Use SmallStr when building target-features LLVM attribute

Author: tmiasko

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};
@@ -382,7 +383,7 @@ pub fn from_fn_attrs<'ll, 'tcx>(
         let val = global_features
             .chain(function_features.iter().map(|s| &s[..]))
             .intersperse(",")
-            .collect::<String>();
+            .collect::<SmallStr<1024>>();
         to_add.push(llvm::CreateAttrStringValue(cx.llcx, "target-features", &val));
     }