Bug Description

Currently, maturin will fill in the version specifier if it is missing from pyproject.toml (it is available in Cargo.toml). However, this is prohibited in the official spec:
https://packaging.python.org/en/latest/specifications/pyproject-toml/#dynamic

If the metadata does not list a key in dynamic, then a build back-end CANNOT fill in the requisite metadata on behalf of the user

So rather than filling in the version, maturin should error and tell the user to add dynamic = ["version"] to their pyproject.toml.

This came up because uv is now enforcing this, and won't install any packages with a missing version specifier in the metadata: astral-sh/uv#9989

Your maturin version (maturin --version)

maturin 1.7.4

Your Python version (python -V)

Python 3.12.2

Your pip version (pip -V)

pip 24.3.1

What bindings you're using

pyo3

Does cargo build work?

• Yes, it works

If on windows, have you checked that you aren't accidentally using unix path (those with the forward slash /)?

• Yes

Steps to Reproduce

You can just go to the official docs:
https://www.maturin.rs/metadata

If you remove the version specifier in the pyproject.toml metadata, it will take the version from the Cargo.toml. This works, but it shouldn't!