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Rollup merge of #119835 - Nadrieril:simplify-empty-logic, r=compiler-errors

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Exhaustiveness: simplify empty pattern logic

The logic that handles empty patterns had gotten quite convoluted. This PR simplifies it a lot. I tried to make the logic as easy as possible to follow; this only does logically equivalent changes.

The first commit is a drive-by comment clarification that was requested after another PR a while back.

r? `@compiler-errors`
This commit is contained in:
Matthias Krüger 2024-01-19 19:27:00 +01:00 • committed by GitHub
commit 2587100a9b
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4 changed files with 30 additions and 51 deletions
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19
compiler/rustc_pattern_analysis/src/constructor.rs
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@ -861,12 +861,14 @@ impl<Cx: TypeCx> ConstructorSet<Cx> {
/// any) are missing; 2/ split constructors to handle non-trivial intersections e.g. on ranges
/// or slices. This can get subtle; see [`SplitConstructorSet`] for details of this operation
/// and its invariants.
#[instrument(level = "debug", skip(self, pcx, ctors), ret)]
#[instrument(level = "debug", skip(self, ctors), ret)]
pub(crate) fn split<'a>(
&self,
pcx: &PlaceCtxt<'a, Cx>,
ctors: impl Iterator<Item = &'a Constructor<Cx>> + Clone,
) -> SplitConstructorSet<Cx> {
) -> SplitConstructorSet<Cx>
where
Cx: 'a,
{
let mut present: SmallVec<[_; 1]> = SmallVec::new();
// Empty constructors found missing.
let mut missing_empty = Vec::new();
@ -1006,17 +1008,6 @@ impl<Cx: TypeCx> ConstructorSet<Cx> {
}
}
// We have now grouped all the constructors into 3 buckets: present, missing, missing_empty.
// In the absence of the `exhaustive_patterns` feature however, we don't count nested empty
// types as empty. Only non-nested `!` or `enum Foo {}` are considered empty.
if !pcx.mcx.tycx.is_exhaustive_patterns_feature_on()
&& !(pcx.is_scrutinee && matches!(self, Self::NoConstructors))
{
// Treat all missing constructors as nonempty.
// This clears `missing_empty`.
missing.append(&mut missing_empty);
}
SplitConstructorSet { present, missing, missing_empty }
}
}