Auto merge of #92601 - camelid:more-intra-doc-cleanup, r=Manishearth

rustdoc: Remove the intra-doc links side channel

The side channel made the code much more complex and harder to
understand. It was added as a temporary workaround in
0c99d80, but it's no longer necessary.

The addition of `UrlFragment` in #92088 was the key to getting rid of
the side channel. The semantic information (rather than the strings that
used to be used for fragments) that is now captured by `UrlFragment` is
enough to obviate the side channel. An additional change had to be made
to `UrlFragment` in this PR to make this possible: it now records
`DefId`s rather than item names.

This PR also consolidates the checks for anchor conflicts into one place.

r? `@Manishearth`

Author: bors

src/librustdoc/clean/types.rs

@@ -1,6 +1,5 @@
 use std::cell::RefCell;
 use std::default::Default;
-use std::fmt::Write;
 use std::hash::Hash;
 use std::lazy::SyncOnceCell as OnceCell;
 use std::path::PathBuf;
@@ -496,7 +495,7 @@ impl Item {
                 if let Ok((mut href, ..)) = href(*did, cx) {
                     debug!(?href);
                     if let Some(ref fragment) = *fragment {
-                        write!(href, "{}", fragment).unwrap()
+                        fragment.render(&mut href, cx.tcx()).unwrap()
                     }
                     Some(RenderedLink {
                         original_text: s.clone(),