Allow conflicting extras in explicit index assignments

[charliermarsh]

Summary

This PR enables something like the "final boss" of PyTorch setups -- explicit support for CPU vs. GPU-enabled variants via extras:

[project]
name = "project"
version = "0.1.0"
requires-python = ">=3.13.0"
dependencies = []

[project.optional-dependencies]
cpu = [
    "torch==2.5.1+cpu",
]
gpu = [
    "torch==2.5.1",
]

[tool.uv.sources]
torch = [
    { index = "torch-cpu", extra = "cpu" },
    { index = "torch-gpu", extra = "gpu" },
]

[[tool.uv.index]]
name = "torch-cpu"
url = "https://download.pytorch.org/whl/cpu"
explicit = true

[[tool.uv.index]]
name = "torch-gpu"
url = "https://download.pytorch.org/whl/cu124"
explicit = true

[tool.uv]
conflicts = [
    [
        { extra = "cpu" },
        { extra = "gpu" },
    ],
]

It builds atop the conflicting extras work to allow sources to be marked as specific to a dedicated extra being enabled or disabled.

As part of this work, sources now have an extra field. If a source has an extra, it means that the source is only applied to the requirement when defined within that optional group. For example, { index = "torch-cpu", extra = "cpu" } above only applies to "torch==2.5.1+cpu".

The extra field does not mean that the source is "enabled" when the extra is activated. For example, this wouldn't work:

[project]
name = "project"
version = "0.1.0"
requires-python = ">=3.13.0"
dependencies = ["torch"]

[tool.uv.sources]
torch = [
    { index = "torch-cpu", extra = "cpu" },
    { index = "torch-gpu", extra = "gpu" },
]

[[tool.uv.index]]
name = "torch-cpu"
url = "https://download.pytorch.org/whl/cpu"
explicit = true

[[tool.uv.index]]
name = "torch-gpu"
url = "https://download.pytorch.org/whl/cu124"
explicit = true

In this case, the sources would effectively be ignored. Extras are really confusing... but I think this is correct? We don't want enabling or disabling extras to affect resolution information that's outside of the relevant optional group.

[charliermarsh]

This needs documentation, but I'm gonna wait for approval.

[zanieb]

I worry about the confusion of

The extra field does not mean that the source is "enabled" when the extra is activated.

But otherwise I'm into it! We could be more verbose and say from-extra to help clarify that case, but I don't know if I love that.

[charliermarsh]

This now also supports dependency groups equally.

[charliermarsh]

Umm. Maybe from = { extra = "cpu" } and from = { extra = "gpu" }?

[charliermarsh]

I could also try to make it such that we error if we see the dependency requested as a production dependency, when it has a source with an extra on it?

[zanieb]

from = { extra = "cpu" } and from = { extra = "gpu" }?

I can't think of a case from from = <package> makes sense which would be a mismatch with our other uses of from. Hm.

I could also try to make it such that we error if we see the dependency requested as a production dependency, when it has a source with an extra on it?

This seems reasonable, or at least a warning.

[charliermarsh]

Ok, I'll start by adding some dedicated errors around this stuff. I think with good error messages (e.g., detect if torch is in project.dependencies but not in the relevant extra) it could be fine.

[charliermarsh]

Ok, I added some dedicated error messages around this.

[konstin]

This is a cool application of conflicting extras!

[konstin]

This is the subset of forks in which this entry occurs, not a conflict we generated?

[konstin]

Is that reachable?

[BurntSushi]

Nice! I think this all makes sense to me.

[BurntSushi]

Should these be Option<&ExtraName> and Option<&GroupName>? It looks like the Clippy lint for this was suppressed explicitly, so I'm guessing there's a reason for it, but it doesn't immediately stand out to me.

[charliermarsh]

They should but I can't for the life of me figure out how to get it to work at the call site.

[charliermarsh]

Figured it out by removing a lifetime.

[BurntSushi]

Ahhh yeah I see now. Nice.

[BurntSushi]

Nice

[zanieb]

Just to clarify, this looks like it includes group = as well as extra = support?

[charliermarsh]

It does yes. I mentioned that here but I guess not in the PR summary.