Stabilize doc_cfg feature

[GuillaumeGomez]

Fixes #43781.

The feature has been around for some time and I think it's now ok to stabilize it.

cc @rust-lang/rustdoc

[rustbot]

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

[rustbot]

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[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

##[endgroup]
[TIMING:end] tool::ToolBuild { build_compiler: Compiler { stage: 0, host: aarch64-unknown-linux-gnu, forced_compiler: false }, target: aarch64-unknown-linux-gnu, tool: "linkchecker", path: "src/tools/linkchecker", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "", cargo_args: [], artifact_kind: Binary } -- 0.150
[TIMING:end] tool::Linkchecker { compiler: Compiler { stage: 0, host: aarch64-unknown-linux-gnu, forced_compiler: false }, target: aarch64-unknown-linux-gnu } -- 0.000
##[group]Testing stage1 Linkcheck (aarch64-unknown-linux-gnu)
cargo/print.html:4456: broken link - `unstable-book/language-features/doc-cfg.html`
cargo/reference/features.html:564: broken link - `unstable-book/language-features/doc-cfg.html`
checked links in: 22.9s
number of HTML files scanned: 46418
number of HTML redirects found: 14548
number of links checked: 2562616
number of links ignored due to external: 123187
number of links ignored due to exceptions: 24
number of intra doc links ignored: 9
errors found: 2
found some broken links
NOTE: if you are adding or renaming a markdown file in a mdBook, don't forget to register the page in SUMMARY.md
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/linkchecker /checkout/obj/build/aarch64-unknown-linux-gnu/doc` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:190:77

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:37:51
  local time: Tue Dec 16 14:52:36 UTC 2025
  network time: Tue, 16 Dec 2025 14:52:36 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[GuillaumeGomez]

That's extremely annoying that we cannot update cargo files directly in here. Sending a PR to their repository then...

EDIT: cargo PR is here: rust-lang/cargo#16404

[fmease]

There are still 14 open issues (w/o the tracking issue) (via). Some of these may be blocking (in the sense that it would be much easier to address them now given the feature isn't stable yet than later when we're a lot more restricted in what we can do).

For example, I'm relatively certain that #147033 should be taken as blocking unless it's a breaking change anyway, need to read up on it again.

[GuillaumeGomez]

Oof, yeah indeed. Can you list them all in the tracking issue then please? Definitely sounds like blockers. Marking this PR as blocked in the meantime.

[fmease]

Moreover, the tracking issue lists two medium-large unresolved questions. I'm not expecting a full stabilization report but it would be nice to list & consider everything that blocks the stabilization (that's the whole purpose of that list, isn't it?) in the PR description plus other concerns & risks. ;'D

[bors]

☔ The latest upstream changes (presumably #149831) made this pull request unmergeable. Please resolve the merge conflicts.