internal/shader: bug fix: a meaningless statement should raise an error (a non-call expression statement)

Updates #1898

internal/shader/expr.go

@@ -373,6 +373,7 @@ func (cs *compileState) parseExpr(block *block, expr ast.Expr, markLocalVariable
 			// TODO: Is this an error?
 		}
 
+		// These local-variable expressions are used for an outside function callers.
 		var exprs []shaderir.Expr
 		for _, p := range outParams {
 			exprs = append(exprs, shaderir.Expr{

internal/shader/stmt.go

@@ -463,6 +463,11 @@ func (cs *compileState) parseStmt(block *block, fname string, stmt ast.Stmt, inP
 		}
 
 	case *ast.ExprStmt:
+		if _, ok := stmt.X.(*ast.CallExpr); !ok {
+			cs.addError(stmt.Pos(), fmt.Sprintf("the statement is evaluated but not used"))
+			return nil, false
+		}
+
 		exprs, _, ss, ok := cs.parseExpr(block, stmt.X, true)
 		if !ok {
 			return nil, false
@@ -470,8 +475,9 @@ func (cs *compileState) parseStmt(block *block, fname string, stmt ast.Stmt, inP
 		stmts = append(stmts, ss...)
 
 		for _, expr := range exprs {
+			// There can be a non-call expr like LocalVariable expressions.
+			// These are necessary to be used as arguments for an outside function callers.
 			if expr.Type != shaderir.Call {
-				// TODO: Should this return an error?
 				continue
 			}
 			if expr.Exprs[0].Type == shaderir.BuiltinFuncExpr {

shader_test.go

@@ -1250,6 +1250,18 @@ func TestShaderMeaninglessSentence(t *testing.T) {
 var Time float
 var ScreenSize vec2
 
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	position
+	return position
+}`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+
+	if _, err := ebiten.NewShader([]byte(`package main
+
+var Time float
+var ScreenSize vec2
+
 func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 	vec2(position)
 	return position