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.

3 files changed, 11 insertions, 0 deletions

diff --git a/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs b/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs
index 8ddc6c924e2..31114190f07 100644
--- a/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs
+++ b/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();
diff --git a/compiler/rustc_mir_build/src/thir/pattern/mod.rs b/compiler/rustc_mir_build/src/thir/pattern/mod.rs
index 8b2a96cff41..5edb2054fd4 100644
--- a/compiler/rustc_mir_build/src/thir/pattern/mod.rs
+++ b/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) => {
diff --git a/compiler/rustc_mir_build/src/thir/print.rs b/compiler/rustc_mir_build/src/thir/print.rs
index e449928a643..547da7e7007 100644
--- a/compiler/rustc_mir_build/src/thir/print.rs
+++ b/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);