Linking to `str::replacen` from `alloc` crate docstring creates a broken link fragment

[WilliamVenner]

Hi, in my PR #97977 I was unable to link to str::replacen from the alloc crate in a docstring:

rust/library/alloc/src/string.rs

Line 1747 in eb1fa3c

/// [`str::replacen(..., 1)`]: str::replacen

Rustbook (x86_64-unknown-linux-gnu) - embedded-book
Rustbook (x86_64-unknown-linux-gnu) - edition-guide
Building stage0 tool linkchecker (x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 0.17s
alloc/string/struct.String.html:842: broken link fragment `#method.replacen` pointing to `core/primitive.str.html`
number of HTML files scanned: 32333
number of HTML redirects found: 10095
number of links checked: 2261468
number of links ignored due to external: 112628

[WilliamVenner]

@rustbot label T-rustdoc

[zachs18]

I don't think this is necessarily a rustdoc issue. Currently, the stdlib only has docs for primitive types in core and std, meaning that inherent methods on primitives that are defined in alloc (like str::replacen) are not documented in alloc (so they are not documented anywhere when building only alloc's docs).

One solution would be to also document (some) primitives in alloc, which would give a place for methods like str::replacen, str::into_string, str::into_boxed_bytes, slice::to_vec, slice::into_vec, etc to be documented without std.

Another "solution" (for the linked PR) is to hardcode the link to go to ../../std/primitive.str.html#method.replacen, so that the link will only be broken when alloc's docs are built without std (This is what I used in my rebased branch)

Another ""solution"" is to just ignore the "broken" link in alloc's docs, since the link is specifically only broken in alloc/string/struct.String.html, not std/string/struct.String.html.