POC: automatically move scope array literals to the stack

compiler/src/dmd/semantic3.d

@@ -1310,6 +1310,26 @@ private extern(C++) final class Semantic3Visitor : Visitor
 
         finishScopeParamInference(funcdecl, f);
 
+        {
+            // Promote array literals like `auto x = [y];` to stack if possible
+            import dmd.foreachvar;
+            void dgVar(VarDeclaration v)
+            {
+                if (v.maybeScope && v._init)
+                    if (auto ei = v._init.isExpInitializer())
+                        if (auto ce = ei.exp.isConstructExp())
+                            if (auto ale = ce.e2.isArrayLiteralExp())
+                                ale.onstack = true;
+            }
+
+            void dgExp(Expression e)
+            {
+                foreachVar(e, &dgVar);
+            }
+
+            foreachExpAndVar(funcdecl.fbody, &dgExp, &dgVar);
+        }
+
         // reset deco to apply inference result to mangled name
         if (f != funcdecl.type)
             f.deco = null;

compiler/test/fail_compilation/test21477.d

@@ -1,16 +1,17 @@
 /* REQUIRED_ARGS: -betterC
 TEST_OUTPUT:
 ---
-fail_compilation/test21477.d(103): Error: expression `[1]` uses the GC and cannot be used with switch `-betterC`
+fail_compilation/test21477.d(14): Error: expression `[1]` uses the GC and cannot be used with switch `-betterC`
 ---
 */
 
 // https://issues.dlang.org/show_bug.cgi?id=21477
 
-#line 100
+int[] global;
 
 int test()
 {
     int[] foo = [1];
+    global = foo;
     return 0;
 }

compiler/test/runnable/betterc.d

@@ -43,6 +43,7 @@ extern (C) void main()
     testRuntimeLowerings();
     test18457();
     test20737();
+    testScopeInferenceArrayLiteral();
 }
 
 /*******************************************/
@@ -221,3 +222,15 @@ void test22427()
     char[] p;
     auto a = cast(int[])p;
 }
+
+/*******************************************/
+// Test that local variable can be inferred scope, moving
+// array literal from GC to stack, allowing usage in betterC
+
+int testScopeInferenceArrayLiteral()
+{
+    auto x = [10, 20, 30] ~ [40];
+    x[0] = 50;
+    assert(x[0] + x[1] == 70);
+    return x[0];
+}