Make "tuple pattern not applied to a tuple" a span_delayed_bug.

Fixes rust-lang#121410.

compiler/rustc_hir_typeck/src/mem_categorization.rs

@@ -582,7 +582,8 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
         match ty.kind() {
             ty::Tuple(args) => Ok(args.len()),
             _ => {
-                self.tcx().dcx().span_bug(span, "tuple pattern not applied to a tuple");
+                self.tcx().dcx().span_delayed_bug(span, "tuple pattern not applied to a tuple");
+                Err(())
             }
         }
     }

tests/ui/typeck/issue-121410.rs

@@ -0,0 +1,15 @@
+fn test_missing_unsafe_warning_on_repr_packed() {
+    struct Foo {
+        x: String,
+    }
+
+    let foo = Foo { x: String::new() };
+
+    let c = || {
+        let (_, t2) = foo.x; //~ ERROR mismatched types
+    };
+
+    c();
+}
+
+fn main() {}

tests/ui/typeck/issue-121410.stderr

@@ -0,0 +1,14 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-121410.rs:9:13
+   |
+LL |         let (_, t2) = foo.x;
+   |             ^^^^^^^   ----- this expression has type `String`
+   |             |
+   |             expected `String`, found `(_, _)`
+   |
+   = note: expected struct `String`
+               found tuple `(_, _)`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.