Rollup merge of #121823 - Nadrieril:never-witnesses, r=compiler-errors

never patterns: suggest `!` patterns on non-exhaustive matches

When a match is non-exhaustive we now suggest never patterns whenever it makes sense.

r? ``@compiler-errors``

1 files changed, 33 insertions, 6 deletions

diff --git a/compiler/rustc_pattern_analysis/src/constructor.rs b/compiler/rustc_pattern_analysis/src/constructor.rs
index bbb89576ef7..5b58d7f43b2 100644
--- a/compiler/rustc_pattern_analysis/src/constructor.rs
+++ b/compiler/rustc_pattern_analysis/src/constructor.rs
@@ -678,15 +678,19 @@ pub enum Constructor<Cx: PatCx> {
     Or,
     /// Wildcard pattern.
     Wildcard,
+    /// Never pattern. Only used in `WitnessPat`. An actual never pattern should be lowered as
+    /// `Wildcard`.
+    Never,
     /// Fake extra constructor for enums that aren't allowed to be matched exhaustively. Also used
-    /// for those types for which we cannot list constructors explicitly, like `f64` and `str`.
+    /// for those types for which we cannot list constructors explicitly, like `f64` and `str`. Only
+    /// used in `WitnessPat`.
     NonExhaustive,
-    /// Fake extra constructor for variants that should not be mentioned in diagnostics.
-    /// We use this for variants behind an unstable gate as well as
-    /// `#[doc(hidden)]` ones.
+    /// Fake extra constructor for variants that should not be mentioned in diagnostics. We use this
+    /// for variants behind an unstable gate as well as `#[doc(hidden)]` ones. Only used in
+    /// `WitnessPat`.
     Hidden,
     /// Fake extra constructor for constructors that are not seen in the matrix, as explained at the
-    /// top of the file.
+    /// top of the file. Only used for specialization.
     Missing,
     /// Fake extra constructor that indicates and empty field that is private. When we encounter one
     /// we skip the column entirely so we don't observe its emptiness. Only used for specialization.
@@ -708,6 +712,7 @@ impl<Cx: PatCx> Clone for Constructor<Cx> {
             Constructor::Str(value) => Constructor::Str(value.clone()),
             Constructor::Opaque(inner) => Constructor::Opaque(inner.clone()),
             Constructor::Or => Constructor::Or,
+            Constructor::Never => Constructor::Never,
             Constructor::Wildcard => Constructor::Wildcard,
             Constructor::NonExhaustive => Constructor::NonExhaustive,
             Constructor::Hidden => Constructor::Hidden,
@@ -1040,10 +1045,32 @@ impl<Cx: PatCx> ConstructorSet<Cx> {
                 // In a `MaybeInvalid` place even an empty pattern may be reachable. We therefore
                 // add a dummy empty constructor here, which will be ignored if the place is
                 // `ValidOnly`.
-                missing_empty.push(NonExhaustive);
+                missing_empty.push(Never);
             }
         }
 
         SplitConstructorSet { present, missing, missing_empty }
     }
+
+    /// Whether this set only contains empty constructors.
+    pub(crate) fn all_empty(&self) -> bool {
+        match self {
+            ConstructorSet::Bool
+            | ConstructorSet::Integers { .. }
+            | ConstructorSet::Ref
+            | ConstructorSet::Union
+            | ConstructorSet::Unlistable => false,
+            ConstructorSet::NoConstructors => true,
+            ConstructorSet::Struct { empty } => *empty,
+            ConstructorSet::Variants { variants, non_exhaustive } => {
+                !*non_exhaustive
+                    && variants
+                        .iter()
+                        .all(|visibility| matches!(visibility, VariantVisibility::Empty))
+            }
+            ConstructorSet::Slice { array_len, subtype_is_empty } => {
+                *subtype_is_empty && matches!(array_len, Some(1..))
+            }
+        }
+    }
 }