bootstrap: Enable rustdoc mergeable CCI for std docs

[camelid]

Ideally we'd enable it for the compiler docs too, though that's blocked on fixing some hacks in bootstrap related to shared build directories.

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

[rustbot]

r? @Mark-Simulacrum

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The reviewer was selected based on:

• Owners of files modified in this PR: bootstrap
• bootstrap expanded to 6 candidates
• Random selection from Mark-Simulacrum, clubby789

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[camelid]

r? @Kobzol
cc #160098

[rustbot]

Kobzol is not on the review rotation at the moment.
They may take a while to respond.

[camelid]

It would be good if we could do some kind of manual testing of the generated docs to make sure everything looks OK. Can that be done with artifacts from a try job?

[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)

##[endgroup]
Doc path: /checkout/obj/build/aarch64-unknown-linux-gnu/doc/alloc/index.html
[TIMING:end] doc::Std { build_compiler: Compiler { stage: 2, host: aarch64-unknown-linux-gnu, forced_compiler: false }, target: aarch64-unknown-linux-gnu, format: Html, crates: [] } -- 58.352
##[group]Testing stage2 rustdoc-js-std (aarch64-unknown-linux-gnu)
Error: ENOENT: no such file or directory, open '/checkout/obj/build/aarch64-unknown-linux-gnu/doc/search.index/root1.99.0.js'
    at Object.readFileSync (node:fs:449:20)
    at readFile (/checkout/src/tools/rustdoc-js/tester.js:30:15)
    at Object.loadRoot (/checkout/src/tools/rustdoc-js/tester.js:468:32)
    at /checkout/obj/build/aarch64-unknown-linux-gnu/doc/static.files/stringdex-b5109af9.js:2:24000
    at new Promise (<anonymous>)
    at Object.loadDatabase (/checkout/obj/build/aarch64-unknown-linux-gnu/doc/static.files/stringdex-b5109af9.js:2:23870)
    at Object.initSearch (/checkout/obj/build/aarch64-unknown-linux-gnu/doc/static.files/search-cf4fc35e.js:5:12018)
    at loadSearchJS (/checkout/src/tools/rustdoc-js/tester.js:458:57)
    at main (/checkout/src/tools/rustdoc-js/tester.js:628:34)
    at Object.<anonymous> (/checkout/src/tools/rustdoc-js/tester.js:656:1) {
  errno: -2,
  code: 'ENOENT',
  syscall: 'open',
  path: '/checkout/obj/build/aarch64-unknown-linux-gnu/doc/search.index/root1.99.0.js'
}
Command `/usr/bin/node /checkout/src/tools/rustdoc-js/tester.js --crate-name std --resource-suffix 1.99.0 --doc-folder /checkout/obj/build/aarch64-unknown-linux-gnu/doc --test-folder /checkout/tests/rustdoc-js-std` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/test.rs:1341:27
Executed at: src/bootstrap/src/core/build_steps/test.rs:1382:17

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip tidy --skip intrinsic-test --skip compiler --skip src`
Currently active steps:
test::RustdocJSStd { build_compiler: Compiler { stage: 2, host: aarch64-unknown-linux-gnu, forced_compiler: false }, target: aarch64-unknown-linux-gnu } at src/bootstrap/src/core/build_steps/test.rs:1332
Build completed unsuccessfully in 0:44:00
  local time: Fri Aug  7 23:01:56 UTC 2026

[notriddle]

I've found two problems,

• one of them is specific to compiler-docs, because it's related to the RustcDocs mode
• the other one is json-related, and needs to be fixed in Cargo

[camelid]

I think we also have to wait for a cargo bootstrap bump so we get the fix for Rustdoc flags... sigh