Auto merge of #121298 - nikic:writable, r=cuviper

Set writable and dead_on_unwind attributes for sret arguments

Set the `writable` and `dead_on_unwind` attributes for `sret` arguments. This allows call slot optimization to remove more memcpy's.

See https://llvm.org/docs/LangRef.html#parameter-attributes for the specification of these attributes. In short, the statement we're making here is that:

 * The return slot is writable.
 * The return slot will not be read if the function unwinds.

Fixes https://github.com/rust-lang/rust/issues/90595.

compiler/rustc_llvm/llvm-wrapper/LLVMWrapper.h

@@ -91,6 +91,8 @@ enum LLVMRustAttribute {
   AllocAlign = 39,
   SanitizeSafeStack = 40,
   FnRetThunkExtern = 41,
+  Writable = 42,
+  DeadOnUnwind = 43,
 };
 
 typedef struct OpaqueRustString *RustStringRef;

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -312,6 +312,16 @@ static Attribute::AttrKind fromRust(LLVMRustAttribute Kind) {
     return Attribute::SafeStack;
   case FnRetThunkExtern:
     return Attribute::FnRetThunkExtern;
+#if LLVM_VERSION_GE(18, 0)
+  case Writable:
+    return Attribute::Writable;
+  case DeadOnUnwind:
+    return Attribute::DeadOnUnwind;
+#else
+  case Writable:
+  case DeadOnUnwind:
+    report_fatal_error("Not supported on this LLVM version");
+#endif
   }
   report_fatal_error("bad AttributeKind");
 }