compiler/rustc_mir_build/src/build/matches/simplify.rs

//! Simplifying Candidates
//!
//! *Simplifying* a match pair `place @ pattern` means breaking it down
//! into bindings or other, simpler match pairs. For example:
//!
//! - `place @ (P1, P2)` can be simplified to `[place.0 @ P1, place.1 @ P2]`
//! - `place @ x` can be simplified to `[]` by binding `x` to `place`
//!
//! The `simplify_candidate` routine just repeatedly applies these
//! sort of simplifications until there is nothing left to
//! simplify. Match pairs cannot be simplified if they require some
//! sort of test: for example, testing which variant an enum is, or
//! testing a value against a constant.

use crate::build::matches::{Ascription, Binding, Candidate, MatchPair};
use crate::build::Builder;
use crate::thir::{self, *};
use rustc_attr::{SignedInt, UnsignedInt};
use rustc_hir::RangeEnd;
use rustc_middle::mir::interpret::truncate;
use rustc_middle::mir::Place;
use rustc_middle::ty;
use rustc_middle::ty::layout::IntegerExt;
use rustc_target::abi::{Integer, Size};

use std::mem;

impl<'a, 'tcx> Builder<'a, 'tcx> {
    /// Simplify a candidate so that all match pairs require a test.
    ///
    /// This method will also split a candidate, in which the only
    /// match-pair is an or-pattern, into multiple candidates.
    /// This is so that
    ///
    /// match x {
    ///     0 | 1 => { ... },
    ///     2 | 3 => { ... },
    /// }
    ///
    /// only generates a single switch. If this happens this method returns
    /// `true`.
    pub(super) fn simplify_candidate<'pat>(
        &mut self,
        candidate: &mut Candidate<'pat, 'tcx>,
    ) -> bool {
        // repeatedly simplify match pairs until fixed point is reached
        loop {
            let match_pairs = mem::take(&mut candidate.match_pairs);

            if let [MatchPair { pattern: Pat { kind: box PatKind::Or { pats }, .. }, place }] =
                *match_pairs
            {
                candidate.subcandidates = self.create_or_subcandidates(candidate, place, pats);
                return true;
            }

            let mut changed = false;
            for match_pair in match_pairs {
                match self.simplify_match_pair(match_pair, candidate) {
                    Ok(()) => {
                        changed = true;
                    }
                    Err(match_pair) => {
                        candidate.match_pairs.push(match_pair);
                    }
                }
            }
            if !changed {
                // Move or-patterns to the end, because they can result in us
                // creating additional candidates, so we want to test them as
                // late as possible.
                candidate
                    .match_pairs
                    .sort_by_key(|pair| matches!(*pair.pattern.kind, PatKind::Or { .. }));
                return false; // if we were not able to simplify any, done.
            }
        }
    }

    /// Given `candidate` that has a single or-pattern for its match-pairs,
    /// creates a fresh candidate for each of its input subpatterns passed via
    /// `pats`.
    fn create_or_subcandidates<'pat>(
        &mut self,
        candidate: &Candidate<'pat, 'tcx>,
        place: Place<'tcx>,
        pats: &'pat [Pat<'tcx>],
    ) -> Vec<Candidate<'pat, 'tcx>> {
        pats.iter()
            .map(|pat| {
                let mut candidate = Candidate::new(place, pat, candidate.has_guard);
                self.simplify_candidate(&mut candidate);
                candidate
            })
            .collect()
    }

    /// Tries to simplify `match_pair`, returning `Ok(())` if
    /// successful. If successful, new match pairs and bindings will
    /// have been pushed into the candidate. If no simplification is
    /// possible, `Err` is returned and no changes are made to
    /// candidate.
    fn simplify_match_pair<'pat>(
        &mut self,
        match_pair: MatchPair<'pat, 'tcx>,
        candidate: &mut Candidate<'pat, 'tcx>,
    ) -> Result<(), MatchPair<'pat, 'tcx>> {
        let tcx = self.hir.tcx();
        match *match_pair.pattern.kind {
            PatKind::AscribeUserType {
                ref subpattern,
                ascription: thir::pattern::Ascription { variance, user_ty, user_ty_span },
            } => {
                // Apply the type ascription to the value at `match_pair.place`, which is the
                // value being matched, taking the variance field into account.
                candidate.ascriptions.push(Ascription {
                    span: user_ty_span,
                    user_ty,
                    source: match_pair.place,
                    variance,
                });

                candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));

                Ok(())
            }

            PatKind::Wild => {
                // nothing left to do
                Ok(())
            }

            PatKind::Binding { name, mutability, mode, var, ty, ref subpattern, is_primary: _ } => {
                candidate.bindings.push(Binding {
                    name,
                    mutability,
                    span: match_pair.pattern.span,
                    source: match_pair.place,
                    var_id: var,
                    var_ty: ty,
                    binding_mode: mode,
                });

                if let Some(subpattern) = subpattern.as_ref() {
                    // this is the `x @ P` case; have to keep matching against `P` now
                    candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));
                }

                Ok(())
            }

            PatKind::Constant { .. } => {
                // FIXME normalize patterns when possible
                Err(match_pair)
            }

            PatKind::Range(PatRange { lo, hi, end }) => {
                let (range, bias) = match *lo.ty.kind() {
                    ty::Char => {
                        (Some(('\u{0000}' as u128, '\u{10FFFF}' as u128, Size::from_bits(32))), 0)
                    }
                    ty::Int(ity) => {
                        let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
                        let max = truncate(u128::MAX, size);
                        let bias = 1u128 << (size.bits() - 1);
                        (Some((0, max, size)), bias)
                    }
                    ty::Uint(uty) => {
                        let size = Integer::from_attr(&tcx, UnsignedInt(uty)).size();
                        let max = truncate(u128::MAX, size);
                        (Some((0, max, size)), 0)
                    }
                    _ => (None, 0),
                };
                if let Some((min, max, sz)) = range {
                    if let (Some(lo), Some(hi)) = (lo.val.try_to_bits(sz), hi.val.try_to_bits(sz)) {
                        // We want to compare ranges numerically, but the order of the bitwise
                        // representation of signed integers does not match their numeric order.
                        // Thus, to correct the ordering, we need to shift the range of signed
                        // integers to correct the comparison. This is achieved by XORing with a
                        // bias (see pattern/_match.rs for another pertinent example of this
                        // pattern).
                        let (lo, hi) = (lo ^ bias, hi ^ bias);
                        if lo <= min && (hi > max || hi == max && end == RangeEnd::Included) {
                            // Irrefutable pattern match.
                            return Ok(());
                        }
                    }
                }
                Err(match_pair)
            }

            PatKind::Slice { ref prefix, ref slice, ref suffix } => {
                if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
                    // irrefutable
                    self.prefix_slice_suffix(
                        &mut candidate.match_pairs,
                        &match_pair.place,
                        prefix,
                        slice.as_ref(),
                        suffix,
                    );
                    Ok(())
                } else {
                    Err(match_pair)
                }
            }

            PatKind::Variant { adt_def, substs, variant_index, ref subpatterns } => {
                let irrefutable = adt_def.variants.iter_enumerated().all(|(i, v)| {
                    i == variant_index || {
                        self.hir.tcx().features().exhaustive_patterns
                            && !v
                                .uninhabited_from(
                                    self.hir.tcx(),
                                    substs,
                                    adt_def.adt_kind(),
                                    self.hir.param_env,
                                )
                                .is_empty()
                    }
                }) && (adt_def.did.is_local()
                    || !adt_def.is_variant_list_non_exhaustive());
                if irrefutable {
                    let place = tcx.mk_place_downcast(match_pair.place, adt_def, variant_index);
                    candidate.match_pairs.extend(self.field_match_pairs(place, subpatterns));
                    Ok(())
                } else {
                    Err(match_pair)
                }
            }

            PatKind::Array { ref prefix, ref slice, ref suffix } => {
                self.prefix_slice_suffix(
                    &mut candidate.match_pairs,
                    &match_pair.place,
                    prefix,
                    slice.as_ref(),
                    suffix,
                );
                Ok(())
            }

            PatKind::Leaf { ref subpatterns } => {
                // tuple struct, match subpats (if any)
                candidate.match_pairs.extend(self.field_match_pairs(match_pair.place, subpatterns));
                Ok(())
            }

            PatKind::Deref { ref subpattern } => {
                let place = tcx.mk_place_deref(match_pair.place);
                candidate.match_pairs.push(MatchPair::new(place, subpattern));
                Ok(())
            }

            PatKind::Or { .. } => Err(match_pair),
        }
    }
}