[clang] Restrict -Wnrvo to C++ code only.

clang/docs/ReleaseNotes.rst

@@ -279,6 +279,8 @@ Improvements to Clang's diagnostics
   ``ACQUIRED_AFTER(...)`` have been moved to the stable feature set and no
   longer require ``-Wthread-safety-beta`` to be used.
 
+- The `-Wnrvo` compiler flag is now ignored in C mode.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/lib/Sema/SemaDecl.cpp

@@ -16174,8 +16174,10 @@ void Sema::computeNRVO(Stmt *Body, FunctionScopeInfo *Scope) {
   for (unsigned I = 0, E = Scope->Returns.size(); I != E; ++I) {
     if (const VarDecl *NRVOCandidate = Returns[I]->getNRVOCandidate()) {
       if (!NRVOCandidate->isNRVOVariable()) {
-        Diag(Returns[I]->getRetValue()->getExprLoc(),
-             diag::warn_not_eliding_copy_on_return);
+        if (getLangOpts().CPlusPlus) {
+          Diag(Returns[I]->getRetValue()->getExprLoc(),
+               diag::warn_not_eliding_copy_on_return);
+        }
         Returns[I]->setNRVOCandidate(nullptr);
       }
     }

clang/test/SemaCXX/no-warn-nrvo-on-c.c

@@ -0,0 +1,41 @@
+// RUN: %clang -std=c23 -Wnrvo -Xclang -verify %s
+// expected-no-diagnostics
+
+#include <stdlib.h>
+
+#define SIZE 20
+
+typedef struct String_s {
+    char*  buf;
+    size_t len;
+} String;
+
+
+void clean(String* s) {
+    free(s->buf);
+}
+
+String randomString() {
+    String s = {};
+
+    s.buf = malloc(SIZE);
+    s.len = SIZE;
+
+    if (!s.buf) {
+        goto fail;
+    }
+
+    return s;
+
+fail:
+    clean(&s);
+    return (String){};
+}
+
+int main(int argc, char** argv)
+{
+    String s= randomString();
+    clean(&s);
+
+    return 0;
+}