Regression test for issue rust-lang#58158.

src/test/ui/wf/wf-packed-on-proj-of-type-as-unimpl-trait.rs

@@ -0,0 +1,31 @@
+// rust-lang/rust#58158: We have special-case code to deal with case
+// when a type is both packed and needs drop glue, (we move the fields
+// out of their potentially unaligned locations before dropping them,
+// which requires they be Sized; see PR #44884).
+//
+// So, we need to check if a given type needs drop-glue. That requires
+// that we actually know that the concrete type, and we guard against
+// the type having unknown parts (i.e. type variables) by ICE'ing in
+// that scenario.
+//
+// But in a case where we have a projection (`Type as Trait::Assoc`)
+// where `Type` does not actually implement `Trait`, we of course
+// cannot have a concrete type, because there is no impl to look up
+// the concrete type for the associated type `Assoc`.
+//
+// So, this test is just making sure that in such a case that we do
+// not immediately ICE, and instead allow the underlying type error to
+// surface.
+
+pub struct Matrix<S>(S);
+pub struct DefaultAllocator;
+
+pub trait Allocator { type Buffer; }
+
+// impl Allocator for DefaultAllocator { type Buffer = (); }
+
+#[repr(packed)]
+struct Foo(Matrix<<DefaultAllocator as Allocator>::Buffer>);
+//~^ ERROR the trait bound `DefaultAllocator: Allocator` is not satisfied
+
+fn main() { }

src/test/ui/wf/wf-packed-on-proj-of-type-as-unimpl-trait.stderr

@@ -0,0 +1,9 @@
+error[E0277]: the trait bound `DefaultAllocator: Allocator` is not satisfied
+  --> $DIR/wf-packed-on-proj-of-type-as-unimpl-trait.rs:28:12
+   |
+LL | struct Foo(Matrix<<DefaultAllocator as Allocator>::Buffer>);
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Allocator` is not implemented for `DefaultAllocator`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.