Lint #[inline(always)] closure in #[target_feature] functions

[calebzulawski]

Make #108655 a future-incompatible lint as suggested in #69098 (comment)

r? @RalfJung

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#16 exporting to docker image format
#16 sending tarball 29.0s done
#16 DONE 41.8s
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Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
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debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
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sccache: Starting the server...
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configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-18', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--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', '--set', 'rust.lld=false', '--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-18/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10
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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
---

error[E0658]: attributes on expressions are experimental
##[error] --> lint_example.rs:6:19
  |
6 |     let closure = #[inline(always)] || {};
  |
  = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
  = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
  = note: this compiler was built on 2024-11-24; consider upgrading it if it is out of date
  = note: this compiler was built on 2024-11-24; consider upgrading it if it is out of date


error[E0658]: `#[target_feature(..)]` can only be applied to `unsafe` functions
  |
  |
4 | #[target_feature(enable = "avx")]
5 | fn example() {
  | ------------ not an `unsafe` function
  |
  = note: see issue #69098 <https://github.com/rust-lang/rust/issues/69098> for more information
  = note: see issue #69098 <https://github.com/rust-lang/rust/issues/69098> for more information
  = help: add `#![feature(target_feature_11)]` to the crate attributes to enable


warning: unused variable: `closure`
##[warning] --> lint_example.rs:6:9
##[warning] --> lint_example.rs:6:9
  |
6 |     let closure = #[inline(always)] || {};
  |         ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_closure`
  = note: `#[warn(unused_variables)]` on by default


error: aborting due to 2 previous errors; 1 warning emitted
---

error[E0658]: attributes on expressions are experimental
##[error] --> lint_example.rs:6:19
  |
6 |     let closure = #[inline(always)] || {};
  |
  = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
  = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
  = note: this compiler was built on 2024-11-24; consider upgrading it if it is out of date
  = note: this compiler was built on 2024-11-24; consider upgrading it if it is out of date


error[E0658]: `#[target_feature(..)]` can only be applied to `unsafe` functions
  |
  |
4 | #[target_feature(enable = "avx")]
5 | fn example() {
  | ------------ not an `unsafe` function
  |
  = note: see issue #69098 <https://github.com/rust-lang/rust/issues/69098> for more information
  = note: see issue #69098 <https://github.com/rust-lang/rust/issues/69098> for more information
  = help: add `#![feature(target_feature_11)]` to the crate attributes to enable


warning: unused variable: `closure`
##[warning] --> lint_example.rs:6:9
##[warning] --> lint_example.rs:6:9
  |
6 |     let closure = #[inline(always)] || {};
  |         ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_closure`
  = note: `#[warn(unused_variables)]` on by default


error: aborting due to 2 previous errors; 1 warning emitted
error: aborting due to 2 previous errors; 1 warning emitted


For more information about this error, try `rustc --explain E0658`.


This error was generated by the lint-docs tool.
This tool extracts documentation for lints from the source code and places
them in the rustc book. See the declare_lint! documentation
https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint_defs/macro.declare_lint.html


To re-run these tests, run: ./x.py test --keep-stage=0 src/tools/lint-docs
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Command has failed. Rerun with -v to see more details.
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[RalfJung]

I am in favor of this lint, but IMO it should get input from some other people as well, so let's

r? compiler

[RalfJung]

In fact since this is a new lint, we'll need approval from @rust-lang/lang. @calebzulawski can you make the PR description a bit more self-contained so t-lang has the required context here?

[RalfJung]

Suggested change

	if let Some(inline_span) = inline_span {
	// Do *not* inherit target features here, that could be unsound.
	if let Some(inline_span) = inline_span {

[RalfJung]

What if we don't have the span? Something is still wrong, so we should still emit a lint -- it can point at the closure instead of the attribute, maybe?