compiler/rustc_mir_build/src/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::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, *};
  20 use rustc_middle::ty;
  21 use rustc_middle::ty::layout::IntegerExt;
  22 use rustc_target::abi::{Integer, Size};
  23
  24 use std::mem;
  25
  26 impl<'a, 'tcx> Builder<'a, 'tcx> {
  27     /// Simplify a candidate so that all match pairs require a test.
  28     ///
  29     /// This method will also split a candidate, in which the only
  30     /// match-pair is an or-pattern, into multiple candidates.
  31     /// This is so that
  32     ///
  33     /// match x {
  34     ///     0 | 1 => { ... },
  35     ///     2 | 3 => { ... },
  36     /// }
  37     ///
  38     /// only generates a single switch. If this happens this method returns
  39     /// `true`.
  40     pub(super) fn simplify_candidate<'pat>(
  41         &mut self,
  42         candidate: &mut Candidate<'pat, 'tcx>,
  43     ) -> bool {
  44         // repeatedly simplify match pairs until fixed point is reached
  45         debug!(?candidate, "simplify_candidate");
  46
  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)`
  50         //
  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
  53         // right)
  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).
  58         //
  59         // example:
  60         // candidate.bindings = [1, 2, 3]
  61         // binding in iter 1: [4, 5]
  62         // binding in iter 2: [6, 7]
  63         //
  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();
  67         loop {
  68             let match_pairs = mem::take(&mut candidate.match_pairs);
  69
  70             if let [MatchPair { pattern: Pat { kind: PatKind::Or { pats }, .. }, place }] =
  71                 &*match_pairs
  72             {
  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);
  77                 return true;
  78             }
  79
  80             let mut changed = false;
  81             for match_pair in match_pairs {
  82                 match self.simplify_match_pair(match_pair, candidate) {
  83                     Ok(()) => {
  84                         changed = true;
  85                     }
  86                     Err(match_pair) => {
  87                         candidate.match_pairs.push(match_pair);
  88                     }
  89                 }
  90             }
  91             // Avoid issue #69971: the binding order should be right to left if there are more
  92             // bindings after `@` to please the borrow checker
  93             // Ex
  94             // struct NonCopyStruct {
  95             //     copy_field: u32,
  96             // }
  97             //
  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;
 102             //     let y = x;
 103             // }
 104             candidate.bindings.extend_from_slice(&new_bindings);
 105             mem::swap(&mut candidate.bindings, &mut new_bindings);
 106             candidate.bindings.clear();
 107
 108             if !changed {
 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
 113                 // late as possible.
 114                 candidate
 115                     .match_pairs
 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.
 119             }
 120         }
 121     }
 122
 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
 125     /// `pats`.
 126     fn create_or_subcandidates<'pat>(
 127         &mut self,
 128         candidate: &Candidate<'pat, 'tcx>,
 129         place: PlaceBuilder<'tcx>,
 130         pats: &'pat [Box<Pat<'tcx>>],
 131     ) -> Vec<Candidate<'pat, 'tcx>> {
 132         pats.iter()
 133             .map(|box pat| {
 134                 let mut candidate = Candidate::new(place.clone(), pat, candidate.has_guard);
 135                 self.simplify_candidate(&mut candidate);
 136                 candidate
 137             })
 138             .collect()
 139     }
 140
 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
 145     /// candidate.
 146     fn simplify_match_pair<'pat>(
 147         &mut self,
 148         match_pair: MatchPair<'pat, 'tcx>,
 149         candidate: &mut Candidate<'pat, 'tcx>,
 150     ) -> Result<(), MatchPair<'pat, 'tcx>> {
 151         let tcx = self.tcx;
 152         match match_pair.pattern.kind {
 153             PatKind::AscribeUserType {
 154                 ref subpattern,
 155                 ascription: thir::Ascription { ref annotation, variance },
 156             } => {
 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)
 160                 {
 161                     candidate.ascriptions.push(Ascription {
 162                         annotation: annotation.clone(),
 163                         source: place_resolved.into_place(self.tcx, self.typeck_results),
 164                         variance,
 165                     });
 166                 }
 167
 168                 candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));
 169
 170                 Ok(())
 171             }
 172
 173             PatKind::Wild => {
 174                 // nothing left to do
 175                 Ok(())
 176             }
 177
 178             PatKind::Binding {
 179                 name: _,
 180                 mutability: _,
 181                 mode,
 182                 var,
 183                 ty: _,
 184                 ref subpattern,
 185                 is_primary: _,
 186             } => {
 187                 if let Ok(place_resolved) =
 188                     match_pair.place.clone().try_upvars_resolved(self.tcx, self.typeck_results)
 189                 {
 190                     candidate.bindings.push(Binding {
 191                         span: match_pair.pattern.span,
 192                         source: place_resolved.into_place(self.tcx, self.typeck_results),
 193                         var_id: var,
 194                         binding_mode: mode,
 195                     });
 196                 }
 197
 198                 if let Some(subpattern) = subpattern.as_ref() {
 199                     // this is the `x @ P` case; have to keep matching against `P` now
 200                     candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern));
 201                 }
 202
 203                 Ok(())
 204             }
 205
 206             PatKind::Constant { .. } => {
 207                 // FIXME normalize patterns when possible
 208                 Err(match_pair)
 209             }
 210
 211             PatKind::Range(box PatRange { lo, hi, end }) => {
 212                 let (range, bias) = match *lo.ty().kind() {
 213                     ty::Char => {
 214                         (Some(('\u{0000}' as u128, '\u{10FFFF}' as u128, Size::from_bits(32))), 0)
 215                     }
 216                     ty::Int(ity) => {
 217                         let size = Integer::from_int_ty(&tcx, ity).size();
 218                         let max = size.truncate(u128::MAX);
 219                         let bias = 1u128 << (size.bits() - 1);
 220                         (Some((0, max, size)), bias)
 221                     }
 222                     ty::Uint(uty) => {
 223                         let size = Integer::from_uint_ty(&tcx, uty).size();
 224                         let max = size.truncate(u128::MAX);
 225                         (Some((0, max, size)), 0)
 226                     }
 227                     _ => (None, 0),
 228                 };
 229                 if let Some((min, max, sz)) = range {
 230                     // We want to compare ranges numerically, but the order of the bitwise
 231                     // representation of signed integers does not match their numeric order. Thus,
 232                     // to correct the ordering, we need to shift the range of signed integers to
 233                     // correct the comparison. This is achieved by XORing with a bias (see
 234                     // pattern/_match.rs for another pertinent example of this pattern).
 235                     //
 236                     // Also, for performance, it's important to only do the second `try_to_bits` if
 237                     // necessary.
 238                     let lo = lo.try_to_bits(sz).unwrap() ^ bias;
 239                     if lo <= min {
 240                         let hi = hi.try_to_bits(sz).unwrap() ^ bias;
 241                         if hi > max || hi == max && end == RangeEnd::Included {
 242                             // Irrefutable pattern match.
 243                             return Ok(());
 244                         }
 245                     }
 246                 }
 247                 Err(match_pair)
 248             }
 249
 250             PatKind::Slice { ref prefix, ref slice, ref suffix } => {
 251                 if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
 252                     // irrefutable
 253                     self.prefix_slice_suffix(
 254                         &mut candidate.match_pairs,
 255                         &match_pair.place,
 256                         prefix,
 257                         slice,
 258                         suffix,
 259                     );
 260                     Ok(())
 261                 } else {
 262                     Err(match_pair)
 263                 }
 264             }
 265
 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
 270                             && !v
 271                                 .uninhabited_from(
 272                                     self.tcx,
 273                                     substs,
 274                                     adt_def.adt_kind(),
 275                                     self.param_env,
 276                                 )
 277                                 .is_empty()
 278                     }
 279                 }) && (adt_def.did().is_local()
 280                     || !adt_def.is_variant_list_non_exhaustive());
 281                 if irrefutable {
 282                     let place_builder = match_pair.place.downcast(adt_def, variant_index);
 283                     candidate
 284                         .match_pairs
 285                         .extend(self.field_match_pairs(place_builder, subpatterns));
 286                     Ok(())
 287                 } else {
 288                     Err(match_pair)
 289                 }
 290             }
 291
 292             PatKind::Array { ref prefix, ref slice, ref suffix } => {
 293                 self.prefix_slice_suffix(
 294                     &mut candidate.match_pairs,
 295                     &match_pair.place,
 296                     prefix,
 297                     slice,
 298                     suffix,
 299                 );
 300                 Ok(())
 301             }
 302
 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));
 306                 Ok(())
 307             }
 308
 309             PatKind::Deref { ref subpattern } => {
 310                 let place_builder = match_pair.place.deref();
 311                 candidate.match_pairs.push(MatchPair::new(place_builder, subpattern));
 312                 Ok(())
 313             }
 314
 315             PatKind::Or { .. } => Err(match_pair),
 316         }
 317     }
 318 }