Auto merge of #94579 - tmiasko:target-features, r=nagisa

Always include global target features in function attributes

This ensures that information about target features configured with
`-C target-feature=...` or detected with `-C target-cpu=native` is
retained for subsequent consumers of LLVM bitcode.

This is crucial for linker plugin LTO, since this information is not
conveyed to the plugin otherwise.

<details><summary>Additional test case demonstrating the issue</summary>

```rust
extern crate core;

#[inline]
#[target_feature(enable = "aes")]
unsafe fn f(a: u128, b: u128) -> u128 {
    use core::arch::x86_64::*;
    use core::mem::transmute;
    transmute(_mm_aesenc_si128(transmute(a), transmute(b)))
}

pub fn g(a: u128, b: u128) -> u128 {
    unsafe { f(a, b) }
}

fn main() {
    let mut args = std::env::args();
    let _ = args.next().unwrap();
    let a: u128 = args.next().unwrap().parse().unwrap();
    let b: u128 = args.next().unwrap().parse().unwrap();
    println!("{}", g(a, b));
}
```

```console
$ rustc --edition=2021 a.rs -Clinker-plugin-lto -Clink-arg=-fuse-ld=lld  -Ctarget-feature=+aes -O
...
  = note: LLVM ERROR: Cannot select: intrinsic %llvm.x86.aesni.aesenc
```

</details>

r? `@nagisa`

1 files changed, 7 insertions, 7 deletions

diff --git a/compiler/rustc_codegen_llvm/src/attributes.rs b/compiler/rustc_codegen_llvm/src/attributes.rs
index 6fd836946ff..101da0012cb 100644
--- a/compiler/rustc_codegen_llvm/src/attributes.rs
+++ b/compiler/rustc_codegen_llvm/src/attributes.rs
@@ -1,6 +1,7 @@
 //! Set and unset common attributes on LLVM values.
 
 use rustc_codegen_ssa::traits::*;
+use rustc_data_structures::small_str::SmallStr;
 use rustc_hir::def_id::DefId;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::ty::{self, TyCtxt};
@@ -377,13 +378,12 @@ pub fn from_fn_attrs<'ll, 'tcx>(
         }
     }
 
-    if !function_features.is_empty() {
-        let global_features = cx.tcx.global_backend_features(()).iter().map(|s| &s[..]);
-        let val = global_features
-            .chain(function_features.iter().map(|s| &s[..]))
-            .intersperse(",")
-            .collect::<String>();
-        to_add.push(llvm::CreateAttrStringValue(cx.llcx, "target-features", &val));
+    let global_features = cx.tcx.global_backend_features(()).iter().map(|s| s.as_str());
+    let function_features = function_features.iter().map(|s| s.as_str());
+    let target_features =
+        global_features.chain(function_features).intersperse(",").collect::<SmallStr<1024>>();
+    if !target_features.is_empty() {
+        to_add.push(llvm::CreateAttrStringValue(cx.llcx, "target-features", &target_features));
     }
 
     attributes::apply_to_llfn(llfn, Function, &to_add);