Issue Description

The example configuration for optional PyTorch dependencies introduced in #9210 is causing errors during execution. Specifically, uv does not recognize the extra key used for specifying sources, resulting in a TOML parse error.

Steps to Reproduce

1. Create the problematic pyproject.toml file:

   [project]
   name = "project"
   version = "0.1.0"
   requires-python = ">=3.12.0"
   dependencies = []
   
   [project.optional-dependencies]
   cpu = [
   "torch>=2.5.1",
   "torchvision>=0.20.1",
   ]
   cu124 = [
   "torch>=2.5.1",
   "torchvision>=0.20.1",
   ]
   
   [tool.uv]
   conflicts = [
   [
       { extra = "cpu" },
       { extra = "cu124" },
   ],
   ]
   
   [tool.uv.sources]
   torch = [
   { index = "pytorch-cpu", extra = "cpu", marker = "platform_system != 'Darwin'" },
   { index = "pytorch-cu124", extra = "cu124" },
   ]
   torchvision = [
   { index = "pytorch-cpu", extra = "cpu", marker = "platform_system != 'Darwin'" },
   { index = "pytorch-cu124", extra = "cu124" },
   ]
   
   [[tool.uv.index]]
   name = "pytorch-cpu"
   url = "https://download.pytorch.org/whl/cpu"
   explicit = true
   
   [[tool.uv.index]]
   name = "pytorch-cu124"
   url = "https://download.pytorch.org/whl/cu124"
   explicit = true

2. Use the following command to sync with the optional PyTorch dependencies:

   $ uv sync --extra cpu --verbose

3. Observe the debug output:

   DEBUG uv 0.5.2
   DEBUG Found project root: `/home/user/dev/project`
   error: Failed to parse: `pyproject.toml`
   Caused by: TOML parse error at line 16, column 30
   |
   16 |     { index = "pytorch-cpu", extra = "cpu", marker = "platform_system != 'Darwin'" },
   |                              ^^^^^
   unknown field `extra`, expected one of `git`, `subdirectory`, `rev`, `tag`, `branch`, `url`, `path`, `editable`, `index`, `workspace`, `marker`

Problematic Code Snippet

The part of the pyproject.toml file that causes the error is as follows:

[tool.uv.dependencies]
my_package = { index = "pytorch-cu124", extra = "cu124" }

For reference, you can find the original example in the documentation here: PyTorch Integration Guide.

Environment Details

• Current uv platform: WSL

  • Distributor ID: Ubuntu
  • Description: Ubuntu 24.04.1 LTS
  • Release: 24.04
  • Codename: noble

• Current uv version:

  $ uv --version
  uv 0.5.2