internal/shader: bug fix: wrong lhs in an assignment

Closes #2910

internal/shader/stmt.go

@@ -622,6 +622,10 @@ func (cs *compileState) assign(block *block, fname string, pos token.Pos, lhs, r
 			}
 
 			if define {
+				if _, ok := e.(*ast.Ident); !ok {
+					cs.addError(pos, "non-name on the left side of :=")
+					return nil, false
+				}
 				name := e.(*ast.Ident).Name
 				if name != "_" {
 					for _, v := range block.vars {

internal/shader/syntax_test.go

@@ -3835,13 +3835,25 @@ func Bar() int {
 	}
 }
 
-// Issue #2891
+// Issue #2891, #2910
 func TestSyntaxTailingUnaryOperator(t *testing.T) {
 	if _, err := compileToIR([]byte(`package main
 
 func Foo() {
 	1 + x := vec2(2)
 }
+`)); err == nil {
+		t.Error("compileToIR must return an error but did not")
+	}
+	if _, err := compileToIR([]byte(`package main
+
+func Foo() {
+	1 + x, y := Bar()
+}
+
+func Bar() (int, int) {
+	return 0, 0
+}
 `)); err == nil {
 		t.Error("compileToIR must return an error but did not")
 	}