rustdoc: some trait implementors don't have src link

[cynecx]

Live: https://doc.rust-lang.org/nightly/std/iter/trait.IntoIterator.html

Some trait implementors don't have src link and anchors:

rustdoc 1.55.0-nightly (7c3872e6b 2021-06-24)

[jyn514]

This is also present on stable:

[GuillaumeGomez]

The ones who don't have the [src] link are generated by the JS because they come from external crates. Also, the display is broken haha.

[jsha]

Related: #85418, #25061

[fmease]

Interestingly, rustdoc does render a source link for e.g. the IntoIterator impl for BinaryHeap on the std::iter::IntoIterator page but not on the core::iter::IntoIterator page. BinaryHeap is defined in alloc and thus this is some kind of cross-crate re-export issue, albeit a bit weird (why is only the core re-export problematic and not the std one?).

@rustbot label A-cross-crate-reexports