build-std: explicit dependencies

[davidtwco]

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]

Is this about adding them to the index (as top-level packages), which isn't needed, or adding dependencies on the standard library to the index format?

[davidtwco]

Just the latter, clarified in 81298d9

[epage]

Still not clear to me why they need to be in the index

[davidtwco]

This was my understanding from @ehuss's earlier feedback but maybe I've misunderstood it?

[davidtwco]

I'm not sure if this resolved your concern but given it's been a few weeks, I'll resolve this conversation thread so it's easier to keep track of what I still need to address. Feel free to reopen or start another thread.

[epage]

Not resolved.

[epage]

@rfcbot fcp concern redundant-no-std

See #3875 (comment)

[epage]

@rfcbot fcp resolve redundant-no-std