internal/shader: bug fix: a meaningless statement should raise an error

Closes #1898

internal/shader/stmt.go

@@ -471,8 +471,13 @@ func (cs *compileState) parseStmt(block *block, fname string, stmt ast.Stmt, inP
 
 		for _, expr := range exprs {
 			if expr.Type != shaderir.Call {
+				// TODO: Should this return an error?
 				continue
 			}
+			if expr.Exprs[0].Type == shaderir.BuiltinFuncExpr {
+				cs.addError(stmt.Pos(), fmt.Sprintf("the statement is evaluated but not used"))
+				return nil, false
+			}
 			stmts = append(stmts, shaderir.Stmt{
 				Type:  shaderir.ExprStmt,
 				Exprs: []shaderir.Expr{expr},

shader_test.go

@@ -1242,3 +1242,18 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 		t.Errorf("error must be non-nil but was nil")
 	}
 }
+
+// Issue #1898
+func TestShaderMeaninglessSentence(t *testing.T) {
+	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
+}`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+}