Rollup merge of #118157 - Nadrieril:never_pat-feature-gate, r=compiler-errors

Add `never_patterns` feature gate

This PR adds the feature gate and most basic parsing for the experimental `never_patterns` feature. See the tracking issue (https://github.com/rust-lang/rust/issues/118155) for details on the experiment.

`@scottmcm` has agreed to be my lang-team liaison for this experiment.

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -827,6 +827,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             PatKind::Constant { .. }
             | PatKind::Range { .. }
             | PatKind::Wild
+            | PatKind::Never
             | PatKind::Error(_) => {}
 
             PatKind::Deref { ref subpattern } => {

compiler/rustc_mir_build/src/build/matches/simplify.rs

@@ -194,6 +194,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 Ok(())
             }
 
+            PatKind::Never => {
+                // A never pattern acts like a load from the place.
+                // FIXME(never_patterns): load from the place
+                Ok(())
+            }
+
             PatKind::Constant { .. } => {
                 // FIXME normalize patterns when possible
                 Err(match_pair)

compiler/rustc_mir_build/src/build/matches/test.rs

@@ -75,6 +75,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | PatKind::Array { .. }
             | PatKind::Wild
             | PatKind::Binding { .. }
+            | PatKind::Never
             | PatKind::Leaf { .. }
             | PatKind::Deref { .. }
             | PatKind::Error(_) => self.error_simplifiable(match_pair),
@@ -107,6 +108,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             PatKind::Slice { .. }
             | PatKind::Array { .. }
             | PatKind::Wild
+            | PatKind::Never
             | PatKind::Or { .. }
             | PatKind::Binding { .. }
             | PatKind::AscribeUserType { .. }

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -247,8 +247,9 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                     self.requires_unsafe(pat.span, AccessToUnionField);
                     return; // we can return here since this already requires unsafe
                 }
-                // wildcard doesn't take anything
+                // wildcard/never don't take anything
                 PatKind::Wild |
+                PatKind::Never |
                 // these just wrap other patterns
                 PatKind::Or { .. } |
                 PatKind::InlineConstant { .. } |

compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs

@@ -1557,6 +1557,12 @@ impl<'p, 'tcx> DeconstructedPat<'p, 'tcx> {
                 let pats = expand_or_pat(pat);
                 fields = Fields::from_iter(cx, pats.into_iter().map(mkpat));
             }
+            PatKind::Never => {
+                // FIXME(never_patterns): handle `!` in exhaustiveness. This is a sane default
+                // in the meantime.
+                ctor = Wildcard;
+                fields = Fields::empty();
+            }
             PatKind::Error(_) => {
                 ctor = Opaque(OpaqueId::new());
                 fields = Fields::empty();

compiler/rustc_mir_build/src/thir/pattern/mod.rs

@@ -251,6 +251,8 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
         let kind = match pat.kind {
             hir::PatKind::Wild => PatKind::Wild,
 
+            hir::PatKind::Never => PatKind::Never,
+
             hir::PatKind::Lit(value) => self.lower_lit(value),
 
             hir::PatKind::Range(ref lo_expr, ref hi_expr, end) => {

compiler/rustc_mir_build/src/thir/print.rs

@@ -642,6 +642,9 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
             PatKind::Wild => {
                 print_indented!(self, "Wild", depth_lvl + 1);
             }
+            PatKind::Never => {
+                print_indented!(self, "Never", depth_lvl + 1);
+            }
             PatKind::AscribeUserType { ascription, subpattern } => {
                 print_indented!(self, "AscribeUserType: {", depth_lvl + 1);
                 print_indented!(self, format!("ascription: {:?}", ascription), depth_lvl + 2);