std Aware Cargo

This repo is a place to focus discussion and work on "std Aware Cargo". This is a feature to allow users of Cargo to build the Rust standard library locally, instead of using the pre-built artifacts shipped with Rust.

We plan to work on multiple RFCs, targeted to specific changes. Work on an experimental implementation will also begin immediately to help work through issues.

Use Cases

It is possible that we will not address all of these use cases, but these are some of the things we are thinking about.

1. Build the standard library with your project with custom profile settings.

   This allows you to use the standard library with different optimization levels, debug settings, etc. See more.

2. Build the standard library (particularly libcore) for an unsupported target.

   See more.

3. Build the standard library with different cfg settings.

   This may be used to disable parts of the standard library, or to select different behaviors. See more.

4. Specify explicit dependencies on sysroot crates in Cargo.toml.

   Primarily this is to remove the need for extern crate. See more.

MVP Implementation

An initial version is available on the latest Rust nightly releases. This version is very minimal, with a simple -Z flag to enable. There are a large number of known issues with this implementation, and it is not intended to work for all targets and should not be used for anything other than experimentation and testing.

Documentation may be found at: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#build-std

RFCs

It is yet to be determined exactly which RFCs will be written to cover the enhancements we want to make. As a rough outline, the following are possibilities:

• Syntax for specifying standard library dependencies in Cargo. #5
• Standard library portability. #8
• Target specification. #6

Getting involved

Everyone is encouraged to jump in to the issue tracker here to discuss specific points. If it looks like something is missing, feel free to open a new issue or leave a comment.

At some point in the future, we will likely start working on the RFCs. If you are interested in helping with that, follow and comment on one of the issues.

Once implementations are available on nightly, helping to test it out is very valuable.

The Cargo team can also be reached on #t-cargo on Zulip.

History

There have been various requests and efforts over the years to define std Aware Cargo and to support building the standard library:

• RFC 1133 proposal
• RFC 2663 proposal
  • Pre-RFC discussion
• #4959 — RFE to add Xargo to Cargo
• #5002 — Explicit standard library dependencies
• #5003 — Implicit standard library dependencies
• Xargo
• cargo-xbuild