For a CL about optimizing, you should probably include a BenchmarkFoo
function.
https://codereview.appspot.com/6842078/diff/7006/src/pkg/bufio/bufio.go
File src/pkg/bufio/bufio.go (right):
https://codereview.appspot.com/6842078/diff/7006/src/pkg/bufio/bufio.go#newcode572
src/pkg/bufio/bufio.go:572: fillFlush := func() error {
I'd make this be a (private) method on *Writer instead, even if it has
to take an explicit (rather than closed-over) io.Reader argument.
Then you can also document its function and return values more, and also
test it stand-alone easier.
Maybe call it "fillFrom".
https://codereview.appspot.com/6842078/