librustc_mir: Fix ICE with slice patterns

If a match arm does not include all fields in a structure and a later
pattern includes a field that is an array, we will attempt to use the
array type from the prior arm. When calculating the field type, treat
a array of an unknown size as a TyErr.

src/librustc_mir/hair/pattern/_match.rs

@@ -1372,7 +1372,14 @@ fn constructor_sub_pattern_tys<'a, 'tcx: 'a>(cx: &MatchCheckCtxt<'a, 'tcx>,
                     let is_visible = adt.is_enum()
                         || field.vis.is_accessible_from(cx.module, cx.tcx);
                     if is_visible {
-                        field.ty(cx.tcx, substs)
+                        let ty = field.ty(cx.tcx, substs);
+                        match ty.sty {
+                            // If the field type returned is an array of an unknown
+                            // size return an TyErr.
+                            ty::Array(_, len) if len.assert_usize(cx.tcx).is_none() =>
+                                cx.tcx.types.err,
+                            _ => ty,
+                        }
                     } else {
                         // Treat all non-visible fields as TyErr. They
                         // can't appear in any other pattern from

src/test/ui/issues/issue-57472.rs

@@ -0,0 +1,35 @@
+#![crate_type="lib"]
+#![deny(unreachable_patterns)]
+
+mod test_struct {
+    // Test the exact copy of the minimal example
+    // posted in the issue.
+    pub struct Punned {
+        foo: [u8; 1],
+        bar: [u8; 1],
+    }
+
+    pub fn test(punned: Punned) {
+        match punned {
+            Punned { foo: [_], .. } => println!("foo"),
+            Punned { bar: [_], .. } => println!("bar"),
+            //~^ ERROR unreachable pattern [unreachable_patterns]
+        }
+    }
+}
+
+mod test_union {
+    // Test the same thing using a union.
+    pub union Punned {
+        foo: [u8; 1],
+        bar: [u8; 1],
+    }
+
+    pub fn test(punned: Punned) {
+        match punned {
+            Punned { foo: [_] } => println!("foo"),
+            Punned { bar: [_] } => println!("bar"),
+            //~^ ERROR unreachable pattern [unreachable_patterns]
+        }
+    }
+}

src/test/ui/issues/issue-57472.stderr

@@ -0,0 +1,20 @@
+error: unreachable pattern
+  --> $DIR/issue-57472.rs:15:13
+   |
+LL |             Punned { bar: [_], .. } => println!("bar"),
+   |             ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/issue-57472.rs:2:9
+   |
+LL | #![deny(unreachable_patterns)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: unreachable pattern
+  --> $DIR/issue-57472.rs:31:13
+   |
+LL |             Punned { bar: [_] } => println!("bar"),
+   |             ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+