Stabilize cargo-script

Summary

Stabilize support for "cargo script", the ability to have a single file that contains both Rust code and a Cargo.toml.

Motivation

Being able to have a Cargo package in a single file can reduce friction in development and communication, improving bug reports, educational material, prototyping, and development of small utilities.

The status quo

Today, at minimum a Cargo package is at least two files (Cargo.toml and either main.rs or lib.rs). The Cargo.toml has several required fields.

To share this in a bug report, people resort to

• Creating a repo and sharing it
• A shell script that cats out to multiple files
• Manually specifying each file
• Under-specifying the reproduction case (likely the most common due to being the eaisest)

To create a utility, a developer will need to run cargo new, update the Cargo.toml and main.rs, and decide on a strategy to run this (e.g. a shell script in the path that calls cargo run --manifest-path ...).

The next six months

The support is already implemented on nightly. The goal is to stabilize support. With RFC #3502 and RFC #3503 approved, the next steps are being tracked in rust-lang/cargo#12207.

At a high-level, this is

• Add support to the compiler for the frontmatter syntax
• Add support in Cargo for scripts as a "source"
• Polish

The "shiny future" we are working towards

Design axioms

• In the trivial case, there should be no boilerplate. The boilerplate should scale with the application's complexity.
• A script with a couple of dependencies should feel pleasant to develop without copy/pasting or scaffolding generators.
• We don't need to support everything that exists today because we have multi-file packages.

Ownership and team asks

Owner: epage

This section defines the specific work items that are planned and who is expected to do them. It should also include what will be needed from Rust teams.

• Subgoal:
  • Describe the work to be done and use ↳ to mark "subitems".
• Owner(s) or team(s):
  • List the owner for this item (who will do the work) or if an owner is needed.
  • If the item is a "team ask" (i.e., approve an RFC), put and the team name(s).
• Status:
  • List if there is an owner but they need support, for example funding.
  • Other needs (e.g., complete, in FCP, etc) are also fine.

Task	Owner(s) or team(s)	Notes
Implementation	Ed Page
Stabilization decision	lang cargo