internal/shader: bug fix: forbide assigning to a uniform variable

Closes #2648

internal/shader/stmt.go

@@ -74,6 +74,11 @@ func (cs *compileState) parseStmt(block *block, fname string, stmt ast.Stmt, inP
 			}
 			stmts = append(stmts, ss...)
 
+			if lhs[0].Type == shaderir.UniformVariable {
+				cs.addError(stmt.Pos(), fmt.Sprintf("a uniform variable cannot be assigned"))
+				return nil, false
+			}
+
 			var op shaderir.Op
 			switch stmt.Tok {
 			case token.ADD_ASSIGN:

internal/shader/syntax_test.go

@@ -3138,3 +3138,38 @@ func bar(x vec2) {
 		t.Error("compileToIR must return an error but did not")
 	}
 }
+
+// Issue #2648
+func TestAssignToUniformVariables(t *testing.T) {
+	if _, err := compileToIR([]byte(`package main
+
+var Foo float
+
+func foo(x vec2) {
+	Foo = 0
+}`)); err == nil {
+		t.Error("compileToIR must return an error but did not")
+	}
+
+	if _, err := compileToIR([]byte(`package main
+
+var Foo float
+
+func foo(x vec2) {
+	var x int
+	x, Foo = 0, 0
+	_ = x
+}`)); err == nil {
+		t.Error("compileToIR must return an error but did not")
+	}
+
+	if _, err := compileToIR([]byte(`package main
+
+var Foo float
+
+func foo(x vec2) {
+	Foo += 0
+}`)); err == nil {
+		t.Error("compileToIR must return an error but did not")
+	}
+}