Respect metadata_directory in build_wheel per PEP 517

[messense]

When a PEP 517 frontend passes metadata_directory to build_wheel, the built wheel should use the pre-generated metadata from that directory. This enables wrapping build backends that modify metadata in prepare_metadata_for_build_wheel.

Python side: when metadata_directory is not None, set MATURIN_PEP517_METADATA_DIR in the subprocess environment.

Rust side: write_dist_info() checks the env var and copies files from the pre-existing .dist-info directory instead of regenerating them. The WHEEL file is always regenerated to ensure correct tags, and RECORD is skipped since it is generated by WheelWriter::finish(). If the env var is set but the expected directory does not exist, an error is raised instead of silently falling back.

Fixes #1973

[Copilot]

Pull request overview

Implements PEP 517 build_wheel(..., metadata_directory=...) support by reusing pre-generated .dist-info metadata created by prepare_metadata_for_build_wheel, enabling wrapper backends to adjust metadata without repacking wheels.

Changes:

• Python PEP 517 wrapper passes metadata_directory to the Rust subprocess via MATURIN_PEP517_METADATA_DIR.
• Rust wheel metadata writer conditionally copies .dist-info contents from MATURIN_PEP517_METADATA_DIR, while always regenerating WHEEL and skipping RECORD.
• Adds a unit test ensuring copied metadata is used and WHEEL/RECORD behaviors are correct.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/module_writer/mod.rs	Adds env-var-driven branch to copy pre-generated .dist-info content into the wheel.
src/module_writer/mock_writer.rs	Adds unit test for env-var metadata directory behavior (copy vs regenerate/skip).
maturin/__init__.py	Propagates metadata_directory into the maturin subprocess environment.

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[Copilot]

Pull request overview

Implements PEP 517’s build_wheel(..., metadata_directory=...) behavior by allowing the wheel build step to reuse pre-generated .dist-info metadata (typically produced by prepare_metadata_for_build_wheel), enabling wrappers that adjust metadata without repacking the wheel.

Changes:

• Python: when metadata_directory is provided to build_wheel, pass it to the Rust subprocess via MATURIN_PEP517_METADATA_DIR.
• Rust: write_dist_info() can copy existing .dist-info contents from disk (skipping RECORD, regenerating WHEEL) instead of regenerating all metadata.
• Tests: add a unit test asserting that copied metadata is used and that WHEEL/RECORD handling is correct.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/module_writer/mod.rs	Adds env-var-controlled path to reuse/copy pre-generated .dist-info metadata during wheel creation.
src/module_writer/mock_writer.rs	Adds a test covering the “reuse pre-generated metadata” path and validates WHEEL regeneration / RECORD omission.
maturin/__init__.py	Sets MATURIN_PEP517_METADATA_DIR for the Rust subprocess when metadata_directory is provided to build_wheel.

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