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, 12 insertions, 0 deletions

diff --git a/compiler/rustc_data_structures/src/small_c_str.rs b/compiler/rustc_data_structures/src/small_c_str.rs
index 33e72914e19..cd902524562 100644
--- a/compiler/rustc_data_structures/src/small_c_str.rs
+++ b/compiler/rustc_data_structures/src/small_c_str.rs
@@ -66,3 +66,15 @@ impl Deref for SmallCStr {
         self.as_c_str()
     }
 }
+
+impl<'a> FromIterator<&'a str> for SmallCStr {
+    fn from_iter<T: IntoIterator<Item = &'a str>>(iter: T) -> Self {
+        let mut data =
+            iter.into_iter().flat_map(|s| s.as_bytes()).copied().collect::<SmallVec<_>>();
+        data.push(0);
+        if let Err(e) = ffi::CStr::from_bytes_with_nul(&data) {
+            panic!("The iterator {:?} cannot be converted into a CStr: {}", data, e);
+        }
+        Self { data }
+    }
+}