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Explain the Opaque special case in specialization

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This commit is contained in:
Nadrieril 2020-10-21 20:15:02 +01:00
parent 3519411700
commit faf87105db
1 changed files with 18 additions and 1 deletions
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19
compiler/rustc_mir_build/src/thir/pattern/_match.rs
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@ -2477,7 +2477,24 @@ fn specialize_one_pattern<'p, 'tcx>(
if let Opaque = constructor {
// Only a wildcard pattern can match an opaque constant, unless we're specializing the
// value against its own constructor.
// value against its own constructor. That happens when we call
// `v.specialize_constructor(ctor)` with `ctor` obtained from `pat_constructor(v.head())`.
// For example, in the following match, when we are dealing with the third branch, we will
// specialize with an `Opaque` ctor. We want to ignore the second branch because opaque
// constants should not be inspected, but we don't want to ignore the current (third)
// branch, as that would cause us to always conclude that such a branch is unreachable.
// ```rust
// #[derive(PartialEq)]
// struct Foo(i32);
// impl Eq for Foo {}
// const FOO: Foo = Foo(42);
//
// match (Foo(0), true) {
// (_, true) => {}
// (FOO, true) => {}
// (FOO, false) => {}
// }
// ```
if is_its_own_ctor || pat.is_wildcard() {
return Some(Fields::empty());
} else {