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 build::Builder;
  16 use build::matches::{Ascription, Binding, MatchPair, Candidate};
  17 use hair::*;
  18 use rustc::ty;
  19 use rustc::ty::layout::{Integer, IntegerExt, Size};
  20 use syntax::attr::{SignedInt, UnsignedInt};
  21 use rustc::hir::RangeEnd;
  22
  23 use std::mem;
  24
  25 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
  26     pub fn simplify_candidate<'pat>(&mut self,
  27                                     candidate: &mut Candidate<'pat, 'tcx>) {
  28         // repeatedly simplify match pairs until fixed point is reached
  29         loop {
  30             let match_pairs = mem::replace(&mut candidate.match_pairs, vec![]);
  31             let mut changed = false;
  32             for match_pair in match_pairs {
  33                 match self.simplify_match_pair(match_pair, candidate) {
  34                     Ok(()) => {
  35                         changed = true;
  36                     }
  37                     Err(match_pair) => {
  38                         candidate.match_pairs.push(match_pair);
  39                     }
  40                 }
  41             }
  42             if !changed {
  43                 return; // if we were not able to simplify any, done.
  44             }
  45         }
  46     }
  47
  48     /// Tries to simplify `match_pair`, returning true if
  49     /// successful. If successful, new match pairs and bindings will
  50     /// have been pushed into the candidate. If no simplification is
  51     /// possible, Err is returned and no changes are made to
  52     /// candidate.
  53     fn simplify_match_pair<'pat>(&mut self,
  54                                  match_pair: MatchPair<'pat, 'tcx>,
  55                                  candidate: &mut Candidate<'pat, 'tcx>)
  56                                  -> Result<(), MatchPair<'pat, 'tcx>> {
  57         let tcx = self.hir.tcx();
  58         match *match_pair.pattern.kind {
  59             PatternKind::AscribeUserType {
  60                 ref subpattern,
  61                 variance,
  62                 ref user_ty,
  63                 user_ty_span
  64             } => {
  65                 // Apply the type ascription to the value at `match_pair.place`, which is the
  66                 // value being matched, taking the variance field into account.
  67                 candidate.ascriptions.push(Ascription {
  68                     span: user_ty_span,
  69                     user_ty: user_ty.clone(),
  70                     source: match_pair.place.clone(),
  71                     variance,
  72                 });
  73
  74                 candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));
  75
  76                 Ok(())
  77             }
  78
  79             PatternKind::Wild => {
  80                 // nothing left to do
  81                 Ok(())
  82             }
  83
  84             PatternKind::Binding { name, mutability, mode, var, ty, ref subpattern } => {
  85                 candidate.bindings.push(Binding {
  86                     name,
  87                     mutability,
  88                     span: match_pair.pattern.span,
  89                     source: match_pair.place.clone(),
  90                     var_id: var,
  91                     var_ty: ty,
  92                     binding_mode: mode,
  93                 });
  94
  95                 if let Some(subpattern) = subpattern.as_ref() {
  96                     // this is the `x @ P` case; have to keep matching against `P` now
  97                     candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));
  98                 }
  99
 100                 Ok(())
 101             }
 102
 103             PatternKind::Constant { .. } => {
 104                 // FIXME normalize patterns when possible
 105                 Err(match_pair)
 106             }
 107
 108             PatternKind::Range(PatternRange { lo, hi, ty, end }) => {
 109                 let (range, bias) = match ty.sty {
 110                     ty::Char => {
 111                         (Some(('\u{0000}' as u128, '\u{10FFFF}' as u128, Size::from_bits(32))), 0)
 112                     }
 113                     ty::Int(ity) => {
 114                         // FIXME(49937): refactor these bit manipulations into interpret.
 115                         let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
 116                         let max = !0u128 >> (128 - size.bits());
 117                         let bias = 1u128 << (size.bits() - 1);
 118                         (Some((0, max, size)), bias)
 119                     }
 120                     ty::Uint(uty) => {
 121                         // FIXME(49937): refactor these bit manipulations into interpret.
 122                         let size = Integer::from_attr(&tcx, UnsignedInt(uty)).size();
 123                         let max = !0u128 >> (128 - size.bits());
 124                         (Some((0, max, size)), 0)
 125                     }
 126                     _ => (None, 0),
 127                 };
 128                 if let Some((min, max, sz)) = range {
 129                     if let (Some(lo), Some(hi)) = (lo.val.try_to_bits(sz), hi.val.try_to_bits(sz)) {
 130                         // We want to compare ranges numerically, but the order of the bitwise
 131                         // representation of signed integers does not match their numeric order.
 132                         // Thus, to correct the ordering, we need to shift the range of signed
 133                         // integers to correct the comparison. This is achieved by XORing with a
 134                         // bias (see pattern/_match.rs for another pertinent example of this
 135                         // pattern).
 136                         let (lo, hi) = (lo ^ bias, hi ^ bias);
 137                         if lo <= min && (hi > max || hi == max && end == RangeEnd::Included) {
 138                             // Irrefutable pattern match.
 139                             return Ok(());
 140                         }
 141                     }
 142                 }
 143                 Err(match_pair)
 144             }
 145
 146             PatternKind::Slice { ref prefix, ref slice, ref suffix } => {
 147                 if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
 148                     // irrefutable
 149                     self.prefix_slice_suffix(&mut candidate.match_pairs,
 150                                              &match_pair.place,
 151                                              prefix,
 152                                              slice.as_ref(),
 153                                              suffix);
 154                     Ok(())
 155                 } else {
 156                     Err(match_pair)
 157                 }
 158             }
 159
 160             PatternKind::Variant { adt_def, substs, variant_index, ref subpatterns } => {
 161                 let irrefutable = adt_def.variants.iter_enumerated().all(|(i, v)| {
 162                     i == variant_index || {
 163                         self.hir.tcx().features().never_type &&
 164                         self.hir.tcx().features().exhaustive_patterns &&
 165                         self.hir.tcx().is_variant_uninhabited_from_all_modules(v, substs)
 166                     }
 167                 });
 168                 if irrefutable {
 169                     let place = match_pair.place.downcast(adt_def, variant_index);
 170                     candidate.match_pairs.extend(self.field_match_pairs(place, subpatterns));
 171                     Ok(())
 172                 } else {
 173                     Err(match_pair)
 174                 }
 175             },
 176
 177             PatternKind::Array { ref prefix, ref slice, ref suffix } => {
 178                 self.prefix_slice_suffix(&mut candidate.match_pairs,
 179                                          &match_pair.place,
 180                                          prefix,
 181                                          slice.as_ref(),
 182                                          suffix);
 183                 Ok(())
 184             }
 185
 186             PatternKind::Leaf { ref subpatterns } => {
 187                 // tuple struct, match subpats (if any)
 188                 candidate.match_pairs
 189                          .extend(self.field_match_pairs(match_pair.place, subpatterns));
 190                 Ok(())
 191             }
 192
 193             PatternKind::Deref { ref subpattern } => {
 194                 let place = match_pair.place.deref();
 195                 candidate.match_pairs.push(MatchPair::new(place, subpattern));
 196                 Ok(())
 197             }
 198         }
 199     }
 200 }