Auto merge of #87402 - nagisa:nagisa/request-feature-requests-for-features, r=estebank

Direct users towards using Rust target feature names in CLI

This PR consists of a couple of changes on how we handle target features.

In particular there is a bug-fix wherein we avoid passing through features that aren't prefixed by `+` or `-` to LLVM. These appear to be causing LLVM to assert, which is pretty poor a behaviour (and also makes it pretty clear we expect feature names to be prefixed).

The other commit, I anticipate to be somewhat more controversial is outputting a warning when users specify a LLVM-specific, or otherwise unknown, feature name on the CLI. In those situations we request users to either replace it with a known Rust feature name (e.g. `bmi` -> `bmi1`) or file a feature request. I've a couple motivations for this: first of all, if users are specifying these features on the command line, I'm pretty confident there is also a need for these features to be usable via `#[cfg(target_feature)]` machinery.  And second, we're growing a fair number of backends recently and having ability to provide some sort of unified-ish interface in this place seems pretty useful to me.

Sponsored by: standard.ai

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -1033,6 +1033,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
     } else {
         tcx.backend_optimization_level(())
     };
+    let backend_features = tcx.global_backend_features(());
     let cgcx = CodegenContext::<B> {
         backend: backend.clone(),
         crate_types: sess.crate_types().to_vec(),
@@ -1054,7 +1055,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
         regular_module_config: regular_config,
         metadata_module_config: metadata_config,
         allocator_module_config: allocator_config,
-        tm_factory: backend.target_machine_factory(tcx.sess, ol),
+        tm_factory: backend.target_machine_factory(tcx.sess, ol, backend_features),
         total_cgus,
         msvc_imps_needed: msvc_imps_needed(tcx),
         is_pe_coff: tcx.sess.target.is_like_windows,

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -4,6 +4,9 @@ use rustc_session::Session;
 use rustc_span::symbol::sym;
 use rustc_span::symbol::Symbol;
 
+/// Features that control behaviour of rustc, rather than the codegen.
+pub const RUSTC_SPECIFIC_FEATURES: &[&str] = &["crt-static"];
+
 // When adding features to the below lists
 // check whether they're named already elsewhere in rust
 // e.g. in stdarch and whether the given name matches LLVM's

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -134,6 +134,7 @@ pub trait ExtraBackendMethods: CodegenBackend + WriteBackendMethods + Sized + Se
         &self,
         sess: &Session,
         opt_level: config::OptLevel,
+        target_features: &[String],
     ) -> TargetMachineFactoryFn<Self>;
     fn target_cpu<'b>(&self, sess: &'b Session) -> &'b str;
     fn tune_cpu<'b>(&self, sess: &'b Session) -> Option<&'b str>;