src/librustc_mir/build/matches/simplify.rs

   1 //! Simplifying Candidates
   2 //!
   3 //! *Simplifying* a match pair `place @ pattern` means breaking it down
   4 //! into bindings or other, simpler match pairs. For example:
   5 //!
   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`
   8 //!
   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.
  14
  15 use crate::build::Builder;
  16 use crate::build::matches::{Ascription, Binding, MatchPair, Candidate};
  17 use crate::hair::{self, *};
  18 use rustc::ty;
  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;
  23
  24 use std::mem;
  25
  26 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, '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
  30         loop {
  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) {
  35                     Ok(()) => {
  36                         changed = true;
  37                     }
  38                     Err(match_pair) => {
  39                         candidate.match_pairs.push(match_pair);
  40                     }
  41                 }
  42             }
  43             if !changed {
  44                 return; // if we were not able to simplify any, done.
  45             }
  46         }
  47     }
  48
  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
  53     /// candidate.
  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 {
  61                 ref subpattern,
  62                 ascription: hair::pattern::Ascription {
  63                     variance,
  64                     ref user_ty,
  65                     user_ty_span,
  66                 },
  67             } => {
  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 {
  71                     span: user_ty_span,
  72                     user_ty: user_ty.clone(),
  73                     source: match_pair.place.clone(),
  74                     variance,
  75                 });
  76
  77                 candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));
  78
  79                 Ok(())
  80             }
  81
  82             PatternKind::Wild => {
  83                 // nothing left to do
  84                 Ok(())
  85             }
  86
  87             PatternKind::Binding { name, mutability, mode, var, ty, ref subpattern } => {
  88                 candidate.bindings.push(Binding {
  89                     name,
  90                     mutability,
  91                     span: match_pair.pattern.span,
  92                     source: match_pair.place.clone(),
  93                     var_id: var,
  94                     var_ty: ty,
  95                     binding_mode: mode,
  96                 });
  97
  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));
 101                 }
 102
 103                 Ok(())
 104             }
 105
 106             PatternKind::Constant { .. } => {
 107                 // FIXME normalize patterns when possible
 108                 Err(match_pair)
 109             }
 110
 111             PatternKind::Range(PatternRange { lo, hi, ty, end }) => {
 112                 let (range, bias) = match ty.sty {
 113                     ty::Char => {
 114                         (Some(('\u{0000}' as u128, '\u{10FFFF}' as u128, Size::from_bits(32))), 0)
 115                     }
 116                     ty::Int(ity) => {
 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)
 121                     }
 122                     ty::Uint(uty) => {
 123                         let size = Integer::from_attr(&tcx, UnsignedInt(uty)).size();
 124                         let max = truncate(u128::max_value(), size);
 125                         (Some((0, max, size)), 0)
 126                     }
 127                     _ => (None, 0),
 128                 };
 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
 136                         // pattern).
 137                         let (lo, hi) = (lo ^ bias, hi ^ bias);
 138                         if lo <= min && (hi > max || hi == max && end == RangeEnd::Included) {
 139                             // Irrefutable pattern match.
 140                             return Ok(());
 141                         }
 142                     }
 143                 }
 144                 Err(match_pair)
 145             }
 146
 147             PatternKind::Slice { ref prefix, ref slice, ref suffix } => {
 148                 if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
 149                     // irrefutable
 150                     self.prefix_slice_suffix(&mut candidate.match_pairs,
 151                                              &match_pair.place,
 152                                              prefix,
 153                                              slice.as_ref(),
 154                                              suffix);
 155                     Ok(())
 156                 } else {
 157                     Err(match_pair)
 158                 }
 159             }
 160
 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                         self.hir.tcx().is_variant_uninhabited_from_all_modules(v, substs)
 167                     }
 168                 });
 169                 if irrefutable {
 170                     let place = match_pair.place.downcast(adt_def, variant_index);
 171                     candidate.match_pairs.extend(self.field_match_pairs(place, subpatterns));
 172                     Ok(())
 173                 } else {
 174                     Err(match_pair)
 175                 }
 176             }
 177
 178             PatternKind::Array { ref prefix, ref slice, ref suffix } => {
 179                 self.prefix_slice_suffix(&mut candidate.match_pairs,
 180                                          &match_pair.place,
 181                                          prefix,
 182                                          slice.as_ref(),
 183                                          suffix);
 184                 Ok(())
 185             }
 186
 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));
 191                 Ok(())
 192             }
 193
 194             PatternKind::Deref { ref subpattern } => {
 195                 let place = match_pair.place.deref();
 196                 candidate.match_pairs.push(MatchPair::new(place, subpattern));
 197                 Ok(())
 198             }
 199         }
 200     }
 201 }