Comment 1:

Seems to happen for any nested package such as crypto/hmac or encoding/binary. Curious:
if you go one level deeper, say encoding/binary/foo, it mkdirs only two, not three
mkdir -p $WORK/encoding/binary/foo/_obj/
mkdir -p $WORK/encoding/binary/
A quick scan of the source doesn't make the reason obvious. It's also very unimportant.

Labels changed: added priority-later, removed priority-triage.

Status changed to Accepted.

Comment 2 by frederic.husson@baleinoid.fr:

In build.go you have a makedir for  the objdir and one for the target and you have in
modeBuild if not a.link : a.target = a.objpkg
    // Make build directory.
    obj := a.objdir
    if err := b.mkdir(obj); err != nil {
        return err
    }
    // make target directory
    dir, _ := filepath.Split(a.target)
    if dir != "" {
        if err := b.mkdir(dir); err != nil {
            return err
        }
    }

Comment 3:

Labels changed: added go1.3maybe.

Comment 4:

Labels changed: added release-none, removed go1.3maybe.

Comment 5:

Labels changed: added repo-main.

This doesn't matter; fixing it would make the code more fragile.