Preserve feature metadata in packaged Cargo.toml

[weihanglo]

Problem

The implementation of feature metadata in #15056 currently normalizes the metadata table to an array of enabled features, which is a lossy normoralization.

• cargo info and cargo metadata cannot retrieve the information for registry dependencies.
• package.rust-version may drift from registry dependencies and the git repo if people want to patch. The latter will require higher MSRV supporting the new syntax.

See #14157 (comment)

Steps

No response

Possible Solution(s)

The published manifest generated by cargo package/cargo publish should contain whatever new metadata fields written in the original Cargo.toml. We may want to additionally check whether package.rust-version is compatible if those fields are present.

Notes

No response

Version

cc5596f058e623d22be718adf541a9bd766d9417

[tgross35]

I noted this in #14157 (comment) but one option is to normalize only if package.rust-version is a version lower than when Cargo started to recognize the syntax. That means you can start adding docs as is convenient rather than waiting for a MSRV bump and get their benefits when running cargo doc/cargo metadata locally, as well as with any tooling that cares to read Cargo.toml.orig. Then a MSRV bump just stops normalizing and makes it easier to use downstream.

But there is a reasonable argument against doing so in the reply to that comment, and I definitely wouldn't prioritize this over anything else.