text/template/parse: nicer error when comment ends before final delimiter

By separating finding the end of the comment from the end of the action,
we can diagnose malformed comments better.
Also tweak the documentation to make the comment syntax clearer.

Fixes #6022.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/12570044

src/pkg/text/template/doc.go

@@ -44,7 +44,8 @@ data, defined in detail below.
 */
 //	{{/* a comment */}}
 //		A comment; discarded. May contain newlines.
-//		Comments do not nest.
+//		Comments do not nest and must start and end at the
+//		delimiters, as shown here.
 /*
 
 	{{pipeline}}

src/pkg/text/template/parse/lex.go

@@ -243,11 +243,16 @@ func lexLeftDelim(l *lexer) stateFn {
 // lexComment scans a comment. The left comment marker is known to be present.
 func lexComment(l *lexer) stateFn {
 	l.pos += Pos(len(leftComment))
-	i := strings.Index(l.input[l.pos:], rightComment+l.rightDelim)
+	i := strings.Index(l.input[l.pos:], rightComment)
 	if i < 0 {
 		return l.errorf("unclosed comment")
 	}
-	l.pos += Pos(i + len(rightComment) + len(l.rightDelim))
+	l.pos += Pos(i + len(rightComment))
+	if !strings.HasPrefix(l.input[l.pos:], l.rightDelim) {
+		return l.errorf("comment ends before closing delimiter")
+
+	}
+	l.pos += Pos(len(l.rightDelim))
 	l.ignore()
 	return lexText
 }

src/pkg/text/template/parse/lex_test.go

@@ -336,6 +336,10 @@ var lexTests = []lexTest{
 		{itemText, 0, "hello-"},
 		{itemError, 0, `unclosed comment`},
 	}},
+	{"text with comment close separted from delim", "hello-{{/* */ }}-world", []item{
+		{itemText, 0, "hello-"},
+		{itemError, 0, `comment ends before closing delimiter`},
+	}},
 }
 
 // collect gathers the emitted items into a slice.