Lower never patterns to Unreachable in mir

compiler/rustc_middle/src/thir.rs

@@ -682,6 +682,23 @@ impl<'tcx> Pat<'tcx> {
             true
         })
     }
+
+    /// Whether this a never pattern.
+    pub fn is_never_pattern(&self) -> bool {
+        let mut is_never_pattern = false;
+        self.walk(|pat| match &pat.kind {
+            PatKind::Never => {
+                is_never_pattern = true;
+                false
+            }
+            PatKind::Or { pats } => {
+                is_never_pattern = pats.iter().all(|p| p.is_never_pattern());
+                false
+            }
+            _ => true,
+        });
+        is_never_pattern
+    }
 }
 
 impl<'tcx> IntoDiagArg for Pat<'tcx> {