Add constraints (and overrides?) to build environments

[charliermarsh]

We should add a --build-constraints that apply to the build environments.

See: #5551 (comment)

[hauntsaninja]

(Note that pip's behaviour is a little weird, only the env var affects the build environments, not the command line flag)

[charliermarsh]

Oh wow, that's interesting. I'm considering providing separate API for this (like --build-constraints).

[orsinium]

In DepHell, I used to use ensurepip with fallbacks to bootstrap build venvs. It uses the setuptools and pip versions bundled with stdlib that are always the same and guaranteed to be stable.

[grant-zietsman]

@charliermarsh Are there any plans to make --build-constraints a project setting (similar to constraint-dependencies)?

Currently build constraints need to be manually specified:

• --build-constraint (seems exclusive to the pip interface).
• UV_BUILD_CONSTRAINT (seems global).

Having a central project setting would be convenient (at least for the use case I have in mind).

Use Case (Context)

The goal is for packages (source distributions) that include protobufs to support compiling these protobufs during build while being constrained to the same major version of protobuf as required by the project into which they are being installed.

Constraining the version of protobuf both during build and via constraint-dependencies (in the end project) should ensure compatibility, if these packages (source distributions) compile protobufs with grpcio-tools during build.

This should also be attainable with no-build-isolation and no-build-isolation-package but it would be preferred to retain build isolation (to avoid unnecessarily constraining any other build requirements).

A workaround that somewhat side steps this is to compile protobufs on import.

[charliermarsh]

Yeah we should add this. I'll make a separate issue.

[charliermarsh]

Tracking here: #6913