rustdoc fails when building rust 1.85.0 for ARM targets

[jpalus]

Running ./x.py dist results in error on ARM 32-bit targets, works fine for aarch64 though:

arm-unknown-linux-gnueabihf:

...
error: target feature `fp-armv8` cannot be toggled with `#[target_feature]`: Rust ties `fp-armv8` to `neon`
     --> library/core/src/../../stdarch/crates/core_arch/src/arm_shared/neon/generated.rs:20668:48
      |
20668 | #[cfg_attr(target_arch = "arm", target_feature(enable = "fp-armv8,v8"))]
      |                                                ^^^^^^^^^^^^^^^^^^^^^^

error: target feature `fp-armv8` cannot be toggled with `#[target_feature]`: Rust ties `fp-armv8` to `neon`
     --> library/core/src/../../stdarch/crates/core_arch/src/arm_shared/neon/generated.rs:20688:48
      |
20688 | #[cfg_attr(target_arch = "arm", target_feature(enable = "fp-armv8,v8"))]
      |                                                ^^^^^^^^^^^^^^^^^^^^^^

error: could not document `core`

Caused by:
  process didn't exit successfully: `/home/users/builder/rpm/BUILD/rust-1.85.0-build/rustc-1.85.0-src/build/bootstrap/debug/rustdoc --edition=2021 --crate-type lib --crate-name core library/core/src/lib.rs --target arm-unknown-linux-gnueabihf -o /home/users/builder/rpm/BUILD/rust-1.85.0-build/rustc-1.85.0-src/build/arm-unknown-linux-gnueabihf/stage2-std/arm-unknown-linux-gnueabihf/doc/arm-unknown-linux-gnueabihf/doc --check-cfg 'cfg(docsrs)' --check-cfg 'cfg(feature, values("debug_refcell", "optimize_for_size", "panic_immediate_abort"))' --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --warn=unexpected_cfgs --check-cfg 'cfg(bootstrap)' --check-cfg 'cfg(no_fp_fmt_parse)' --check-cfg 'cfg(stdarch_intel_sde)' --check-cfg 'cfg(target_arch, values("xtensa"))' --check-cfg 'cfg(feature, values(any()))' -C metadata=cc6cca0d841652cf -L dependency=/home/users/builder/rpm/BUILD/rust-1.85.0-build/rustc-1.85.0-src/build/arm-unknown-linux-gnueabihf/stage2-std/arm-unknown-linux-gnueabihf/doc/arm-unknown-linux-gnueabihf/release/deps -L dependency=/home/users/builder/rpm/BUILD/rust-1.85.0-build/rustc-1.85.0-src/build/arm-unknown-linux-gnueabihf/stage2-std/arm-unknown-linux-gnueabihf/doc/release/deps -C debuginfo=0 -C strip=none -Csymbol-mangling-version=legacy '--check-cfg=cfg(feature,values(any()))' -Zunstable-options '--check-cfg=cfg(bootstrap)' '--check-cfg=cfg(test)' -Dwarnings '-Wrustdoc::invalid_codeblock_attributes' --crate-version '1.85.0	(4d91de4e4	    2025-02-17)	(builtfrom	   a	    source	tarball)' '-Zcrate-attr=doc(html_root_url="https://doc.rust-lang.org/1.85.0/")' '-Zcrate-attr=warn(rust_2018_idioms)' --extern-html-root-url 'std_detect=https://docs.rs/std_detect/latest/' --extern-html-root-takes-precedence --resource-suffix 1.85.0 --markdown-css rust.css --markdown-no-toc --index-page /home/users/builder/rpm/BUILD/rust-1.85.0-build/rustc-1.85.0-src/src/doc/index.md -Zunstable-options` (exit status: 1)
warning: build failed, waiting for other jobs to finish...

armv7-unknown-linux-gnueabihf:

...
error: target feature `neon` cannot be toggled with `#[target_feature]`: unsound on hard-float targets because it changes float ABI
    --> library/core/src/../../stdarch/crates/core_arch/src/arm/neon.rs:1398:18
     |
1398 | #[target_feature(enable = "neon,v7,aes")]
     |                  ^^^^^^^^^^^^^^^^^^^^^^

error: target feature `neon` cannot be toggled with `#[target_feature]`: unsound on hard-float targets because it changes float ABI
    --> library/core/src/../../stdarch/crates/core_arch/src/arm/neon.rs:1415:18
     |
1415 | #[target_feature(enable = "neon,v7,aes")]
     |                  ^^^^^^^^^^^^^^^^^^^^^^

error: could not document `core`

Caused by:
  process didn't exit successfully: `/home/users/builder/rpm/BUILD/rust-1.85.0-build/rustc-1.85.0-src/build/bootstrap/debug/rustdoc --edition=2021 --crate-type lib --crate-name core library/core/src/lib.rs --target armv7-unknown-linux-gnueabihf -o /home/users/builder/rpm/BUILD/rust-1.85.0-build/rustc-1.85.0-src/build/armv7-unknown-linux-gnueabihf/stage2-std/armv7-unknown-linux-gnueabihf/doc/armv7-unknown-linux-gnueabihf/doc --check-cfg 'cfg(docsrs)' --check-cfg 'cfg(feature, values("debug_refcell", "optimize_for_size", "panic_immediate_abort"))' --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --warn=unexpected_cfgs --check-cfg 'cfg(bootstrap)' --check-cfg 'cfg(no_fp_fmt_parse)' --check-cfg 'cfg(stdarch_intel_sde)' --check-cfg 'cfg(target_arch, values("xtensa"))' --check-cfg 'cfg(feature, values(any()))' -C metadata=4ab4e812d278f910 -L dependency=/home/users/builder/rpm/BUILD/rust-1.85.0-build/rustc-1.85.0-src/build/armv7-unknown-linux-gnueabihf/stage2-std/armv7-unknown-linux-gnueabihf/doc/armv7-unknown-linux-gnueabihf/release/deps -L dependency=/home/users/builder/rpm/BUILD/rust-1.85.0-build/rustc-1.85.0-src/build/armv7-unknown-linux-gnueabihf/stage2-std/armv7-unknown-linux-gnueabihf/doc/release/deps -C debuginfo=0 -C strip=none -Csymbol-mangling-version=legacy '--check-cfg=cfg(feature,values(any()))' -Zunstable-options '--check-cfg=cfg(bootstrap)' '--check-cfg=cfg(test)' -Dwarnings '-Wrustdoc::invalid_codeblock_attributes' --crate-version '1.85.0	    (4d91de4e4	  2025-02-17)	    (built	from	a	   source	tarball)' '-Zcrate-attr=doc(html_root_url="https://doc.rust-lang.org/1.85.0/")' '-Zcrate-attr=warn(rust_2018_idioms)' --extern-html-root-url 'std_detect=https://docs.rs/std_detect/latest/' --extern-html-root-takes-precedence --resource-suffix 1.85.0 --markdown-css rust.css --markdown-no-toc --index-page /home/users/builder/rpm/BUILD/rust-1.85.0-build/rustc-1.85.0-src/src/doc/index.md -Zunstable-options` (exit status: 1)
warning: build failed, waiting for other jobs to finish...

Build is performed on aarch64 host although I think it shouldn't really matter.

[ehuss]

Thanks for the report! I'm able to reproduce this (on latest master, too).

What I'm thinking is that here it is collecting a map of all features. However, ARM and AARCH64 both enable features of the same name, but with different Stability settings. Thus there are key collisions in the map.

cc @RalfJung since it looks like you've been doing some work here. (Presumably this is from #133417)

[RalfJung]

Hm... fp-armv8 is unstable (and allowed) as an ARM target feature, but "forbidden" as an aarch64 target feature. It used to just not be listed for aarch64, which would also make it a hard error when used with #[target_feature]. Now it is listed as "forbidden". And somehow that seems to leak over onto the ARM side?

@ehuss yeah I think you found the culprit, for rustdoc we're merging all features into one map and that just doesn't make sense, features of the same name on different targets will overwrite each other and we'll get some "random" final idea of what is and is not stable. It's kind of surprising that this worked for so long.

Cc @workingjubilee @rust-lang/rustdoc

[RalfJung]

The immediate short-term fix I can think of is to do that merging in a more clever way, where the "more stable" feature takes precedent.

But really I feel this needs to be redesigned, it's just not coherent to say "use all features from all targets". We probably want to only enable them all for cfg but the rust_target_features query should still return something sensible for the rest of the compiler.

[truboxl]

armv7-linux-androideabi and thumbv7neon-linux-androideabi are also affected

Related: #134511, #136755

[RalfJung]

FWIW there is a potential stability hazard here: rustdoc will (have to) accept code with #[target_feature(enable = "foo")] on all targets once foo is stable on one target. I hope our stability policy says that code which builds in rustdoc but not rustc is not considered stable.

[RalfJung]

#137632 should fix the issue (but not the stability hazard).

[notriddle]

I hope our stability policy says that code which builds in rustdoc but not rustc is not considered stable.

Is there any reason we can't emit docs for target_features that don't even exist? It doesn't seem to affect type checking, and we already have to allow a bunch of other code in rustdoc that rustc would've rejected.

[RalfJung]

I don't understand the question / comment, sorry. Too many negations.^^ This is about #[target_feature], not cfg(target_feature).

I don't even know what exactly rustdoc currently does for cfg here, whether it does anything special.