starlark: skip last frame of stack backtrace if built-in (google#319)

Instead, the name of the built-in function is append to the
error message ("Error in foo: ...").

starlark/eval.go

@@ -217,7 +217,9 @@ func (stack *CallStack) Pop() CallFrame {
 // String returns a user-friendly description of the stack.
 func (stack CallStack) String() string {
 	out := new(strings.Builder)
-	fmt.Fprintf(out, "Traceback (most recent call last):\n")
+	if len(stack) > 0 {
+		fmt.Fprintf(out, "Traceback (most recent call last):\n")
+	}
 	for _, fr := range stack {
 		fmt.Fprintf(out, "  %s: in %s\n", fr.Pos, fr.Name)
 	}
@@ -259,7 +261,15 @@ func (e *EvalError) Error() string { return e.Msg }
 // Backtrace returns a user-friendly error message describing the stack
 // of calls that led to this error.
 func (e *EvalError) Backtrace() string {
-	return fmt.Sprintf("%sError: %s", e.CallStack, e.Msg)
+	// If the topmost stack frame is a built-in function,
+	// remove it from the stack and add print "Error in fn:".
+	stack := e.CallStack
+	suffix := ""
+	if last := len(stack) - 1; last >= 0 && stack[last].Pos.Filename() == builtinFilename {
+		suffix = " in " + stack[last].Name
+		stack = stack[:last]
+	}
+	return fmt.Sprintf("%sError%s: %s", stack, suffix, e.Msg)
 }
 
 func (e *EvalError) Unwrap() error { return e.cause }

starlark/eval_test.go

@@ -512,7 +512,6 @@ i()
 `
 	thread := new(starlark.Thread)
 	_, err := starlark.ExecFile(thread, "crash.star", src, nil)
-	// Compiled code currently has no column information.
 	const want = `Traceback (most recent call last):
   crash.star:6:2: in <toplevel>
   crash.star:5:18: in i
@@ -527,6 +526,9 @@ Error: floored division by zero`
 
 	// Additionally, ensure that errors originating in
 	// Starlark and/or Go each have an accurate frame.
+	// The topmost frame, if built-in, is not shown,
+	// but the name of the built-in function is shown
+	// as "Error in fn: ...".
 	//
 	// This program fails in Starlark (f) if x==0,
 	// or in Go (string.join) if x is non-zero.
@@ -542,8 +544,7 @@ Error: floored division by zero`,
 		1: `Traceback (most recent call last):
   crash.star:3:2: in <toplevel>
   crash.star:2:17: in f
-  <builtin>: in join
-Error: join: in list, want string, got int`,
+Error in join: join: in list, want string, got int`,
 	} {
 		globals := starlark.StringDict{"i": starlark.MakeInt(i)}
 		_, err := starlark.ExecFile(thread, "crash.star", src2, globals)