Ah, you fell into a trap already! Using foo/* like that is generally "weird", since you let the shell expand. It will, for example, miss files that start with a dot ("."). But not necessarily, it depends on what the shell does!

As for the "trailing /" syntax, I know what you mean, but once you've internalized it, it's easy.

Without trailing slash: Copy the directory (and its contents). With trailing slash: Copy what's in the directory. This makes sense, because with the / you say you want to go "into the directory" and copy then.

And the latter includes files starting with . and all of that, you don't have to worry about it, or about what the shell would do, it's all the directory contents.

So what you want for your example is:

    rsync -a foo/ bar

    rsync -avP foo bar   # puts foo inside bar so bar/foo/* 
    rsync -avP foo/ bar  # puts the the contents of foo, in bar (without making foo)

That means "bar/" is just a contraction of "bar/."

For some reason I find the presence of the "." makes it much easier to reason about. Probably that because turns it into the same pattern as "rsync foo/x bar/y". Most people know what that does, even if "foo/x" or "foo/y" are directories.

Also --delay-updates is useful to minimize disruption when copying into a live directory read by a webserver or other program where you want to have a consisten state.