Rollup merge of #99581 - nnethercote:improve-derive-packed-errors, r=estebank

Improve error messages involving `derive` and `packed`.

There are two errors involving `derive` and `packed`.

```
`#[derive]` can't be derived on a `#[repr(packed)]` struct with type or const parameters
`#[derive]` can't be derived on a `#[repr(packed)]` struct that does not derive Copy
```
The second one overstates things. It is possible to use derive on a
repr(packed) struct that doesn't derive Copy in two cases.
- If all the fields within the struct meet the required alignment: 1 for
  `repr(packed)`, or `N` for `repr(packed(N))`.
- If `Default` is the only trait derived.

This commit improves things in a few ways.
- Changes the errors to say `this trait can't be derived on this ...`.
  This is more accurate, because it's just *this* trait and *this*
  packed struct that are a problem, not *all* derived traits on *all*
  packed structs.
- Adds more details to the "ERROR" lines in the test case, enough to
  distinguish between the two error messages.
- Adds more cases to the test case that don't cause errors, e.g. `Default`
  derives.
- Uses a wider variety of builtin traits in the test case, for better coverage.

r? `@estebank`

1 files changed, 8 insertions, 5 deletions

diff --git a/compiler/rustc_mir_transform/src/check_packed_ref.rs b/compiler/rustc_mir_transform/src/check_packed_ref.rs
index 2eb38941f1a..c21c1efe991 100644
--- a/compiler/rustc_mir_transform/src/check_packed_ref.rs
+++ b/compiler/rustc_mir_transform/src/check_packed_ref.rs
@@ -36,13 +36,16 @@ fn unsafe_derive_on_repr_packed(tcx: TyCtxt<'_>, def_id: LocalDefId) {
     tcx.struct_span_lint_hir(UNALIGNED_REFERENCES, lint_hir_id, tcx.def_span(def_id), |lint| {
         // FIXME: when we make this a hard error, this should have its
         // own error code.
-        let message = if tcx.generics_of(def_id).own_requires_monomorphization() {
-            "`#[derive]` can't be used on a `#[repr(packed)]` struct with \
-             type or const parameters (error E0133)"
+        let extra = if tcx.generics_of(def_id).own_requires_monomorphization() {
+            "with type or const parameters"
         } else {
-            "`#[derive]` can't be used on a `#[repr(packed)]` struct that \
-             does not derive Copy (error E0133)"
+            "that does not derive `Copy`"
         };
+        let message = format!(
+            "`{}` can't be derived on this `#[repr(packed)]` struct {} (error E0133)",
+            tcx.item_name(tcx.trait_id_of_impl(def_id.to_def_id()).expect("derived trait name")),
+            extra
+        );
         lint.build(message).emit();
     });
 }