Rollup merge of #89441 - Nadrieril:fix-89393, r=tmandry

Rollup merge of #89441 - Nadrieril:fix-89393, r=tmandry

Normalize after substituting via `field.ty()`

Back in https://github.com/rust-lang/rust/issues/72476 I hadn't understood where the problem was coming from, and only worked around the issue. What happens is that calling `field.ty()` on a field of a generic struct substitutes the appropriate generics but doesn't normalize the resulting type.
As a consumer of types I'm surprised that one would substitute without normalizing, feels like a footgun, so I added a comment.

Fixes https://github.com/rust-lang/rust/issues/89393.