Rollup merge of #53541 - GuillaumeGomez:fix-impl-trait-ret-type, r=oli-obk

Fix missing impl trait display as ret type

I need to convert a `TraitPredicate` into a `TraitBound` to get the returned impl trait. So far, didn't find how or even if it was the good way to do it.

cc @eddyb @oli-obk (since you're the one behind the change apparently 😉)

diff --cc src/librustdoc/clean/mod.rs
Simple merge