fix 21756 - make cast(immutable)arrayLiteral @nogc

src/dmd/expressionsem.d

@@ -7331,6 +7331,29 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
 
         if (!exp.to) // Handle cast(const) and cast(immutable), etc.
         {
+
+            // turns `cast(immutable)[<elems...>]` into a static decl, @nogc compatible
+            ArrayLiteralExp ale = exp.e1.isArrayLiteralExp();
+            if (exp.mod == MODFlags.immutable_ && ale && sc.func)
+            {
+                Identifier id       = Identifier.generateId("__arr");
+                ExpInitializer ei   = new ExpInitializer(exp.e1.loc, exp.e1);
+                VarDeclaration vd   = new VarDeclaration(exp.e1.loc, exp.e1.type, id, ei, STC.immutable_ | STC.gshared );
+                VarExp ve           = new VarExp(exp.e1.loc, vd);
+
+                ScopeDsymbol sds    = sc.scopesym.toParentDecl().isScopeDsymbol();
+                assert(sds);
+                if (!sds.symtab)
+                    sds.symtab = new DsymbolTable();
+                vd.addMember(sc, sds);
+
+                DeclarationExp de   = new DeclarationExp(exp.e1.loc, vd);
+                CommaExp ce         = new CommaExp(exp.e1.loc, de, ve);
+
+                result              = expressionSemantic(ce, sc);
+                return;
+            }
+
             exp.to = exp.e1.type.castMod(exp.mod);
             exp.to = exp.to.typeSemantic(exp.loc, sc);
 

test/runnable/issue21756.d

@@ -0,0 +1,28 @@
+module issue21756;
+
+auto test1() @nogc
+{
+    return  cast(immutable)['a', 'b', 'c'];
+}
+
+auto test2() @nogc
+{
+    return cast(immutable)[1, 2, 3];
+}
+
+struct S
+{
+    int a,b,c;
+}
+
+auto test3() @nogc
+{
+    return cast(immutable)[S(1,2,3), S(1,2,3)];
+}
+
+void main() @nogc
+{
+    assert(test1() == "abc");
+    assert(test2()[2] == 3);
+    assert(test3()[0] == test3()[1]);
+}