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

1 files changed, 9 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index c299dacfc92..1dc01e12252 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -1944,4 +1944,13 @@ rustc_queries! {
         no_hash
         desc { "performing HIR wf-checking for predicate {:?} at item {:?}", key.0, key.1 }
     }
+
+
+    /// The list of backend features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,
+    /// `--target` and similar).
+    query global_backend_features(_: ()) -> Vec<String> {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        desc { "computing the backend features for CLI flags" }
+    }
 }