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::Builder;
16 use crate::build::matches::{Ascription, Binding, MatchPair, Candidate};
17 use crate::hair::{self, *};
19 use rustc::ty::layout::{Integer, IntegerExt, Size};
20 use syntax::attr::{SignedInt, UnsignedInt};
21 use rustc::hir::RangeEnd;
22 use rustc::mir::interpret::truncate;
26 impl<'a, 'tcx> Builder<'a, 'tcx> {
27 pub fn simplify_candidate<'pat>(&mut self,
28 candidate: &mut Candidate<'pat, 'tcx>) {
29 // repeatedly simplify match pairs until fixed point is reached
31 let match_pairs = mem::replace(&mut candidate.match_pairs, vec![]);
32 let mut changed = false;
33 for match_pair in match_pairs {
34 match self.simplify_match_pair(match_pair, candidate) {
39 candidate.match_pairs.push(match_pair);
44 return; // if we were not able to simplify any, done.
49 /// Tries to simplify `match_pair`, returning `Ok(())` if
50 /// successful. If successful, new match pairs and bindings will
51 /// have been pushed into the candidate. If no simplification is
52 /// possible, `Err` is returned and no changes are made to
54 fn simplify_match_pair<'pat>(&mut self,
55 match_pair: MatchPair<'pat, 'tcx>,
56 candidate: &mut Candidate<'pat, 'tcx>)
57 -> Result<(), MatchPair<'pat, 'tcx>> {
58 let tcx = self.hir.tcx();
59 match *match_pair.pattern.kind {
60 PatternKind::AscribeUserType {
62 ascription: hair::pattern::Ascription {
68 // Apply the type ascription to the value at `match_pair.place`, which is the
69 // value being matched, taking the variance field into account.
70 candidate.ascriptions.push(Ascription {
72 user_ty: user_ty.clone(),
73 source: match_pair.place.clone(),
77 candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));
82 PatternKind::Wild => {
87 PatternKind::Binding { name, mutability, mode, var, ty, ref subpattern } => {
88 candidate.bindings.push(Binding {
91 span: match_pair.pattern.span,
92 source: match_pair.place.clone(),
98 if let Some(subpattern) = subpattern.as_ref() {
99 // this is the `x @ P` case; have to keep matching against `P` now
100 candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));
106 PatternKind::Constant { .. } => {
107 // FIXME normalize patterns when possible
111 PatternKind::Range(PatternRange { lo, hi, ty, end }) => {
112 let (range, bias) = match ty.sty {
114 (Some(('\u{0000}' as u128, '\u{10FFFF}' as u128, Size::from_bits(32))), 0)
117 let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
118 let max = truncate(u128::max_value(), size);
119 let bias = 1u128 << (size.bits() - 1);
120 (Some((0, max, size)), bias)
123 let size = Integer::from_attr(&tcx, UnsignedInt(uty)).size();
124 let max = truncate(u128::max_value(), size);
125 (Some((0, max, size)), 0)
129 if let Some((min, max, sz)) = range {
130 if let (Some(lo), Some(hi)) = (lo.val.try_to_bits(sz), hi.val.try_to_bits(sz)) {
131 // We want to compare ranges numerically, but the order of the bitwise
132 // representation of signed integers does not match their numeric order.
133 // Thus, to correct the ordering, we need to shift the range of signed
134 // integers to correct the comparison. This is achieved by XORing with a
135 // bias (see pattern/_match.rs for another pertinent example of this
137 let (lo, hi) = (lo ^ bias, hi ^ bias);
138 if lo <= min && (hi > max || hi == max && end == RangeEnd::Included) {
139 // Irrefutable pattern match.
147 PatternKind::Slice { ref prefix, ref slice, ref suffix } => {
148 if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
150 self.prefix_slice_suffix(&mut candidate.match_pairs,
161 PatternKind::Variant { adt_def, substs, variant_index, ref subpatterns } => {
162 let irrefutable = adt_def.variants.iter_enumerated().all(|(i, v)| {
163 i == variant_index || {
164 self.hir.tcx().features().never_type &&
165 self.hir.tcx().features().exhaustive_patterns &&
166 !v.uninhabited_from(self.hir.tcx(), substs, adt_def.adt_kind()).is_empty()
170 let place = match_pair.place.downcast(adt_def, variant_index);
171 candidate.match_pairs.extend(self.field_match_pairs(place, subpatterns));
178 PatternKind::Array { ref prefix, ref slice, ref suffix } => {
179 self.prefix_slice_suffix(&mut candidate.match_pairs,
187 PatternKind::Leaf { ref subpatterns } => {
188 // tuple struct, match subpats (if any)
189 candidate.match_pairs
190 .extend(self.field_match_pairs(match_pair.place, subpatterns));
194 PatternKind::Deref { ref subpattern } => {
195 let place = match_pair.place.deref();
196 candidate.match_pairs.push(MatchPair::new(place, subpattern));