Change unexpected_cfg to allow by default

[tbu-]

This allows to find solutions to the false positives that were found in the ecosystem before turning it to warn by default again.

Most projects hit by this seem to just disable the warning, which indicates that it isn't working as expected.

CC #124800
CC #124804
CC #124821
CC hyperium/hyper#3660
CC microsoft/windows-rs#3022
CC rust-bitcoin/rust-bitcoin#2748
CC tokio-rs/tokio#6538

[rustbot]

r? @lcnr

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[rustbot]

Some changes occurred in tests/ui/check-cfg

cc @Urgau

[rust-log-analyzer]

The job x86_64-gnu-llvm-17 failed! Check out the build log: (web) (plain)

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

#17 exporting to docker image format
#17 sending tarball 33.8s done
#17 DONE 38.7s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-17]
---
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-17', '--enable-llvm-link-shared', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'change-id=99999999', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-17/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.thin-lto-import-instr-limit := 10
configure: change-id            := 99999999
---
  Downloaded boml v0.3.1
   Compiling boml v0.3.1
   Compiling y v0.1.0 (/checkout/compiler/rustc_codegen_gcc/build_system)
    Finished `release` profile [optimized] target(s) in 3.50s
     Running `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-codegen/x86_64-unknown-linux-gnu/release/y test --use-system-gcc --use-backend gcc --out-dir /checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/cg_gcc --release --mini-tests --std-tests`
Using system GCC
[BUILD] example
[AOT] mini_core_hello_world
/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/cg_gcc/mini_core_hello_world
abc
---
##[group]Building stage0 tool linkchecker (x86_64-unknown-linux-gnu)
    Finished `release` profile [optimized] target(s) in 0.13s
##[endgroup]
##[group]Testing stage0 Linkcheck (x86_64-unknown-linux-gnu)
cargo/print.html:5396: broken link fragment `#unexpected-cfgs` pointing to `rustc/lints/listing/warn-by-default.html`
cargo/print.html:5411: broken link fragment `#unexpected-cfgs` pointing to `rustc/lints/listing/warn-by-default.html`
cargo/reference/build-scripts.html:370: broken link fragment `#unexpected-cfgs` pointing to `rustc/lints/listing/warn-by-default.html`
cargo/reference/build-scripts.html:385: broken link fragment `#unexpected-cfgs` pointing to `rustc/lints/listing/warn-by-default.html`
number of HTML files scanned: 38355
number of HTML redirects found: 12087
number of links checked: 3070942
number of links ignored due to external: 73485

[Urgau]

Most projects hit by this seem to just disable the warning, which indicates that it isn't working as expected.

This just represent some projects which are far from being representative of the ecosystem.

We also have received appreciation that the feature is working as expected.

Anyway, this PR premature to say the least, discussion is still happening on #124800.

@rustbot label +S-blocked -S-waiting-on-review

[tbu-]

We also have received appreciation that the feature is working as expected.

Note that this isn't removing the feature altogether. I believe that one could significantly improve the false-positive rate and/or make creating exceptions easier so that the feature could be appreciated by everyone. I'm sure the projects mentioned above (like e.g. tokio) also like getting warnings about actually wrong #[cfg(…)].

[epage]

imo this is a conversation best to be had on the issue to not fragment thingsd

[bors]

☔ The latest upstream changes (presumably #124706) made this pull request unmergeable. Please resolve the merge conflicts.

[tbu-]

Superseded by #125219.