src/librustc_mir/build/matches/simplify.rs

   1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Simplifying Candidates
  12 //!
  13 //! *Simplifying* a match pair `place @ pattern` means breaking it down
  14 //! into bindings or other, simpler match pairs. For example:
  15 //!
  16 //! - `place @ (P1, P2)` can be simplified to `[place.0 @ P1, place.1 @ P2]`
  17 //! - `place @ x` can be simplified to `[]` by binding `x` to `place`
  18 //!
  19 //! The `simplify_candidate` routine just repeatedly applies these
  20 //! sort of simplifications until there is nothing left to
  21 //! simplify. Match pairs cannot be simplified if they require some
  22 //! sort of test: for example, testing which variant an enum is, or
  23 //! testing a value against a constant.
  24
  25 use build::{BlockAnd, BlockAndExtension, Builder};
  26 use build::matches::{Ascription, Binding, MatchPair, Candidate};
  27 use hair::*;
  28 use rustc::mir::*;
  29
  30 use std::mem;
  31
  32 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
  33     pub fn simplify_candidate<'pat>(&mut self,
  34                                     block: BasicBlock,
  35                                     candidate: &mut Candidate<'pat, 'tcx>)
  36                                     -> BlockAnd<()> {
  37         // repeatedly simplify match pairs until fixed point is reached
  38         loop {
  39             let match_pairs = mem::replace(&mut candidate.match_pairs, vec![]);
  40             let mut progress = match_pairs.len(); // count how many were simplified
  41             for match_pair in match_pairs {
  42                 match self.simplify_match_pair(match_pair, candidate) {
  43                     Ok(()) => {}
  44                     Err(match_pair) => {
  45                         candidate.match_pairs.push(match_pair);
  46                         progress -= 1; // this one was not simplified
  47                     }
  48                 }
  49             }
  50             if progress == 0 {
  51                 return block.unit(); // if we were not able to simplify any, done.
  52             }
  53         }
  54     }
  55
  56     /// Tries to simplify `match_pair`, returning true if
  57     /// successful. If successful, new match pairs and bindings will
  58     /// have been pushed into the candidate. If no simplification is
  59     /// possible, Err is returned and no changes are made to
  60     /// candidate.
  61     fn simplify_match_pair<'pat>(&mut self,
  62                                  match_pair: MatchPair<'pat, 'tcx>,
  63                                  candidate: &mut Candidate<'pat, 'tcx>)
  64                                  -> Result<(), MatchPair<'pat, 'tcx>> {
  65         match *match_pair.pattern.kind {
  66             PatternKind::AscribeUserType { ref subpattern, user_ty } => {
  67                 candidate.ascriptions.push(Ascription {
  68                     span: match_pair.pattern.span,
  69                     user_ty,
  70                     source: match_pair.place.clone(),
  71                 });
  72
  73                 candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));
  74
  75                 Ok(())
  76             }
  77
  78             PatternKind::Wild => {
  79                 // nothing left to do
  80                 Ok(())
  81             }
  82
  83             PatternKind::Binding { name, mutability, mode, var, ty, ref subpattern } => {
  84                 candidate.bindings.push(Binding {
  85                     name,
  86                     mutability,
  87                     span: match_pair.pattern.span,
  88                     source: match_pair.place.clone(),
  89                     var_id: var,
  90                     var_ty: ty,
  91                     binding_mode: mode,
  92                 });
  93
  94                 if let Some(subpattern) = subpattern.as_ref() {
  95                     // this is the `x @ P` case; have to keep matching against `P` now
  96                     candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));
  97                 }
  98
  99                 Ok(())
 100             }
 101
 102             PatternKind::Constant { .. } => {
 103                 // FIXME normalize patterns when possible
 104                 Err(match_pair)
 105             }
 106
 107             PatternKind::Range { .. } => {
 108                 Err(match_pair)
 109             }
 110
 111             PatternKind::Slice { ref prefix, ref slice, ref suffix } => {
 112                 if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
 113                     // irrefutable
 114                     self.prefix_slice_suffix(&mut candidate.match_pairs,
 115                                              &match_pair.place,
 116                                              prefix,
 117                                              slice.as_ref(),
 118                                              suffix);
 119                     Ok(())
 120                 } else {
 121                     Err(match_pair)
 122                 }
 123             }
 124
 125             PatternKind::Variant { adt_def, substs, variant_index, ref subpatterns } => {
 126                 let irrefutable = adt_def.variants.iter().enumerate().all(|(i, v)| {
 127                     i == variant_index || {
 128                         self.hir.tcx().features().never_type &&
 129                         self.hir.tcx().features().exhaustive_patterns &&
 130                         self.hir.tcx().is_variant_uninhabited_from_all_modules(v, substs)
 131                     }
 132                 });
 133                 if irrefutable {
 134                     let place = match_pair.place.downcast(adt_def, variant_index);
 135                     candidate.match_pairs.extend(self.field_match_pairs(place, subpatterns));
 136                     Ok(())
 137                 } else {
 138                     Err(match_pair)
 139                 }
 140             },
 141
 142             PatternKind::Array { ref prefix, ref slice, ref suffix } => {
 143                 self.prefix_slice_suffix(&mut candidate.match_pairs,
 144                                          &match_pair.place,
 145                                          prefix,
 146                                          slice.as_ref(),
 147                                          suffix);
 148                 Ok(())
 149             }
 150
 151             PatternKind::Leaf { ref subpatterns } => {
 152                 // tuple struct, match subpats (if any)
 153                 candidate.match_pairs
 154                          .extend(self.field_match_pairs(match_pair.place, subpatterns));
 155                 Ok(())
 156             }
 157
 158             PatternKind::Deref { ref subpattern } => {
 159                 let place = match_pair.place.deref();
 160                 candidate.match_pairs.push(MatchPair::new(place, subpattern));
 161                 Ok(())
 162             }
 163         }
 164     }
 165 }