1 //! Simplifying Candidates
3 //! *Simplifying* a match pair `place @ pattern` means breaking it down
4 //! into bindings or other, simpler match pairs. For example:
6 //! - `place @ (P1, P2)` can be simplified to `[place.0 @ P1, place.1 @ P2]`
7 //! - `place @ x` can be simplified to `[]` by binding `x` to `place`
9 //! The `simplify_candidate` routine just repeatedly applies these
10 //! sort of simplifications until there is nothing left to
11 //! simplify. Match pairs cannot be simplified if they require some
12 //! sort of test: for example, testing which variant an enum is, or
13 //! testing a value against a constant.
15 use crate::build::expr::as_place::PlaceBuilder;
16 use crate::build::matches::{Ascription, Binding, Candidate, MatchPair};
17 use crate::build::Builder;
18 use rustc_hir::RangeEnd;
19 use rustc_middle::thir::{self, *};
21 use rustc_middle::ty::layout::IntegerExt;
22 use rustc_target::abi::{Integer, Size};
26 impl<'a, 'tcx> Builder<'a, 'tcx> {
27 /// Simplify a candidate so that all match pairs require a test.
29 /// This method will also split a candidate, in which the only
30 /// match-pair is an or-pattern, into multiple candidates.
38 /// only generates a single switch. If this happens this method returns
40 pub(super) fn simplify_candidate<'pat>(
42 candidate: &mut Candidate<'pat, 'tcx>,
44 // repeatedly simplify match pairs until fixed point is reached
45 debug!(?candidate, "simplify_candidate");
47 // existing_bindings and new_bindings exists to keep the semantics in order.
48 // Reversing the binding order for bindings after `@` changes the binding order in places
49 // it shouldn't be changed, for example `let (Some(a), Some(b)) = (x, y)`
51 // To avoid this, the binding occurs in the following manner:
52 // * the bindings for one iteration of the following loop occurs in order (i.e. left to
54 // * the bindings from the previous iteration of the loop is prepended to the bindings from
55 // the current iteration (in the implementation this is done by mem::swap and extend)
56 // * after all iterations, these new bindings are then appended to the bindings that were
57 // preexisting (i.e. `candidate.binding` when the function was called).
60 // candidate.bindings = [1, 2, 3]
61 // binding in iter 1: [4, 5]
62 // binding in iter 2: [6, 7]
64 // final binding: [1, 2, 3, 6, 7, 4, 5]
65 let mut existing_bindings = mem::take(&mut candidate.bindings);
66 let mut new_bindings = Vec::new();
68 let match_pairs = mem::take(&mut candidate.match_pairs);
70 if let [MatchPair { pattern: Pat { kind: box PatKind::Or { pats }, .. }, place }] =
73 existing_bindings.extend_from_slice(&new_bindings);
74 mem::swap(&mut candidate.bindings, &mut existing_bindings);
75 candidate.subcandidates =
76 self.create_or_subcandidates(candidate, place.clone(), pats);
80 let mut changed = false;
81 for match_pair in match_pairs {
82 match self.simplify_match_pair(match_pair, candidate) {
87 candidate.match_pairs.push(match_pair);
91 // Avoid issue #69971: the binding order should be right to left if there are more
92 // bindings after `@` to please the borrow checker
94 // struct NonCopyStruct {
98 // fn foo1(x: NonCopyStruct) {
99 // let y @ NonCopyStruct { copy_field: z } = x;
100 // // the above should turn into
101 // let z = x.copy_field;
104 candidate.bindings.extend_from_slice(&new_bindings);
105 mem::swap(&mut candidate.bindings, &mut new_bindings);
106 candidate.bindings.clear();
109 existing_bindings.extend_from_slice(&new_bindings);
110 mem::swap(&mut candidate.bindings, &mut existing_bindings);
111 // Move or-patterns to the end, because they can result in us
112 // creating additional candidates, so we want to test them as
116 .sort_by_key(|pair| matches!(*pair.pattern.kind, PatKind::Or { .. }));
117 debug!(simplified = ?candidate, "simplify_candidate");
118 return false; // if we were not able to simplify any, done.
123 /// Given `candidate` that has a single or-pattern for its match-pairs,
124 /// creates a fresh candidate for each of its input subpatterns passed via
126 fn create_or_subcandidates<'pat>(
128 candidate: &Candidate<'pat, 'tcx>,
129 place: PlaceBuilder<'tcx>,
130 pats: &'pat [Pat<'tcx>],
131 ) -> Vec<Candidate<'pat, 'tcx>> {
134 let mut candidate = Candidate::new(place.clone(), pat, candidate.has_guard);
135 self.simplify_candidate(&mut candidate);
141 /// Tries to simplify `match_pair`, returning `Ok(())` if
142 /// successful. If successful, new match pairs and bindings will
143 /// have been pushed into the candidate. If no simplification is
144 /// possible, `Err` is returned and no changes are made to
146 fn simplify_match_pair<'pat>(
148 match_pair: MatchPair<'pat, 'tcx>,
149 candidate: &mut Candidate<'pat, 'tcx>,
150 ) -> Result<(), MatchPair<'pat, 'tcx>> {
152 match *match_pair.pattern.kind {
153 PatKind::AscribeUserType {
155 ascription: thir::Ascription { variance, user_ty, user_ty_span },
157 // Apply the type ascription to the value at `match_pair.place`, which is the
158 if let Ok(place_resolved) =
159 match_pair.place.clone().try_upvars_resolved(self.tcx, self.typeck_results)
161 candidate.ascriptions.push(Ascription {
164 source: place_resolved.into_place(self.tcx, self.typeck_results),
169 candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));
175 // nothing left to do
188 if let Ok(place_resolved) =
189 match_pair.place.clone().try_upvars_resolved(self.tcx, self.typeck_results)
191 candidate.bindings.push(Binding {
192 span: match_pair.pattern.span,
193 source: place_resolved.into_place(self.tcx, self.typeck_results),
199 if let Some(subpattern) = subpattern.as_ref() {
200 // this is the `x @ P` case; have to keep matching against `P` now
201 candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));
207 PatKind::Constant { .. } => {
208 // FIXME normalize patterns when possible
212 PatKind::Range(PatRange { lo, hi, end }) => {
213 let (range, bias) = match *lo.ty().kind() {
215 (Some(('\u{0000}' as u128, '\u{10FFFF}' as u128, Size::from_bits(32))), 0)
218 let size = Integer::from_int_ty(&tcx, ity).size();
219 let max = size.truncate(u128::MAX);
220 let bias = 1u128 << (size.bits() - 1);
221 (Some((0, max, size)), bias)
224 let size = Integer::from_uint_ty(&tcx, uty).size();
225 let max = size.truncate(u128::MAX);
226 (Some((0, max, size)), 0)
230 if let Some((min, max, sz)) = range {
231 if let (Some(lo), Some(hi)) =
232 (lo.val().try_to_bits(sz), hi.val().try_to_bits(sz))
234 // We want to compare ranges numerically, but the order of the bitwise
235 // representation of signed integers does not match their numeric order.
236 // Thus, to correct the ordering, we need to shift the range of signed
237 // integers to correct the comparison. This is achieved by XORing with a
238 // bias (see pattern/_match.rs for another pertinent example of this
240 let (lo, hi) = (lo ^ bias, hi ^ bias);
241 if lo <= min && (hi > max || hi == max && end == RangeEnd::Included) {
242 // Irrefutable pattern match.
250 PatKind::Slice { ref prefix, ref slice, ref suffix } => {
251 if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
253 self.prefix_slice_suffix(
254 &mut candidate.match_pairs,
266 PatKind::Variant { adt_def, substs, variant_index, ref subpatterns } => {
267 let irrefutable = adt_def.variants.iter_enumerated().all(|(i, v)| {
268 i == variant_index || {
269 self.tcx.features().exhaustive_patterns
279 }) && (adt_def.did.is_local()
280 || !adt_def.is_variant_list_non_exhaustive());
282 let place_builder = match_pair.place.downcast(adt_def, variant_index);
285 .extend(self.field_match_pairs(place_builder, subpatterns));
292 PatKind::Array { ref prefix, ref slice, ref suffix } => {
293 self.prefix_slice_suffix(
294 &mut candidate.match_pairs,
303 PatKind::Leaf { ref subpatterns } => {
304 // tuple struct, match subpats (if any)
305 candidate.match_pairs.extend(self.field_match_pairs(match_pair.place, subpatterns));
309 PatKind::Deref { ref subpattern } => {
310 let place_builder = match_pair.place.deref();
311 candidate.match_pairs.push(MatchPair::new(place_builder, subpattern));
315 PatKind::Or { .. } => Err(match_pair),