Rollup merge of #138238 - compiler-errors:dyn-suggestion-in-struct, r=nnethercote

Fix dyn -> param suggestion in struct ICEs

Makes the logic from #138042 a bit less ICEy and more clean. Also fixes an incorrect suggestion when the struct already has generics. I'll point out the major changes and observations in the code.

Fixes #138229
Fixes #138211

r? nnethercote since you reviewed the original pr, or re-roll if you don't want to review this

Author: matthiaskrgr