Rollup merge of #117616 - RalfJung:unstable-target-features, r=compiler-errors

warn when using an unstable feature with -Ctarget-feature

Setting or unsetting the wrong target features can cause ABI incompatibility (https://github.com/rust-lang/rust/issues/116344, https://github.com/rust-lang/rust/issues/116558). We need to carefully audit features for their ABI impact before stabilization. I just learned that we currently accept arbitrary unstable features on stable and if they are in the list of Rust target features, even unstable, then we don't even warn about that!1 That doesn't seem great, so I propose we introduce a warning here.

This has an obvious loophole via `-Ctarget-cpu`. I'm not sure how to best deal with that, but it seems better to fix what we can and think about the other cases later, maybe once we have a better idea for how to resolve the general mess that are ABI-affecting target features.

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -23,6 +23,15 @@ pub const RUSTC_SPECIFIC_FEATURES: &[&str] = &["crt-static"];
 // check whether they're named already elsewhere in rust
 // e.g. in stdarch and whether the given name matches LLVM's
 // if it doesn't, to_llvm_feature in llvm_util in rustc_codegen_llvm needs to be adapted
+//
+// When adding a new feature, be particularly mindful of features that affect function ABIs. Those
+// need to be treated very carefully to avoid introducing unsoundness! This often affects features
+// that enable/disable hardfloat support (see https://github.com/rust-lang/rust/issues/116344 for an
+// example of this going wrong), but features enabling new SIMD registers are also a concern (see
+// https://github.com/rust-lang/rust/issues/116558 for an example of this going wrong).
+//
+// Stabilizing a target feature (setting the 2nd component of the pair to `None`) requires t-lang
+// approval.
 
 const ARM_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
     // tidy-alphabetical-start