diff options
Diffstat (limited to 'compiler/rustc_pattern_analysis/src/usefulness.rs')
| -rw-r--r-- | compiler/rustc_pattern_analysis/src/usefulness.rs | 26 |
1 files changed, 17 insertions, 9 deletions
diff --git a/compiler/rustc_pattern_analysis/src/usefulness.rs b/compiler/rustc_pattern_analysis/src/usefulness.rs index 0de48c9f3c0..a627bdeef81 100644 --- a/compiler/rustc_pattern_analysis/src/usefulness.rs +++ b/compiler/rustc_pattern_analysis/src/usefulness.rs @@ -548,11 +548,12 @@ //! [`ValidityConstraint::specialize`]. //! //! Having said all that, in practice we don't fully follow what's been presented in this section. -//! Under `exhaustive_patterns`, we allow omitting empty arms even in `!known_valid` places, for -//! backwards-compatibility until we have a better alternative. Without `exhaustive_patterns`, we -//! mostly treat empty types as inhabited, except specifically a non-nested `!` or empty enum. In -//! this specific case we also allow the empty match regardless of place validity, for -//! backwards-compatibility. Hopefully we can eventually deprecate this. +//! Let's call "toplevel exception" the case where the match scrutinee itself has type `!` or +//! `EmptyEnum`. First, on stable rust, we require `_` patterns for empty types in all cases apart +//! from the toplevel exception. The `exhaustive_patterns` and `min_exaustive_patterns` allow +//! omitting patterns in the cases described above. There's a final detail: in the toplevel +//! exception or with the `exhaustive_patterns` feature, we ignore place validity when checking +//! whether a pattern is required for exhaustiveness. I (Nadrieril) hope to deprecate this behavior. //! //! //! @@ -1442,10 +1443,17 @@ fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>( // We treat match scrutinees of type `!` or `EmptyEnum` differently. let is_toplevel_exception = is_top_level && matches!(ctors_for_ty, ConstructorSet::NoConstructors); - // Whether empty patterns can be omitted for exhaustiveness. - let can_omit_empty_arms = is_toplevel_exception || mcx.tycx.is_exhaustive_patterns_feature_on(); - // Whether empty patterns are counted as useful or not. - let empty_arms_are_unreachable = place_validity.is_known_valid() && can_omit_empty_arms; + // Whether empty patterns are counted as useful or not. We only warn an empty arm unreachable if + // it is guaranteed unreachable by the opsem (i.e. if the place is `known_valid`). + let empty_arms_are_unreachable = place_validity.is_known_valid() + && (is_toplevel_exception + || mcx.tycx.is_exhaustive_patterns_feature_on() + || mcx.tycx.is_min_exhaustive_patterns_feature_on()); + // Whether empty patterns can be omitted for exhaustiveness. We ignore place validity in the + // toplevel exception and `exhaustive_patterns` cases for backwards compatibility. + let can_omit_empty_arms = empty_arms_are_unreachable + || is_toplevel_exception + || mcx.tycx.is_exhaustive_patterns_feature_on(); // Analyze the constructors present in this column. let ctors = matrix.heads().map(|p| p.ctor()); |