Confusing interaction between rootdir and pytest.ini

From https://github.com/pytest-dev/pytest/issues/6359#issue-540390553:

Unfortunately I agree that the current interaction between rootdir and ini files is far from clear, and someone just looking at --rootdir would assume that it should point to the root of your repository/project, and from there on pytest would find pytest.ini files and start collection, but this is currently not the case.

I've seen more than one issue popping up about this confusion: people assume that --rootdir mydir implies that pytest will look for pytest.ini files and start collection from mydir, which is not the case at all currently.

I think we should change the meaning of --rootdir the option somehow, because I believe it is the main source of confusion, while I don't think it happens when rootdir is found automatically.

An initial proposal:

--rootdir when given:

• Will set config.rootdir to that value (of course).
• Will search from --rootdir and upwards for pytest.ini files.
• If no paths are given in the command line, it will act like testpaths, where collection will be assumed to start from --rootdir.

Thoughts?

It is a change in existing behavior, but I think --rootdir is rarely used, and when used people are usually confused by it.

Read the referenced issue #6359 and investigate the current --rootdir behavior, including how pytest.ini discovery and collection roots are selected. Resolve the proposed semantics before implementation; done means the behavior is explicitly agreed and matches the resulting --rootdir rules.