diff options
| -rw-r--r-- | compiler/rustc_mir_build/src/build/matches/simplify.rs | 64 |
1 files changed, 20 insertions, 44 deletions
diff --git a/compiler/rustc_mir_build/src/build/matches/simplify.rs b/compiler/rustc_mir_build/src/build/matches/simplify.rs index 9fab61803e8..3eb39d68d74 100644 --- a/compiler/rustc_mir_build/src/build/matches/simplify.rs +++ b/compiler/rustc_mir_build/src/build/matches/simplify.rs @@ -39,28 +39,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> { candidate: &mut Candidate<'pat, 'tcx>, ) -> bool { debug!("{candidate:#?}"); - // `original_bindings` and `new_bindings` exist to keep the semantics in order. - // Reversing the binding order for bindings after `@` changes the binding order in places - // where it shouldn't be changed, for example `let (Some(a), Some(b)) = (x, y)`. + // In order to please the borrow checker, in a pattern like `x @ pat` we must lower the + // bindings in `pat` before `x`. E.g. (#69971): // - // To avoid this, the binding occurs in the following manner: - // * the bindings for one iteration of the loop occurs in order (i.e. left to right) - // * the bindings from the previous iteration of the loop is prepended to the bindings from - // the current iteration (in the implementation this is done by mem::swap and extend) - // * after all iterations, these new bindings are then appended to the bindings that were - // preexisting (i.e. `candidate.binding` when the function was called). - // - // example: - // candidate.bindings = [1, 2, 3] - // binding in iter 1: [4, 5] - // binding in iter 2: [6, 7] - // - // final binding: [1, 2, 3, 6, 7, 4, 5] - // - // This is because we treat refutable and irrefutable bindings differently. The binding - // order should be right-to-left if there are more _irrefutable_ bindings after `@` to - // please the borrow checker (#69971) - // Ex // struct NonCopyStruct { // copy_field: u32, // } @@ -72,23 +53,20 @@ impl<'a, 'tcx> Builder<'a, 'tcx> { // let y = x; // } // - // If however the bindings are refutable, i.e. under a test, then we keep the bindings - // left-to-right. - // Ex - // enum NonCopyEnum { - // Variant { copy_field: u32 }, - // None, - // } + // We can't just reverse the binding order, because we must preserve pattern-order + // otherwise, e.g. in `let (Some(a), Some(b)) = (x, y)`. Our rule then is: deepest-first, + // and bindings at the same depth stay in source order. // - // fn foo2(x: NonCopyEnum) { - // let y @ NonCopyEnum::Variant { copy_field: z } = x else { return }; - // // turns into - // let y = x; - // let z = (x as Variant).copy_field; - // // and raises an error - // } - let original_bindings = mem::take(&mut candidate.bindings); - let mut new_bindings = Vec::new(); + // To do this, every time around the loop we prepend the newly found bindings to the + // bindings we already had. + // + // example: + // candidate.bindings = [1, 2, 3] + // bindings in iter 1: [4, 5] + // bindings in iter 2: [6, 7] + // + // final bindings: [6, 7, 4, 5, 1, 2, 3] + let mut accumulated_bindings = mem::take(&mut candidate.bindings); // Repeatedly simplify match pairs until fixed point is reached loop { let mut changed = false; @@ -103,9 +81,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> { } } - // This does: new_bindings = candidate.bindings.take() ++ new_bindings - candidate.bindings.extend_from_slice(&new_bindings); - mem::swap(&mut candidate.bindings, &mut new_bindings); + // This does: accumulated_bindings = candidate.bindings.take() ++ accumulated_bindings + candidate.bindings.extend_from_slice(&accumulated_bindings); + mem::swap(&mut candidate.bindings, &mut accumulated_bindings); candidate.bindings.clear(); if !changed { @@ -114,10 +92,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> { } } - // Restore original bindings and append the new ones. - // This does: candidate.bindings = new_bindings ++ original_bindings - mem::swap(&mut candidate.bindings, &mut new_bindings); - candidate.bindings.extend_from_slice(&original_bindings); + // Store computed bindings back in `candidate`. + mem::swap(&mut candidate.bindings, &mut accumulated_bindings); let did_expand_or = if let [MatchPair { pattern: Pat { kind: PatKind::Or { pats }, .. }, place }] = |