feat: Add new field build.requires

[LecrisUT]

TODO:

• Define a new field to be inserted in build-system.requires
• Document new feature
• Add support for {root:uri}
  • should we copy hatchling's implementation or resolve it only if hatchling is added to dependency?
• Do we add a special case for sdist building or document that the user should define it? Postponed because it would require a more involved process to validate.

[henryiii]

Do we need {root:uri} if we have overrides? I thought that was a way to control the dependencies based on how it was being built?

And for sdist-building, I think we already have an override selector for that case?

[LecrisUT]

And for sdist-building, I think we already have an override selector for that case?

Yes. The question for this PR, is if we should provide a sane default if the user did not provide one, or we leave it completely to the user

Do we need {root:uri} if we have overrides? I thought that was a way to control the dependencies based on how it was being built?

Kind-of. The issue with file: is that relative paths are expanded relative to where pip install is called, so you cannot make a consistent pip install for the users without a more specialized handling. Although, I guess the user could do

[[tool.scikit-build.overrides]]
if.env.BUILD_ROOT = true
build.build-requires = ["foo @ file:${BUILD_ROOT}/foo"]

and make sure BUILD_ROOT=$(pwd). Not as convenient as having a {root:uri} setup and relying on default CI environment variable, but maybe it's a better initial format for it.

[henryiii]

I guess my question is, in what cases would this be incorrect?

[[tool.scikit-build.overrides]]
if.from-sdist = false
build.build-requires = ["foo @ ./foo"]

[[tool.scikit-build.overrides]]
if.from-sdist = true
build.build-requires = ["foo"]

[LecrisUT]

I guess my question is, in what cases would this be incorrect?

[[tool.scikit-build.overrides]]
if.from-sdist = false
build.build-requires = ["foo @ ./foo"]

[[tool.scikit-build.overrides]]
if.from-sdist = true
build.build-requires = ["foo"]

$ pip install .
(this works fine assuming there is /foo)
$ cd bar && pip install ../
(this breaks because it is checking for /bar/foo)

(don't we need to specify @ file:./foo instead of just @ ./foo. Don't remember if I checked if there was a difference there)

[henryiii]

Okay, since we already use Python formatting in build-dir, we could do it here too. And implementing {root:uri} should be easy. Not so sure about {root:parent:uri}. Hmm, it seems __format__ passes the colon in. Okay, guess we could do it:

class DispPath(Path):
    def __format__(self, fmt):
        command, _, rest = fmt.partition(":")
        if command == "parent":
            return self.parent.__format__(rest)
        if command == "uri" and rest == "":
            return self.as_uri()
        if command == "" and rest == "":
            return str(self)
        raise ...