rewrite: support insert at end of file

In some rare situations Encore would try to add code at the end
of a file, which would panic because no segment exists at that pos.

Thanks Rob Gray for the report.

v2/codegen/internal/rewrite/rewrite.go

@@ -34,6 +34,17 @@ func (r *Rewriter) ReplaceNode(node ast.Node, data []byte) {
 }
 
 func (r *Rewriter) Insert(start token.Pos, data []byte) {
+	// If the pos is at the very end of the file, insert a new segment directly,
+	// since calling r.seg(start) would panic.
+	if len(r.segs) > 0 && r.segs[len(r.segs)-1].end == int(start) {
+		r.segs = append(r.segs, segment{
+			start: int(start),
+			end:   int(start) + len(data),
+			data:  data,
+		})
+		return
+	}
+
 	si, so := r.seg(start)
 	r.replace(si, so, si, so, data)
 }
@@ -93,10 +104,12 @@ func (r *Rewriter) seg(pos token.Pos) (idx int, offset int) {
 			return i, int(p - seg.start)
 		}
 	}
+
 	panic(fmt.Sprintf("original file does not contain pos %v", pos))
 }
 
 type segment struct {
-	start, end int
-	data       []byte
+	start int // inclusive
+	end   int // exclusive
+	data  []byte
 }

v2/codegen/internal/rewrite/rewrite_test.go

@@ -43,3 +43,15 @@ func TestDelete(t *testing.T) {
 		t.Errorf("got data %s, want %s", got, want)
 	}
 }
+
+func TestInsertAtEnd(t *testing.T) {
+	rw := New([]byte(""), 1)
+	rw.Insert(1, []byte("// test"))
+	if got, want := rw.Data(), []byte("// test"); !bytes.Equal(got, want) {
+		t.Errorf("got data %s, want %s", got, want)
+	}
+	rw.Delete(1, 4)
+	if got, want := rw.Data(), []byte("test"); !bytes.Equal(got, want) {
+		t.Errorf("got data %s, want %s", got, want)
+	}
+}