//
// In that case, the inexhaustive tips of the decision tree
// can't be reached - terminate them with an `unreachable`.
let source_info = self.source_info(span);
let mut otherwise = otherwise;
otherwise.sort();
otherwise.dedup(); // variant switches can introduce duplicate target blocks
for block in otherwise {
self.cfg.terminate(block, source_info, TerminatorKind::Unreachable);
}
}
// all the arm blocks will rejoin here
let end_block = self.cfg.start_new_block();
let outer_source_info = self.source_info(span);
for (arm_index, (body, source_scope)) in arm_bodies.into_iter().enumerate() {
let mut arm_block = arm_blocks.blocks[arm_index];
// Re-enter the source scope we created the bindings in.
self.source_scope = source_scope;
unpack!(arm_block = self.into(destination, arm_block, body));
self.cfg.terminate(arm_block, outer_source_info,
TerminatorKind::Goto { target: end_block });
}
self.source_scope = outer_source_info.scope;
end_block.unit()
}
pub fn user_assert_ty(&mut self, block: BasicBlock, hir_id: hir::HirId,
var: NodeId, span: Span) {
if self.hir.tcx().sess.opts.debugging_opts.disable_nll_user_type_assert { return; }
let local_id = self.var_local_id(var, OutsideGuard);
let source_info = self.source_info(span);
debug!("user_assert_ty: local_id={:?}", hir_id.local_id);
if let Some(c_ty) = self.hir.tables.user_provided_tys().get(hir_id) {
debug!("user_assert_ty: c_ty={:?}", c_ty);
self.cfg.push(block, Statement {
source_info,
kind: StatementKind::UserAssertTy(*c_ty, local_id),
});
}
}
pub fn expr_into_pattern(&mut self,
mut block: BasicBlock,
ty: Option<hir::HirId>,
irrefutable_pat: Pattern<'tcx>,
initializer: ExprRef<'tcx>)
-> BlockAnd<()> {
// optimize the case of `let x = ...`
match *irrefutable_pat.kind {
PatternKind::Binding { mode: BindingMode::ByValue,
var,
subpattern: None, .. } => {
let place = self.storage_live_binding(block, var, irrefutable_pat.span,
OutsideGuard);
if let Some(ty) = ty {
self.user_assert_ty(block, ty, var, irrefutable_pat.span);
}
unpack!(block = self.into(&place, block, initializer));
self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
block.unit()
}
_ => {
let place = unpack!(block = self.as_place(block, initializer));
self.place_into_pattern(block, irrefutable_pat, &place, true)
}
}
}
pub fn place_into_pattern(&mut self,
mut block: BasicBlock,
irrefutable_pat: Pattern<'tcx>,
initializer: &Place<'tcx>,
set_match_place: bool)
-> BlockAnd<()> {
// create a dummy candidate
let mut candidate = Candidate {
span: irrefutable_pat.span,
match_pairs: vec![MatchPair::new(initializer.clone(), &irrefutable_pat)],
bindings: vec![],
guard: None,
// since we don't call `match_candidates`, next fields is unused
arm_index: 0,
pat_index: 0,
pre_binding_block: block,
next_candidate_pre_binding_block: block
};
// Simplify the candidate. Since the pattern is irrefutable, this should
// always convert all match-pairs into bindings.
unpack!(block = self.simplify_candidate(block, &mut candidate));
if !candidate.match_pairs.is_empty() {
span_bug!(candidate.match_pairs[0].pattern.span,
"match pairs {:?} remaining after simplifying \
irrefutable pattern",
candidate.match_pairs);
}
// for matches and function arguments, the place that is being matched
// can be set when creating the variables. But the place for
// let PATTERN = ... might not even exist until we do the assignment.
// so we set it here instead
if set_match_place {
for binding in &candidate.bindings {
let local = self.var_local_id(binding.var_id, OutsideGuard);
if let Some(ClearCrossCrate::Set(BindingForm::Var(
VarBindingForm {opt_match_place: Some((ref mut match_place, _)), .. }
))) = self.local_decls[local].is_user_variable
{
*match_place = Some(initializer.clone());
} else {
bug!("Let binding to non-user variable.")
}
}
}
// now apply the bindings, which will also declare the variables
self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
block.unit()
}
/// Declares the bindings of the given patterns and returns the visibility
/// scope for the bindings in these patterns, if such a scope had to be
/// created. NOTE: Declaring the bindings should always be done in their
/// drop scope.
pub fn declare_bindings(&mut self,
mut visibility_scope: Option<SourceScope>,
scope_span: Span,
lint_level: LintLevel,
patterns: &[Pattern<'tcx>],
has_guard: ArmHasGuard,
opt_match_place: Option<(Option<&Place<'tcx>>, Span)>)
-> Option<SourceScope> {
assert!(!(visibility_scope.is_some() && lint_level.is_explicit()),
"can't have both a visibility and a lint scope at the same time");
let mut scope = self.source_scope;
let num_patterns = patterns.len();
self.visit_bindings(&patterns[0], &mut |this, mutability, name, mode, var, span, ty| {
if visibility_scope.is_none() {
visibility_scope = Some(this.new_source_scope(scope_span,
LintLevel::Inherited,
None));
// If we have lints, create a new source scope
// that marks the lints for the locals. See the comment
// on the `source_info` field for why this is needed.
if lint_level.is_explicit() {
scope =
this.new_source_scope(scope_span, lint_level, None);
}
}
let source_info = SourceInfo {
span,
scope,
};
let visibility_scope = visibility_scope.unwrap();
this.declare_binding(source_info, visibility_scope, mutability, name, mode,
num_patterns, var, ty, has_guard,
opt_match_place.map(|(x, y)| (x.cloned(), y)),
patterns[0].span);
});
visibility_scope
}
pub fn storage_live_binding(&mut self,
block: BasicBlock,
var: NodeId,
span: Span,
for_guard: ForGuard)
-> Place<'tcx>
{
let local_id = self.var_local_id(var, for_guard);
let source_info = self.source_info(span);
self.cfg.push(block, Statement {
source_info,
kind: StatementKind::StorageLive(local_id)
});
let place = Place::Local(local_id);
let var_ty = self.local_decls[local_id].ty;
let hir_id = self.hir.tcx().hir.node_to_hir_id(var);
let region_scope = self.hir.region_scope_tree.var_scope(hir_id.local_id);
self.schedule_drop(
span, region_scope, &place, var_ty,
DropKind::Storage,
);
place
}
pub fn schedule_drop_for_binding(&mut self,
var: NodeId,
span: Span,
for_guard: ForGuard) {
let local_id = self.var_local_id(var, for_guard);
let var_ty = self.local_decls[local_id].ty;
let hir_id = self.hir.tcx().hir.node_to_hir_id(var);
let region_scope = self.hir.region_scope_tree.var_scope(hir_id.local_id);
self.schedule_drop(
span, region_scope, &Place::Local(local_id), var_ty,
DropKind::Value {
cached_block: CachedBlock::default(),
},
);
}
pub fn visit_bindings<F>(&mut self, pattern: &Pattern<'tcx>, f: &mut F)
where F: FnMut(&mut Self, Mutability, Name, BindingMode, NodeId, Span, Ty<'tcx>)
{
match *pattern.kind {
PatternKind::Binding { mutability, name, mode, var, ty, ref subpattern, .. } => {
f(self, mutability, name, mode, var, pattern.span, ty);
if let Some(subpattern) = subpattern.as_ref() {
self.visit_bindings(subpattern, f);
}
}
PatternKind::Array { ref prefix, ref slice, ref suffix } |
PatternKind::Slice { ref prefix, ref slice, ref suffix } => {
for subpattern in prefix.iter().chain(slice).chain(suffix) {
self.visit_bindings(subpattern, f);
}
}
PatternKind::Constant { .. } | PatternKind::Range { .. } | PatternKind::Wild => {
}
PatternKind::Deref { ref subpattern } => {
self.visit_bindings(subpattern, f);
}
PatternKind::Leaf { ref subpatterns } |
PatternKind::Variant { ref subpatterns, .. } => {
for subpattern in subpatterns {
self.visit_bindings(&subpattern.pattern, f);
}
}
}
}
}
/// List of blocks for each arm (and potentially other metadata in the
/// future).
struct ArmBlocks {
blocks: Vec<BasicBlock>,
}
#[derive(Clone, Debug)]
pub struct Candidate<'pat, 'tcx:'pat> {
// span of the original pattern that gave rise to this candidate
span: Span,
// all of these must be satisfied...
match_pairs: Vec<MatchPair<'pat, 'tcx>>,
// ...these bindings established...
bindings: Vec<Binding<'tcx>>,
// ...and the guard must be evaluated...
guard: Option<ExprRef<'tcx>>,
// ...and then we branch to arm with this index.
arm_index: usize,
// ...and the blocks for add false edges between candidates
pre_binding_block: BasicBlock,
next_candidate_pre_binding_block: BasicBlock,
// This uniquely identifies this candidate *within* the arm.
pat_index: usize,
}
#[derive(Clone, Debug)]
struct Binding<'tcx> {
span: Span,
source: Place<'tcx>,
name: Name,
var_id: NodeId,
var_ty: Ty<'tcx>,
mutability: Mutability,
binding_mode: BindingMode<'tcx>,
}
#[derive(Clone, Debug)]
pub struct MatchPair<'pat, 'tcx:'pat> {
// this place...
place: Place<'tcx>,
// ... must match this pattern.
pattern: &'pat Pattern<'tcx>,
// HACK(eddyb) This is used to toggle whether a Slice pattern
// has had its length checked. This is only necessary because
// the "rest" part of the pattern right now has type &[T] and
// as such, it requires an Rvalue::Slice to be generated.
// See RFC 495 / issue #23121 for the eventual (proper) solution.
slice_len_checked: bool
}
#[derive(Clone, Debug, PartialEq)]
enum TestKind<'tcx> {
// test the branches of enum
Switch {
adt_def: &'tcx ty::AdtDef,
variants: BitArray<usize>,
},
// test the branches of enum
SwitchInt {
switch_ty: Ty<'tcx>,
options: Vec<u128>,
indices: FxHashMap<&'tcx ty::Const<'tcx>, usize>,
},
// test for equality
Eq {
value: &'tcx ty::Const<'tcx>,
ty: Ty<'tcx>,
},
// test whether the value falls within an inclusive or exclusive range
Range {
lo: &'tcx ty::Const<'tcx>,
hi: &'tcx ty::Const<'tcx>,
ty: Ty<'tcx>,
end: hir::RangeEnd,
},
// test length of the slice is equal to len
Len {
len: u64,
op: BinOp,
},
}
#[derive(Debug)]
pub struct Test<'tcx> {
span: Span,
kind: TestKind<'tcx>,
}
///////////////////////////////////////////////////////////////////////////
// Main matching algorithm
impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
/// The main match algorithm. It begins with a set of candidates
/// `candidates` and has the job of generating code to determine
/// which of these candidates, if any, is the correct one. The
/// candidates are sorted such that the first item in the list
/// has the highest priority. When a candidate is found to match
/// the value, we will generate a branch to the appropriate
/// block found in `arm_blocks`.
///
/// The return value is a list of "otherwise" blocks. These are
/// points in execution where we found that *NONE* of the
/// candidates apply. In principle, this means that the input
/// list was not exhaustive, though at present we sometimes are
/// not smart enough to recognize all exhaustive inputs.
///
/// It might be surprising that the input can be inexhaustive.
/// Indeed, initially, it is not, because all matches are
/// exhaustive in Rust. But during processing we sometimes divide
/// up the list of candidates and recurse with a non-exhaustive
/// list. This is important to keep the size of the generated code
/// under control. See `test_candidates` for more details.
fn match_candidates<'pat>(&mut self,
span: Span,
arm_blocks: &mut ArmBlocks,
mut candidates: Vec<Candidate<'pat, 'tcx>>,
mut block: BasicBlock)
-> Vec<BasicBlock>
{
debug!("matched_candidate(span={:?}, block={:?}, candidates={:?})",
span, block, candidates);
// Start by simplifying candidates. Once this process is
// complete, all the match pairs which remain require some
// form of test, whether it be a switch or pattern comparison.
for candidate in &mut candidates {
unpack!(block = self.simplify_candidate(block, candidate));
}
// The candidates are sorted by priority. Check to see
// whether the higher priority candidates (and hence at
// the front of the vec) have satisfied all their match
// pairs.
let fully_matched =
candidates.iter().take_while(|c| c.match_pairs.is_empty()).count();
debug!("match_candidates: {:?} candidates fully matched", fully_matched);
let mut unmatched_candidates = candidates.split_off(fully_matched);
let fully_matched_with_guard =
candidates.iter().take_while(|c| c.guard.is_some()).count();
let unreachable_candidates = if fully_matched_with_guard + 1 < candidates.len() {
candidates.split_off(fully_matched_with_guard + 1)
} else {
vec![]
};
for candidate in candidates {
// If so, apply any bindings, test the guard (if any), and
// branch to the arm.
if let Some(b) = self.bind_and_guard_matched_candidate(block, arm_blocks, candidate) {
block = b;
} else {
// if None is returned, then any remaining candidates
// are unreachable (at least not through this path).
// Link them with false edges.
debug!("match_candidates: add false edges for unreachable {:?} and unmatched {:?}",
unreachable_candidates, unmatched_candidates);
for candidate in unreachable_candidates {
let source_info = self.source_info(candidate.span);
let target = self.cfg.start_new_block();
if let Some(otherwise) = self.bind_and_guard_matched_candidate(target,
arm_blocks,
candidate) {
self.cfg.terminate(otherwise, source_info, TerminatorKind::Unreachable);
}
}
if unmatched_candidates.is_empty() {
return vec![]
} else {
let target = self.cfg.start_new_block();
return self.match_candidates(span, arm_blocks, unmatched_candidates, target);
}
}
}
// If there are no candidates that still need testing, we're done.
// Since all matches are exhaustive, execution should never reach this point.
if unmatched_candidates.is_empty() {
return vec![block];
}
// Test candidates where possible.
let (otherwise, tested_candidates) =
self.test_candidates(span, arm_blocks, &unmatched_candidates, block);
// If the target candidates were exhaustive, then we are done.
// But for borrowck continue build decision tree.
// If all candidates were sorted into `target_candidates` somewhere, then
// the initial set was inexhaustive.
let untested_candidates = unmatched_candidates.split_off(tested_candidates);
if untested_candidates.len() == 0 {
return otherwise;
}
// Otherwise, let's process those remaining candidates.
let join_block = self.join_otherwise_blocks(span, otherwise);
self.match_candidates(span, arm_blocks, untested_candidates, join_block)
}
fn join_otherwise_blocks(&mut self,
span: Span,
mut otherwise: Vec<BasicBlock>)
-> BasicBlock
{
let source_info = self.source_info(span);
otherwise.sort();
otherwise.dedup(); // variant switches can introduce duplicate target blocks
if otherwise.len() == 1 {
otherwise[0]
} else {
let join_block = self.cfg.start_new_block();
for block in otherwise {
self.cfg.terminate(block, source_info,
TerminatorKind::Goto { target: join_block });
}
join_block
}
}
/// This is the most subtle part of the matching algorithm. At
/// this point, the input candidates have been fully simplified,
/// and so we know that all remaining match-pairs require some
/// sort of test. To decide what test to do, we take the highest
/// priority candidate (last one in the list) and extract the
/// first match-pair from the list. From this we decide what kind
/// of test is needed using `test`, defined in the `test` module.
///
/// *Note:* taking the first match pair is somewhat arbitrary, and
/// we might do better here by choosing more carefully what to
/// test.
///
/// For example, consider the following possible match-pairs:
///
/// 1. `x @ Some(P)` -- we will do a `Switch` to decide what variant `x` has
/// 2. `x @ 22` -- we will do a `SwitchInt`
/// 3. `x @ 3..5` -- we will do a range test
/// 4. etc.
///
/// Once we know what sort of test we are going to perform, this
/// test may also help us with other candidates. So we walk over
/// the candidates (from high to low priority) and check. This
/// gives us, for each outcome of the test, a transformed list of
/// candidates. For example, if we are testing the current
/// variant of `x.0`, and we have a candidate `{x.0 @ Some(v), x.1
/// @ 22}`, then we would have a resulting candidate of `{(x.0 as
/// Some).0 @ v, x.1 @ 22}`. Note that the first match-pair is now
/// simpler (and, in fact, irrefutable).
///
/// But there may also be candidates that the test just doesn't
/// apply to. The classical example involves wildcards:
///
/// ```
/// # let (x, y, z) = (true, true, true);
/// match (x, y, z) {
/// (true, _, true) => true, // (0)
/// (_, true, _) => true, // (1)
/// (false, false, _) => false, // (2)
/// (true, _, false) => false, // (3)
/// }
/// ```
///
/// In that case, after we test on `x`, there are 2 overlapping candidate
/// sets:
///
/// - If the outcome is that `x` is true, candidates 0, 1, and 3
/// - If the outcome is that `x` is false, candidates 1 and 2
///
/// Here, the traditional "decision tree" method would generate 2
/// separate code-paths for the 2 separate cases.
///
/// In some cases, this duplication can create an exponential amount of
/// code. This is most easily seen by noticing that this method terminates
/// with precisely the reachable arms being reachable - but that problem
/// is trivially NP-complete:
///
/// ```rust
/// match (var0, var1, var2, var3, ..) {
/// (true, _, _, false, true, ...) => false,
/// (_, true, true, false, _, ...) => false,
/// (false, _, false, false, _, ...) => false,
/// ...
/// _ => true
/// }
/// ```
///
/// Here the last arm is reachable only if there is an assignment to
/// the variables that does not match any of the literals. Therefore,
/// compilation would take an exponential amount of time in some cases.
///
/// That kind of exponential worst-case might not occur in practice, but
/// our simplistic treatment of constants and guards would make it occur
/// in very common situations - for example #29740:
///
/// ```rust
/// match x {
/// "foo" if foo_guard => ...,
/// "bar" if bar_guard => ...,
/// "baz" if baz_guard => ...,
/// ...
/// }
/// ```
///
/// Here we first test the match-pair `x @ "foo"`, which is an `Eq` test.
///
/// It might seem that we would end up with 2 disjoint candidate
/// sets, consisting of the first candidate or the other 3, but our
/// algorithm doesn't reason about "foo" being distinct from the other
/// constants; it considers the latter arms to potentially match after
/// both outcomes, which obviously leads to an exponential amount
/// of tests.
///
/// To avoid these kinds of problems, our algorithm tries to ensure
/// the amount of generated tests is linear. When we do a k-way test,
/// we return an additional "unmatched" set alongside the obvious `k`
/// sets. When we encounter a candidate that would be present in more
/// than one of the sets, we put it and all candidates below it into the
/// "unmatched" set. This ensures these `k+1` sets are disjoint.
///
/// After we perform our test, we branch into the appropriate candidate
/// set and recurse with `match_candidates`. These sub-matches are
/// obviously inexhaustive - as we discarded our otherwise set - so
/// we set their continuation to do `match_candidates` on the
/// "unmatched" set (which is again inexhaustive).
///
/// If you apply this to the above test, you basically wind up
/// with an if-else-if chain, testing each candidate in turn,
/// which is precisely what we want.
///
/// In addition to avoiding exponential-time blowups, this algorithm
/// also has nice property that each guard and arm is only generated
/// once.
fn test_candidates<'pat>(&mut self,
span: Span,
arm_blocks: &mut ArmBlocks,
candidates: &[Candidate<'pat, 'tcx>],
block: BasicBlock)
-> (Vec<BasicBlock>, usize)
{
// extract the match-pair from the highest priority candidate
let match_pair = &candidates.first().unwrap().match_pairs[0];
let mut test = self.test(match_pair);
// most of the time, the test to perform is simply a function
// of the main candidate; but for a test like SwitchInt, we
// may want to add cases based on the candidates that are
// available
match test.kind {
TestKind::SwitchInt { switch_ty, ref mut options, ref mut indices } => {
for candidate in candidates.iter() {
if !self.add_cases_to_switch(&match_pair.place,
candidate,
switch_ty,
options,
indices) {
break;
}
}
}
TestKind::Switch { adt_def: _, ref mut variants} => {
for candidate in candidates.iter() {
if !self.add_variants_to_switch(&match_pair.place,
candidate,
variants) {
break;
}
}
}
_ => { }
}
// perform the test, branching to one of N blocks. For each of
// those N possible outcomes, create a (initially empty)
// vector of candidates. Those are the candidates that still
// apply if the test has that particular outcome.
debug!("match_candidates: test={:?} match_pair={:?}", test, match_pair);
let target_blocks = self.perform_test(block, &match_pair.place, &test);
let mut target_candidates: Vec<_> = (0..target_blocks.len()).map(|_| vec![]).collect();
// Sort the candidates into the appropriate vector in
// `target_candidates`. Note that at some point we may
// encounter a candidate where the test is not relevant; at
// that point, we stop sorting.
let tested_candidates =
candidates.iter()
.take_while(|c| self.sort_candidate(&match_pair.place,
&test,
c,
&mut target_candidates))
.count();
assert!(tested_candidates > 0); // at least the last candidate ought to be tested
debug!("tested_candidates: {}", tested_candidates);
debug!("untested_candidates: {}", candidates.len() - tested_candidates);
// For each outcome of test, process the candidates that still
// apply. Collect a list of blocks where control flow will
// branch if one of the `target_candidate` sets is not
// exhaustive.
let otherwise: Vec<_> =
target_blocks.into_iter()
.zip(target_candidates)
.flat_map(|(target_block, target_candidates)| {
self.match_candidates(span,
arm_blocks,
target_candidates,
target_block)
})
.collect();
(otherwise, tested_candidates)
}
/// Initializes each of the bindings from the candidate by
/// moving/copying/ref'ing the source as appropriate. Tests the
/// guard, if any, and then branches to the arm. Returns the block
/// for the case where the guard fails.
///
/// Note: we check earlier that if there is a guard, there cannot
/// be move bindings. This isn't really important for the
/// self-consistency of this fn, but the reason for it should be
/// clear: after we've done the assignments, if there were move
/// bindings, further tests would be a use-after-move (which would
/// in turn be detected by the borrowck code that runs on the
/// MIR).
fn bind_and_guard_matched_candidate<'pat>(&mut self,
mut block: BasicBlock,
arm_blocks: &mut ArmBlocks,
candidate: Candidate<'pat, 'tcx>)
-> Option<BasicBlock> {
debug!("bind_and_guard_matched_candidate(block={:?}, candidate={:?})",
block, candidate);
debug_assert!(candidate.match_pairs.is_empty());
let arm_block = arm_blocks.blocks[candidate.arm_index];
let candidate_source_info = self.source_info(candidate.span);
self.cfg.terminate(block, candidate_source_info,
TerminatorKind::Goto { target: candidate.pre_binding_block });
block = self.cfg.start_new_block();
self.cfg.terminate(candidate.pre_binding_block, candidate_source_info,
TerminatorKind::FalseEdges {
real_target: block,
imaginary_targets:
vec![candidate.next_candidate_pre_binding_block],
});
// rust-lang/rust#27282: The `autoref` business deserves some
// explanation here.
//
// The intent of the `autoref` flag is that when it is true,
// then any pattern bindings of type T will map to a `&T`
// within the context of the guard expression, but will
// continue to map to a `T` in the context of the arm body. To
// avoid surfacing this distinction in the user source code
// (which would be a severe change to the language and require
// far more revision to the compiler), when `autoref` is true,
// then any occurrence of the identifier in the guard
// expression will automatically get a deref op applied to it.
//
// So an input like:
//
// ```
// let place = Foo::new();
// match place { foo if inspect(foo)
// => feed(foo), ... }
// ```
//
// will be treated as if it were really something like:
//
// ```
// let place = Foo::new();
// match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
// => { let tmp2 = place; feed(tmp2) }, ... }
//
// And an input like:
//
// ```
// let place = Foo::new();
// match place { ref mut foo if inspect(foo)
// => feed(foo), ... }
// ```
//
// will be treated as if it were really something like:
//
// ```
// let place = Foo::new();
// match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
// => { let tmp2 = &mut place; feed(tmp2) }, ... }
// ```
//
// In short, any pattern binding will always look like *some*
// kind of `&T` within the guard at least in terms of how the
// MIR-borrowck views it, and this will ensure that guard
// expressions cannot mutate their the match inputs via such
// bindings. (It also ensures that guard expressions can at
// most *copy* values from such bindings; non-Copy things
// cannot be moved via pattern bindings in guard expressions.)
//
// ----
//
// Implementation notes (under assumption `autoref` is true).
//
// To encode the distinction above, we must inject the
// temporaries `tmp1` and `tmp2`.
//
// There are two cases of interest: binding by-value, and binding by-ref.
//
// 1. Binding by-value: Things are simple.
//
// * Establishing `tmp1` creates a reference into the
// matched place. This code is emitted by
// bind_matched_candidate_for_guard.
//
// * `tmp2` is only initialized "lazily", after we have
// checked the guard. Thus, the code that can trigger
// moves out of the candidate can only fire after the
// guard evaluated to true. This initialization code is
// emitted by bind_matched_candidate_for_arm.
//
// 2. Binding by-reference: Things are tricky.
//
// * Here, the guard expression wants a `&&` or `&&mut`
// into the original input. This means we need to borrow
// a reference that we do not immediately have at hand
// (because all we have is the places associated with the
// match input itself; it is up to us to create a place
// holding a `&` or `&mut` that we can then borrow).
let autoref = self.hir.tcx().all_pat_vars_are_implicit_refs_within_guards();
if let Some(guard) = candidate.guard {
if autoref {
self.bind_matched_candidate_for_guard(
block, candidate.pat_index, &candidate.bindings);
let guard_frame = GuardFrame {
locals: candidate.bindings.iter()
.map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode))
.collect(),
};
debug!("Entering guard building context: {:?}", guard_frame);
self.guard_context.push(guard_frame);
} else {
self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
}
// the block to branch to if the guard fails; if there is no
// guard, this block is simply unreachable
let guard = self.hir.mirror(guard);
let source_info = self.source_info(guard.span);
let cond = unpack!(block = self.as_local_operand(block, guard));
if autoref {
let guard_frame = self.guard_context.pop().unwrap();
debug!("Exiting guard building context with locals: {:?}", guard_frame);
}
let false_edge_block = self.cfg.start_new_block();
// We want to ensure that the matched candidates are bound
// after we have confirmed this candidate *and* any
// associated guard; Binding them on `block` is too soon,
// because that would be before we've checked the result
// from the guard.
//
// But binding them on `arm_block` is *too late*, because
// then all of the candidates for a single arm would be
// bound in the same place, that would cause a case like:
//
// ```rust
// match (30, 2) {
// (mut x, 1) | (2, mut x) if { true } => { ... }
// ... // ^^^^^^^ (this is `arm_block`)
// }
// ```
//
// would yield a `arm_block` something like:
//
// ```
// StorageLive(_4); // _4 is `x`
// _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case
// _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case
// ```
//
// and that is clearly not correct.
let post_guard_block = self.cfg.start_new_block();
self.cfg.terminate(block, source_info,
TerminatorKind::if_(self.hir.tcx(), cond, post_guard_block,
false_edge_block));
if autoref {
self.bind_matched_candidate_for_arm_body(post_guard_block, &candidate.bindings);
}
self.cfg.terminate(post_guard_block, source_info,
TerminatorKind::Goto { target: arm_block });
let otherwise = self.cfg.start_new_block();
self.cfg.terminate(false_edge_block, source_info,
TerminatorKind::FalseEdges {
real_target: otherwise,
imaginary_targets:
vec![candidate.next_candidate_pre_binding_block],
});
Some(otherwise)
} else {
// (Here, it is not too early to bind the matched
// candidate on `block`, because there is no guard result
// that we have to inspect before we bind them.)
self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
self.cfg.terminate(block, candidate_source_info,
TerminatorKind::Goto { target: arm_block });
None
}
}
// Only called when all_pat_vars_are_implicit_refs_within_guards,
// and thus all code/comments assume we are in that context.
fn bind_matched_candidate_for_guard(&mut self,
block: BasicBlock,
pat_index: usize,
bindings: &[Binding<'tcx>]) {
debug!("bind_matched_candidate_for_guard(block={:?}, pat_index={:?}, bindings={:?})",
block, pat_index, bindings);
// Assign each of the bindings. Since we are binding for a
// guard expression, this will never trigger moves out of the
// candidate.
let re_empty = self.hir.tcx().types.re_empty;
for binding in bindings {
let source_info = self.source_info(binding.span);
// For each pattern ident P of type T, `ref_for_guard` is
// a reference R: &T pointing to the location matched by
// the pattern, and every occurrence of P within a guard
// denotes *R.
let ref_for_guard = self.storage_live_binding(
block, binding.var_id, binding.span, RefWithinGuard);
// Question: Why schedule drops if bindings are all
// shared-&'s? Answer: Because schedule_drop_for_binding
// also emits StorageDead's for those locals.
self.schedule_drop_for_binding(binding.var_id, binding.span, RefWithinGuard);
match binding.binding_mode {
BindingMode::ByValue => {
let rvalue = Rvalue::Ref(re_empty, BorrowKind::Shared, binding.source.clone());
self.cfg.push_assign(block, source_info, &ref_for_guard, rvalue);
}
BindingMode::ByRef(region, borrow_kind) => {
// Tricky business: For `ref id` and `ref mut id`
// patterns, we want `id` within the guard to
// correspond to a temp of type `& &T` or `& &mut
// T` (i.e. a "borrow of a borrow") that is
// implicitly dereferenced.
//
// To borrow a borrow, we need that inner borrow
// to point to. So, create a temp for the inner
// borrow, and then take a reference to it.
//
// Note: the temp created here is *not* the one
// used by the arm body itself. This eases
// observing two-phase borrow restrictions.
let val_for_guard = self.storage_live_binding(
block, binding.var_id, binding.span, ValWithinGuard(pat_index));
self.schedule_drop_for_binding(
binding.var_id, binding.span, ValWithinGuard(pat_index));
// rust-lang/rust#27282: We reuse the two-phase
// borrow infrastructure so that the mutable
// borrow (whose mutabilty is *unusable* within
// the guard) does not conflict with the implicit
// borrow of the whole match input. See additional
// discussion on rust-lang/rust#49870.
let borrow_kind = match borrow_kind {
BorrowKind::Shared | BorrowKind::Unique => borrow_kind,
BorrowKind::Mut { .. } => BorrowKind::Mut { allow_two_phase_borrow: true },
};
let rvalue = Rvalue::Ref(region, borrow_kind, binding.source.clone());
self.cfg.push_assign(block, source_info, &val_for_guard, rvalue);
let rvalue = Rvalue::Ref(region, BorrowKind::Shared, val_for_guard);
self.cfg.push_assign(block, source_info, &ref_for_guard, rvalue);
}
}
}
}
fn bind_matched_candidate_for_arm_body(&mut self,
block: BasicBlock,
bindings: &[Binding<'tcx>]) {
debug!("bind_matched_candidate_for_arm_body(block={:?}, bindings={:?}", block, bindings);
// Assign each of the bindings. This may trigger moves out of the candidate.
for binding in bindings {
let source_info = self.source_info(binding.span);
let local = self.storage_live_binding(block, binding.var_id, binding.span,
OutsideGuard);
self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
let rvalue = match binding.binding_mode {
BindingMode::ByValue => {
Rvalue::Use(self.consume_by_copy_or_move(binding.source.clone()))
}
BindingMode::ByRef(region, borrow_kind) => {
Rvalue::Ref(region, borrow_kind, binding.source.clone())
}
};
self.cfg.push_assign(block, source_info, &local, rvalue);
}
}
/// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where
/// the bound `var` has type `T` in the arm body) in a pattern
/// maps to `2+N` locals. The first local is a binding for
/// occurrences of `var` in the guard, which will all have type
/// `&T`. The N locals are bindings for the `T` that is referenced
/// by the first local; they are not used outside of the
/// guard. The last local is a binding for occurrences of `var` in
/// the arm body, which will have type `T`.
///
/// The reason we have N locals rather than just 1 is to
/// accommodate rust-lang/rust#51348: If the arm has N candidate
/// patterns, then in general they can correspond to distinct
/// parts of the matched data, and we want them to be distinct
/// temps in order to simplify checks performed by our internal
/// leveraging of two-phase borrows).
fn declare_binding(&mut self,
source_info: SourceInfo,
visibility_scope: SourceScope,
mutability: Mutability,
name: Name,
mode: BindingMode,
num_patterns: usize,
var_id: NodeId,
var_ty: Ty<'tcx>,
has_guard: ArmHasGuard,
opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
pat_span: Span)
{
debug!("declare_binding(var_id={:?}, name={:?}, mode={:?}, var_ty={:?}, \
visibility_scope={:?}, source_info={:?})",
var_id, name, mode, var_ty, visibility_scope, source_info);
let tcx = self.hir.tcx();
let binding_mode = match mode {
BindingMode::ByValue => ty::BindingMode::BindByValue(mutability.into()),
BindingMode::ByRef { .. } => ty::BindingMode::BindByReference(mutability.into()),
};
let local = LocalDecl::<'tcx> {
mutability,
ty: var_ty.clone(),
name: Some(name),
source_info,
visibility_scope,
internal: false,
is_user_variable: Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
binding_mode,
// hypothetically, `visit_bindings` could try to unzip
// an outermost hir::Ty as we descend, matching up
// idents in pat; but complex w/ unclear UI payoff.
// Instead, just abandon providing diagnostic info.
opt_ty_info: None,
opt_match_place,
pat_span,
}))),
};
let for_arm_body = self.local_decls.push(local.clone());
let locals = if has_guard.0 && tcx.all_pat_vars_are_implicit_refs_within_guards() {
let mut vals_for_guard = Vec::with_capacity(num_patterns);
for _ in 0..num_patterns {
let val_for_guard_idx = self.local_decls.push(LocalDecl {
// This variable isn't mutated but has a name, so has to be
// immutable to avoid the unused mut lint.
mutability: Mutability::Not,
..local.clone()
});
vals_for_guard.push(val_for_guard_idx);
}
let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
// See previous comment.
mutability: Mutability::Not,
ty: tcx.mk_imm_ref(tcx.types.re_empty, var_ty),
name: Some(name),
source_info,
visibility_scope,
// FIXME: should these secretly injected ref_for_guard's be marked as `internal`?
internal: false,
is_user_variable: Some(ClearCrossCrate::Set(BindingForm::RefForGuard)),
});
LocalsForNode::ForGuard { vals_for_guard, ref_for_guard, for_arm_body }
} else {
LocalsForNode::One(for_arm_body)
};
debug!("declare_binding: vars={:?}", locals);
self.var_indices.insert(var_id, locals);
}
}