rustdoc: In specific circumstances, links to `core` items in item signatures are now broken (beta → nightly regression)

[fmease]

Noticed while browsing r-l/r's API docs:

On 1.94.0-beta.4 (172dc09 2026-02-18), type Bound in function rustc_parse::Parser::parse_expr_assoc_with (beta) correctly links to https://doc.rust-lang.org/beta/core/ops/range/enum.Bound.html. However, on 1.95.0-nightly
(7f99507 2026-02-19), type Bound in function rustc_parse::Parser::parse_expr_assoc_with (nightly) links to (BROKEN) https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/core/ops/range/enum.Bound.html (notice how the URL doesn't just contain the target crate name core but also the source crate name rustc_parse before that; some path manipulations seem to go awry here).

I couldn't minimize it so far or find similar regressions. I couldn't find similar reports. I don't know if it only concerns links to core / sysroot crates.

[eggyal]

In the successful case, the hyperlink is absolute href="https://doc.rust-lang.org/beta/core/ops/range/enum.Bound.html". In the failing case, it is relative href="../core/ops/range/enum.Bound.html".

[eggyal]

Pretty sure the regression is from #152243 cc @arferreira @GuillaumeGomez

[eggyal]

RUSTDOCFLAGS now includes --extern-html-root-url core=../, but if (as here) that root url is a relative path then obviously it needs adjusting to work from the current location/depth; whereas I suspect it's just being used directly as if it were absolute, with no adjustment.