Validate build dependencies when using `--no-build-isolation`

What’s the problem this feature will solve?

Prevent builds from running when all required build dependencies are not present.

Describe the solution you’d like

pip fails to build with a descriptive message when a build dependency is missing, providing a nicer user experience (IMHO).

Alternative Solutions

1. Use python-build which does exactly that.
2. Manually parse pyproject.toml + generate egg info, get additional build requires and validate those are in the environment.

Additional context I see the pip docs on --no-build-isolation state “Build dependencies specified by PEP 518 must be already installed if this option is used.”. I was wondering whether pip would like to mimic the behaviour that python-build has on --no-isolation(Fail nicely if build dependencies are not present) and if so I’m happy to try and push a patch.

Related: https://github.com/pypa/pip/issues/6718

[uranusjr]

A sanity check to make sure the user actually sets up the environment correctly makes sense to me.

[pradyunsg]

The change needed here is calling build_env.check_requirements(build_deps) at some point in the codebase, since https://github.com/pypa/pip/blob/main/src/pip/_internal/build_env.py#L158 has the relevant logic already IIUC.

I’m pretty sure the things to figure out here are:

1. Where to make that call.
2. How to get build_deps to make that call.