text/template: fix newline counting in raw strings

lexRawQuote already uses the next method, which keeps track of newlines
on a character by character basis. Adding up newlines in emit again
results in the newlines being counted twice, which can mean bad position
information in error messages.

Fix that, and add a test.

Fixes #27319.

Change-Id: Id803be065c541412dc808d388bc6d8a86a0de41e
Reviewed-on: https://go-review.googlesource.com/131996
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/text/template/parse/lex.go

@@ -155,7 +155,7 @@ func (l *lexer) emit(t itemType) {
 	l.items <- item{t, l.start, l.input[l.start:l.pos], l.line}
 	// Some items contain text internally. If so, count their newlines.
 	switch t {
-	case itemText, itemRawString, itemLeftDelim, itemRightDelim:
+	case itemText, itemLeftDelim, itemRightDelim:
 		l.line += strings.Count(l.input[l.start:l.pos], "\n")
 	}
 	l.start = l.pos

src/text/template/parse/parse_test.go

@@ -447,6 +447,9 @@ var errorTests = []parseTest{
 	{"emptypipeline",
 		`{{ ( ) }}`,
 		hasError, `missing value for parenthesized pipeline`},
+	{"multilinerawstring",
+		"{{ $v := `\n` }} {{",
+		hasError, `multilinerawstring:2: unexpected unclosed action`},
 }
 
 func TestErrors(t *testing.T) {