build-std: explicit dependencies

[davidtwco]

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Allow users to add explicit dependencies on standard library crates in the Cargo.toml. This enables Cargo to determine which standard library crates are required by the crate graph without build-std-crates being set and for different crates to require different standard library crates.

This RFC is is part of the build-std project goal and a series of build-std RFCs:

1. build-std context (build-std: context #3873)
2. build-std="always" (build-std: always #3874)
3. Explicit standard library dependencies (this RFC)
4. build-std="compatible" (RFC not opened yet)
5. build-std="match-profile" (RFC not opened yet)

There is also a Zulip channel where you can ask questions about any of the build-std RFCs. This series of RFCs was drafted over many months with the help of stakeholders from many Rust project teams, we thank them for their help!

There are some details that have been noted as being worth mentioning in any eventual tracking issues:

• build-std: explicit dependencies #3875 (comment)

FCP

Rendered

[epage]

@rfcbot fcp resolve redundant-no-std

[weihanglo]

Thanks and sorry for the late review.

View changes since this review

[weihanglo]

@rfcbot reviewed

[arlosi]

Do we need to consider what happens when a crate with builtin deps is uploaded to an existing 3rd party registry without explicit support for it?

View changes since the review

[davidtwco]

I'm not sure that this is any different than any other change or evolution of the registry index format such that it needs addressed specifically in this RFC. As I understand it, the format has been evolved in the past so could be again and there are means for doing so?

[davidtwco]

@rust-rfcbot is having some issues, so leaving this comment to note that all but two checkboxes in the FCP have been checked, which means this RFC should be entering its 10 day final comment period.

[rust-rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔