rustdoc: fix cross-crate impl Sized & impl ?Sized

[fmease]

Previously, cross-crate impl-Trait (APIT, RPIT, etc.) that only consists of a single Sized bound (modulo outlives-bounds) and ones that are ?Sized were incorrectly rendered. To give you a taste (before vs. after):

- fn sized(x: impl ) -> impl 
+ fn sized(x: impl Sized) -> impl Sized

- fn sized_outlives<'a>(x: impl 'a) -> impl 'a
+ fn sized_outlives<'a>(x: impl Sized + 'a) -> impl Sized + 'a

- fn maybe_sized(x: &impl ) -> &impl 
+ fn maybe_sized(x: &impl ?Sized) -> &impl ?Sized

- fn debug_maybe_sized(x: &impl Debug) -> &impl ?Sized + Debug
+ fn debug_maybe_sized(x: &(impl Debug + ?Sized)) -> &(impl Debug + ?Sized)

Moreover, we now surround impl-Trait that has multiple bounds with parentheses if they're the pointee of a reference or raw pointer type. This affects both local and cross-crate docs. The current output isn't correct (rustc would emit the error ambiguous + in a type if we fed the rendered code back to it).

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Best reviewed commit by commit :)

@rustbot label A-cross-crate-reexports

[rustbot]

r? @jsha

(rustbot has picked a reviewer for you, use r? to override)

[rustbot]

Some changes occurred in src/librustdoc/clean/types.rs

cc @camelid

[fmease]

This is just a refactor. I found the map followed by and_then, try-operator and if-let quite hairy.

[fmease]

This further enlarges the preexisting bound cleaning duplication of clean_ty_generics and clean_middle_opaque_bounds. Let me know if this is okay for the time being, I plan on consolidating these implementations when I overhaul bound cleaning next to fix #113015.

[fmease]

Looking at this, I don't think it handles ~const Sized (TraitBoundModifier::MaybeConst) correctly.
I pretty sure it results in has_sized == false making us add + ?Sized to it.
Let me know if I should fix it here or in a follow-up (I guess there might be some more misuses of is_sized_bound).

Edit: impl ~const Sized is actually not allowed (at the moment?) since Sized is not a #[const_trait].

[GuillaumeGomez]

That answers the question then. ;)

[GuillaumeGomez]

Thanks!

@bors r+ rollup

[bors]

📌 Commit 28d40f1 has been approved by GuillaumeGomez

It is now in the queue for this repository.