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Auto merge of #78638 - vn-ki:bindigs-after-at-issue-69971, r=oli-obk

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reverse binding order in matches to allow the subbinding of copyable fields in bindings after @

Fixes #69971

### TODO

- [x] Regression tests

r? `@oli-obk`
This commit is contained in:
bors 2020-11-05 13:26:08 +00:00
commit b1d9f31e04
22 changed files with 572 additions and 599 deletions
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44
compiler/rustc_mir_build/src/build/matches/simplify.rs
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@ -43,12 +43,36 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
candidate: &mut Candidate<'pat, 'tcx>,
) -> bool {
// repeatedly simplify match pairs until fixed point is reached
debug!(?candidate, "simplify_candidate");
// existing_bindings and new_bindings exists to keep the semantics in order.
// Reversing the binding order for bindings after `@` changes the binding order in places
// it shouldn't be changed, for example `let (Some(a), Some(b)) = (x, y)`
//
// To avoid this, the binding occurs in the following manner:
// * the bindings for one iteration of the following 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
// prexisting (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]
let mut existing_bindings = mem::take(&mut candidate.bindings);
let mut new_bindings = Vec::new();
loop {
let match_pairs = mem::take(&mut candidate.match_pairs);
if let [MatchPair { pattern: Pat { kind: box PatKind::Or { pats }, .. }, place }] =
*match_pairs
{
existing_bindings.extend_from_slice(&new_bindings);
mem::swap(&mut candidate.bindings, &mut existing_bindings);
candidate.subcandidates = self.create_or_subcandidates(candidate, place, pats);
return true;
}
@ -64,13 +88,33 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
}
}
}
// Avoid issue #69971: the binding order should be right to left if there are more
// bindings after `@` to please the borrow checker
// Ex
// struct NonCopyStruct {
// copy_field: u32,
// }
//
// fn foo1(x: NonCopyStruct) {
// let y @ NonCopyStruct { copy_field: z } = x;
// // the above should turn into
// let z = x.copy_field;
// let y = x;
// }
candidate.bindings.extend_from_slice(&new_bindings);
mem::swap(&mut candidate.bindings, &mut new_bindings);
candidate.bindings.clear();
if !changed {
existing_bindings.extend_from_slice(&new_bindings);
mem::swap(&mut candidate.bindings, &mut existing_bindings);
// Move or-patterns to the end, because they can result in us
// creating additional candidates, so we want to test them as
// late as possible.
candidate
.match_pairs
.sort_by_key(|pair| matches!(*pair.pattern.kind, PatKind::Or { .. }));
debug!(simplified = ?candidate, "simplify_candidate");
return false; // if we were not able to simplify any, done.
}
}