go/token: add (*File).Lines method

This method returns the array updated by SetLines, for
use in exporter packages.

Fixes golang#57708

Change-Id: I12ed5e7e1bae7517f40cb25e76e51997c25d84f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/464515
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Alan Donovan <adonovan@google.com>

api/next/57708.txt

@@ -0,0 +1 @@
+pkg go/token, method (*File) Lines() []int #57708

src/go/token/position.go

@@ -159,6 +159,15 @@ func (f *File) MergeLine(line int) {
 	f.lines = f.lines[:len(f.lines)-1]
 }
 
+// Lines returns the effective line offset table of the form described by SetLines.
+// Callers must not mutate the result.
+func (f *File) Lines() []int {
+	f.mutex.Lock()
+	lines := f.lines
+	f.mutex.Unlock()
+	return lines
+}
+
 // SetLines sets the line offsets for a file and reports whether it succeeded.
 // The line offsets are the offsets of the first character of each line;
 // for instance for the content "ab\nc\n" the line offsets are {0, 3}.

src/go/token/position_test.go

@@ -130,6 +130,9 @@ func TestPositions(t *testing.T) {
 		if f.LineCount() != len(test.lines) {
 			t.Errorf("%s, SetLines: got line count %d; want %d", f.Name(), f.LineCount(), len(test.lines))
 		}
+		if !reflect.DeepEqual(f.Lines(), test.lines) {
+			t.Errorf("%s, Lines after SetLines(v): got %v; want %v", f.Name(), f.Lines(), test.lines)
+		}
 		verifyPositions(t, fset, f, test.lines)
 
 		// add lines with SetLinesForContent and verify all positions