Rollup merge of #103885 - fmease:rustdoc-various-cross-crate-reexport-fixes, r=cjgillot,GuillaumeGomez

rustdoc: various cross-crate reexport fixes

Fixes for various smaller cross-crate reexport issues.
The PR is split into several commits for easier review. Will be squashed after approval.

Most notable changes:

* We finally render late-bound lifetimes in the generic parameter list of cross-crate functions & methods.
  Previously, we would display the re-export of `pub fn f<'s>(x: &'s str) {}` as `pub fn f(x: &'s str)`
* We now render unnamed parameters of cross-crate functions and function pointers as underscores
  since that's exactly what we do for local definitions, too. Mentioned as a bug in #44306.
* From now on, the rendering of cross-crate trait-object types is more correct:
  * `for<>` parameter lists (for higher-ranked lifetimes) are now shown
  * the return type of `Fn{,Mut,Once}` trait bounds is now displayed

Regarding the last list item, here is a diff for visualization (before vs. after):

```patch
- dyn FnOnce(&'any str) + 'static
+ dyn for<'any> FnOnce(&'any str) -> bool + 'static
```

The redundant `+ 'static` will be removed in a follow-up PR that will hide trait-object lifetime-bounds if they coincide with [their default](https://doc.rust-lang.org/reference/lifetime-elision.html#default-trait-object-lifetimes) (see [Zulip discussion](https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/clean_middle_ty.3A.20I.20need.20to.20add.20a.20parameter/near/307143097)). `FIXME(fmease)`s were added.

``@rustbot`` label A-cross-crate-reexports
r? ``@GuillaumeGomez``

1 files changed, 11 insertions, 2 deletions

diff --git a/src/librustdoc/clean/inline.rs b/src/librustdoc/clean/inline.rs
index 841c4f9d530..8a5463c10f2 100644
--- a/src/librustdoc/clean/inline.rs
+++ b/src/librustdoc/clean/inline.rs
@@ -243,10 +243,19 @@ pub(crate) fn build_external_trait(cx: &mut DocContext<'_>, did: DefId) -> clean
 fn build_external_function<'tcx>(cx: &mut DocContext<'tcx>, did: DefId) -> Box<clean::Function> {
     let sig = cx.tcx.fn_sig(did);
 
-    let predicates = cx.tcx.predicates_of(did);
+    let late_bound_regions = sig.bound_vars().into_iter().filter_map(|var| match var {
+        ty::BoundVariableKind::Region(ty::BrNamed(_, name)) if name != kw::UnderscoreLifetime => {
+            Some(clean::GenericParamDef::lifetime(name))
+        }
+        _ => None,
+    });
+
+    let predicates = cx.tcx.explicit_predicates_of(did);
     let (generics, decl) = clean::enter_impl_trait(cx, |cx| {
         // NOTE: generics need to be cleaned before the decl!
-        let generics = clean_ty_generics(cx, cx.tcx.generics_of(did), predicates);
+        let mut generics = clean_ty_generics(cx, cx.tcx.generics_of(did), predicates);
+        // FIXME: This does not place parameters in source order (late-bound ones come last)
+        generics.params.extend(late_bound_regions);
         let decl = clean_fn_decl_from_did_and_sig(cx, Some(did), sig);
         (generics, decl)
     });