go/token: add (*File).LineStart, which returns Pos for a given line

LineStart returns the position of the start of a given line.
Like MergeLine, it panics if the 1-based line number is invalid.

This function is especially useful in programs that occasionally
handle non-Go files such as assembly but wish to use the token.Pos
mechanism to identify file positions.

Change-Id: I5f774c0690074059553cdb38c0f681f5aafc8da1
Reviewed-on: https://go-review.googlesource.com/134075
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/token/position.go

@@ -146,7 +146,7 @@ func (f *File) AddLine(offset int) {
 // MergeLine will panic if given an invalid line number.
 //
 func (f *File) MergeLine(line int) {
-	if line <= 0 {
+	if line < 1 {
 		panic("illegal line number (line numbering starts at 1)")
 	}
 	f.mutex.Lock()
@@ -209,6 +209,21 @@ func (f *File) SetLinesForContent(content []byte) {
 	f.mutex.Unlock()
 }
 
+// LineStart returns the Pos value of the start of the specified line.
+// It ignores any alternative positions set using AddLineColumnInfo.
+// LineStart panics if the 1-based line number is invalid.
+func (f *File) LineStart(line int) Pos {
+	if line < 1 {
+		panic("illegal line number (line numbering starts at 1)")
+	}
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+	if line > len(f.lines) {
+		panic("illegal line number")
+	}
+	return Pos(f.base + f.lines[line-1])
+}
+
 // A lineInfo object describes alternative file, line, and column
 // number information (such as provided via a //line directive)
 // for a given file offset.

src/go/token/position_test.go

@@ -324,3 +324,18 @@ done
 		checkPos(t, "3. Position", got3, want)
 	}
 }
+
+func TestLineStart(t *testing.T) {
+	const src = "one\ntwo\nthree\n"
+	fset := NewFileSet()
+	f := fset.AddFile("input", -1, len(src))
+	f.SetLinesForContent([]byte(src))
+
+	for line := 1; line <= 3; line++ {
+		pos := f.LineStart(line)
+		position := fset.Position(pos)
+		if position.Line != line || position.Column != 1 {
+			t.Errorf("LineStart(%d) returned wrong pos %d: %s", line, pos, position)
+		}
+	}
+}