assertions_on_constants is falsely reported on is_x86_feature_detected depending on target-cpu configuration

Start with the assertions_on_constants lint implementation and its tests, then reproduce the report using src/main.rs and the .cargo/config.toml target-cpu=x86-64-v2 configuration. Done means configuration-dependent is_x86_feature_detected! assertions are not falsely warned about, while ordinary constant assertions retain their existing behavior.

Summary

When a -Ctarget-cpu option is passed to rustc, certain is_x86_feature_detected! invocations become compile-time constants. Clippy flags this code with clippy::assertions_on_constants, despite the constantness of the expression being dependent on the build configuration.

Lint Name

assertions_on_constants

Reproducer

I tried this code:

fn main() {
    assert!(std::arch::is_x86_feature_detected!("sse4.2"));
}

By default, no clippy assertion is issued (which is correct). If you then add the following to .cargo/config.toml:

[target.'cfg(target_arch = "x86_64")']
rustflags = ["-Ctarget-cpu=x86-64-v2"]

This now occurs

warning: this assertion has a constant value
 --> src/main.rs:2:5
  |
2 |     assert!(std::arch::is_x86_feature_detected!("sse4.2"));
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: consider moving this into a const block: `const { assert!(..) }`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.92.0/index.html#assertions_on_constants
  = note: `#[warn(clippy::assertions_on_constants)]` on by default

Further note that following the suggestion fails:

error[E0015]: cannot call non-const function `std_detect::detect::arch::x86::__is_feature_detected::sse4_2` in constants
 --> src/main.rs:2:21
  |
2 |     const { assert!(std::arch::is_x86_feature_detected!("sse4.2")) };
  |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: calls in constants are limited to constant functions, tuple structs and tuple variants
  = note: this error originates in the macro `std::arch::is_x86_feature_detected` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0015`.
error: could not compile `tmp-warning` (bin "tmp-warning") due to 1 previous error

Version

rustc 1.92.0 (ded5c06cf 2025-12-08)
binary: rustc
commit-hash: ded5c06cf21d2b93bffd5d884aa6e96934ee4234
commit-date: 2025-12-08
host: x86_64-unknown-linux-gnu
release: 1.92.0
LLVM version: 21.1.3

Additional Labels

No response