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_passes/src/hir_stats.rs

@@ -286,7 +286,21 @@ impl<'v> hir_visit::Visitor<'v> for StatCollector<'v> {
     fn visit_pat(&mut self, p: &'v hir::Pat<'v>) {
         record_variants!(
             (self, p, p.kind, Id::Node(p.hir_id), hir, Pat, PatKind),
-            [Wild, Binding, Struct, TupleStruct, Or, Path, Tuple, Box, Ref, Lit, Range, Slice]
+            [
+                Wild,
+                Binding,
+                Struct,
+                TupleStruct,
+                Or,
+                Never,
+                Path,
+                Tuple,
+                Box,
+                Ref,
+                Lit,
+                Range,
+                Slice
+            ]
         );
         hir_visit::walk_pat(self, p)
     }
@@ -554,6 +568,7 @@ impl<'v> ast_visit::Visitor<'v> for StatCollector<'v> {
                 Range,
                 Slice,
                 Rest,
+                Never,
                 Paren,
                 MacCall
             ]