> interface). So if pip calls build_sdist and then build_wheel, there will be
> two source distributions built (one by pip and one by setuptools) before
> building a wheel.

This is precisely the question - whose responsibility is it to ensure
that sdist->wheel vs build_wheel equivalence is maintained? The PEP
says it's the backend's responsibility, so pip shouldn't need to do
anything.

But, we're currently having debates about "return None" which hinge on
"if backends have bugs, return None might not work well". Sure - but
if backends have bugs, they may not maintain wheel equivalence.

 To that extent, I'm with Donald - pip going
sdist->wheel protects the user against the known-to-be-an-issue bug
that the backend doesn't ensure wheel equivalence.

* Chris Barker has pointed out that backends have no reason to support
sdists now.

* Nathaniel is pushing a means of notifying "I can't build a sdist"
that protects against backends accidentally not following the spec.

of "trust the backend" is simply telling the user that the problem
behaviour they found is not pip's issue and should be reported to the
backend. Is that a sufficiently bad user experience that we should try
to improve it (experience with setuptools says that it is - why are we
assuming that developers of new backends will be so much more
conscientious and careful than the setuptools developers? *I*
certainly don't feel that the setuptools developers are unusually bad
- quite the opposite!)

Having a more robust means of saying "I can't build a sdist" than
"return None" is protecting the user from issues with the backend. So
is building via sdist.