Add Constructor::Never

Author: Nadrieril

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -678,15 +678,19 @@ pub enum Constructor<Cx: TypeCx> {
     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: TypeCx> 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,

compiler/rustc_pattern_analysis/src/pat.rs

@@ -189,6 +189,7 @@ impl<Cx: TypeCx> fmt::Debug for DeconstructedPat<Cx> {
                 }
                 Ok(())
             }
+            Never => write!(f, "!"),
             Wildcard | Missing | NonExhaustive | Hidden | PrivateUninhabited => {
                 write!(f, "_ : {:?}", pat.ty())
             }

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -251,7 +251,7 @@ impl<'p, 'tcx: 'p> RustcMatchCheckCtxt<'p, 'tcx> {
                 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty),
             },
             Bool(..) | IntRange(..) | F32Range(..) | F64Range(..) | Str(..) | Opaque(..)
-            | NonExhaustive | Hidden | Missing | PrivateUninhabited | Wildcard => &[],
+            | Never | NonExhaustive | Hidden | Missing | PrivateUninhabited | Wildcard => &[],
             Or => {
                 bug!("called `Fields::wildcards` on an `Or` ctor")
             }
@@ -279,7 +279,7 @@ impl<'p, 'tcx: 'p> RustcMatchCheckCtxt<'p, 'tcx> {
             Ref => 1,
             Slice(slice) => slice.arity(),
             Bool(..) | IntRange(..) | F32Range(..) | F64Range(..) | Str(..) | Opaque(..)
-            | NonExhaustive | Hidden | Missing | PrivateUninhabited | Wildcard => 0,
+            | Never | NonExhaustive | Hidden | Missing | PrivateUninhabited | Wildcard => 0,
             Or => bug!("The `Or` constructor doesn't have a fixed arity"),
         }
     }
@@ -807,7 +807,8 @@ impl<'p, 'tcx: 'p> RustcMatchCheckCtxt<'p, 'tcx> {
                 }
             }
             &Str(value) => PatKind::Constant { value },
-            Wildcard | NonExhaustive | Hidden | PrivateUninhabited => PatKind::Wild,
+            Never if self.tcx.features().never_patterns => PatKind::Never,
+            Never | Wildcard | NonExhaustive | Hidden | PrivateUninhabited => PatKind::Wild,
             Missing { .. } => bug!(
                 "trying to convert a `Missing` constructor into a `Pat`; this is probably a bug,
                 `Missing` should have been processed in `apply_constructors`"