Rollup merge of #122503 - compiler-errors:trait-alias-wf, r=Nilstrieb

Make `SubdiagMessageOp` well-formed

`WF(Diag<'_, G>)` requires `G: EmissionGuarantee`, but we don't currently check this is true due to limitations in the solver. Probably still worth enforcing.

r? `@nnethercote` (or anyone can r+ this, really)

Author: matthiaskrgr