Supporting sdists and source trees in PEP 665

Can’t the locker also pin dynamic build dependencies (i.e. what get_requires_for_build_wheel returns). Is there something else ?

I don’t follow this. If the source tree is pinned can’t we assume it will have the same static and dynamic build requirements in a subsequent build (at least when building on the same platform) ? In which case the build requirements will be available in the pinned build dependencies and the installer can blindly pre-install them in the build environment, trusting that the build requirements have been correctly resolved by the locker.