Remove context dependant ! fallback

[WaffleLapkin]

... and minor cleanup.

r? lcnr

[WaffleLapkin]

I have really low confidence in the never type tests, IMO they require a major refactoring, as a lot of them are based on outdated ideas and features...

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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

[TIMING:end] tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
fmt: checked 6544 files
tidy check
tidy [ui_tests (tests)]: Stray file with UI testing output: "/checkout/tests/ui/never_type/feature-gate-never_type_fallback.stderr"
tidy [ui_tests (tests)]: FAIL
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [features]: /checkout/compiler/rustc_feature/src/removed.rs:190: feature needs_allocator is not sorted by feature name (should be between mut_preserve_binding_mode_2024 and never_type_fallback)
tidy [features]: FAIL
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'venv'
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'virtualenv'
Requirement already satisfied: pip in ./build/venv/lib/python3.10/site-packages (25.2)
Collecting pip
---
linting javascript files and applying suggestions
Running eslint on rustdoc JS files
info: ES-Check: there were no ES version matching errors!  🎉
typechecking javascript files
tidy: The following checks failed: features, ui_tests (tests)
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1549:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1279:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:02:49
  local time: Wed Nov 12 16:40:32 UTC 2025
  network time: Wed, 12 Nov 2025 16:40:32 GMT
##[error]Process completed with exit code 1.

[jackh726]

For the tests, I would expect us to have revisions for edition 2021 and edition 2024 (for the different fallbacks)