rustdoc does not detect broken markdown due to missing links (except for links without a space)

I changed [allocated object] to [allocation] in docs, and strangely that triggered an error. Turns out the docs had a broken link, which you can see live at https://doc.rust-lang.org/1.87.0/std/primitive.pointer.html#method.split_at_mut_unchecked (note the "[allocated object]") -- but rustdoc somehow wouldn't complain unless the link text has no spaces? Something very strange is going on here.

See https://github.com/rust-lang/rust/pull/141224 for context. The error emitted after the change is below:

  error: unresolved link to `allocation`
      --> library/core/src/ptr/mut_ptr.rs:1851:54
       |
  1851 |     /// `mid` must be [in-bounds] of the underlying [allocation].
       |                                                      ^^^^^^^^^^ no item named `allocation` in scope
       |
       = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
       = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
       = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`

Possibly related: https://github.com/rust-lang/rust/issues/130126

Start with the broken link example in library/core/src/ptr/mut_ptr.rs:1851 and compare rustdoc behavior for link text with and without spaces. Read PR 141224 for context, then review related issue 130126. Done means the reported broken-link case is consistently diagnosed with regression coverage.