rustdoc: accept attribute@ disambiguator for attribute macros

[sankalpsthakur]

Summary

Rustdoc already suggested attribute@ when an intra-doc link resolved to an attribute macro with the wrong kind (for example example!), but Disambiguator::from_str did not recognize attribute, so applying the suggestion produced unknown disambiguator \attribute``.

This change:

• accepts attribute@ as a kind disambiguator for MacroKinds::ATTR (symmetric with existing derive@)
• documents the prefix in the rustdoc book disambiguator list
• adds a rustdoc-ui regression test for Invalid broken link lint suggestion when referencing disambiguous attribute macro #159771

Validation

• Local full bootstrap was not run here (sparse/partial checkout; ~31 GiB free disk; no stage1 rustdoc build available).
• Change is intentionally small (parser + docs + ui test). Please rely on CI ./x.py test tests/rustdoc-ui --stage 1 (or the relevant rustdoc jobs).

Fixes #159771

AI/LLM disclosure

• AI coding tools (including Grok and/or Codex agent-assisted editing) were used to help draft or modify code and this PR description.
• I reviewed the complete change, understand the reasoning, and prepared the reported local validation notes before submitting.
• This submission is original work of authorship under the project CLA / contributor terms; AI output was not pasted unreviewed.

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[oli-obk]

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[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

To only update this specific test, also pass `--test-args intra-doc/issue-159771-attribute-disambiguator.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustdoc" "/checkout/tests/rustdoc-ui/intra-doc/issue-159771-attribute-disambiguator.rs" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-o" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/rustdoc-ui/intra-doc/issue-159771-attribute-disambiguator" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Cdebuginfo=0" "--edition=2024"
stdout: none
--- stderr -------------------------------
error: incompatible link kind for `example`
##[error]  --> /checkout/tests/rustdoc-ui/intra-doc/issue-159771-attribute-disambiguator.rs:17:32
   |
LL | /// Wrong bang disambiguator: [example!]
   |                                ^^^^^^^^ this link resolved to an attribute macro, which is not a macro
   |
note: the lint level is defined here
  --> /checkout/tests/rustdoc-ui/intra-doc/issue-159771-attribute-disambiguator.rs:3:9
   |
LL | #![deny(rustdoc::broken_intra_doc_links)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: to link to the attribute macro, prefix with `attribute@`
   |
LL - /// Wrong bang disambiguator: [example!]
LL + /// Wrong bang disambiguator: [attribute@example]
   |

error: aborting due to 1 previous error
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