parserErrorDetail: fix '^' location of lines starting with tabs

Fixes #1129 for the most part.

What still is wrongly put, but also harder to fix, is the case where
there's tabs INSIDE the line:

	p = TAB true TAB { TAB as }

or a space before the tab, like

	SPACE TAB p = true { as }

will still have a misaligned "^", as in that case, not all tabs are
fully expanded.

This merely trims leading tabs, and fixes the error maker location if
there's no other tabs used in the line.

However, that should fit common usage: Leading tabs is what `opa fmt`
proposes; and I'm doubtful of too many uses of tabs in other places
in Rego code.

Signed-off-by: Stephan Renatus <srenatus@chef.io>

ast/parser_ext.go

@@ -749,7 +749,9 @@ func newParserErrorDetail(bs []byte, pos position) *parserErrorDetail {
 }
 
 func (d parserErrorDetail) Lines() []string {
-	return []string{d.line, strings.Repeat(" ", d.idx) + "^"}
+	line := strings.TrimLeft(d.line, "\t") // remove leading tabs
+	tabCount := len(d.line) - len(line)
+	return []string{line, strings.Repeat(" ", d.idx-tabCount) + "^"}
 }
 
 func isNewLineChar(b byte) bool {

ast/parser_test.go

@@ -1417,6 +1417,7 @@ func TestParseErrorDetails(t *testing.T) {
 	tests := []struct {
 		note  string
 		exp   *parserErrorDetail
+		err   string
 		input string
 	}{
 		{
@@ -1474,6 +1475,32 @@ p { }`},
 			input: `
 package test
 p = "foo`},
+		{
+			note: "rule with error begins with one tab",
+			exp: &parserErrorDetail{
+				line: "\tas",
+				idx:  2,
+			},
+			input: `
+package test
+	as`,
+			err: `1 error occurred: test.rego:3: rego_parse_error: no match found
+	as
+	 ^`},
+		{
+			note: "rule term with error begins with two tabs",
+			exp: &parserErrorDetail{
+				line: "\t\tas",
+				idx:  3,
+			},
+			input: `
+package test
+p = true {
+		as
+}`,
+			err: `1 error occurred: test.rego:5: rego_parse_error: no match found
+	as
+	 ^`},
 	}
 
 	for _, tc := range tests {
@@ -1485,6 +1512,9 @@ p = "foo`},
 		if !reflect.DeepEqual(detail, tc.exp) {
 			t.Fatalf("Expected %v but got: %v", tc.exp, detail)
 		}
+		if tc.err != "" && tc.err != err.Error() {
+			t.Fatalf("Expected error string %q but got: %q", tc.err, err.Error())
+		}
 	}
 }