rust-latest

A CLI tool to determine the latest version of the Rust toolchain. For nightly, it finds the latest version to include builds of all the builtin components (clippy, rustfmt, rls, etc.).

$ rust-latest
1.34.1
$ rust-latest -c nightly
nightly-2019-05-04

This tool does not actually download the toolchain; that job is still left to rustup. This tool is intended to be used to set your project's rust-toolchain file. Read more about what this is below.

$ rustup show active-toolchain
stable-x86_64-unknown-linux-gnu (default)
$ rust-latest -c nightly > rust-toolchain
$ rustup show active-toolchain
nightly-2019-05-04-x86_64-unknown-linux-gnu (overridden by '/path/to/project/rust-toolchain')

The Problem

The Rust language has a lot of updates, which can be both a good thing and a bad thing. It's a good thing because new features are being added to the language constantly. It's a bad thing because (especially on nightly), these frequent changes can break your builds without your code changing at all!

Thankfully, rustup allows you to pin the Rust version for your project with the rust-toolchain file. This enables a few things:

First, it prevents the version of Rust your project builds with from changing when a new version is installed on your system. This allows you to control exactly when you want to upgrade the version of Rust you're using for that project. It enables a sort of "virtual environment" for your project which is independent of the system-wide Rust toolchain (rustup default).

Second, it lets you declare a version of Rust that your project is known to be compatible with, much like Cargo.toml does for your dependencies. This is a bit different from Cargo.toml because you use a specific pinned version instead of a Semver range, but it serves a similar purpose.

Third, it ensures repeatable builds of your project: everyone who builds your project, whether it be you, another contributor, you a month from now, or your CI server, will build it with exactly the same (or a compatible set of) dependencies and toolchain.

"rust-toolchain sounds great!" you say, "but what do I put in it?" That's the central problem this tool aims to solve.

The Solution

The first step to deciding what you should put in your rust-toolchain file is to decide what release channel you want to use. This decision is outside the scope of this tool, but you can read up on the differences between stable, beta, and nightly here and here.

The next step, which this tool solves for you, is to decide which release in that channel to use. When you don't have a particular preference for a version, or just want the latest version, how do determine the actual label?

If you're using nightly, you can go look at the rustup components history and pick a date from there. Usually I prefer a nightly build that has all the package components available, since I need clippy, rls, and rust-src for development. So I scan the table for the last date that has green boxes for all those components. But if the last release to contain the package was older than a week, it wouldn't show up on the table. Also, not every day has a nightly release.

If you're using stable, you can look at the Rust release history to get a release version number to use.

I've never really used beta, so I'm not actually sure how you would normally find a list of releases for it.

But I don't want to have to do any of that manual work! I wanted a tool that could perform all of these checks automatically, which is what rust-latest does. Given a release channel, a list of desired components, and a list of build targets, it finds the last release on that channel which had all of the components available for every target. To make this a bit easier on the user, it actually checks for releases containing everything except a few components which are platform-specific. If you run the tool on one of these platforms and specify --target current, it will detect this case and include those components. By default, the tool will look in the stable channel back at most 90 days for releases which include the components for all Tier 1 targets.

Usage

Installing the tool is as easy as cargo install rust-latest!

$ rust-latest --help
rust-latest 1.3.0
Daniel Beckwith <djbsnx@gmail.com>
Determines the last known complete build of a Rust toolchain.

USAGE:
    rust-latest [FLAGS] [OPTIONS]

FLAGS:
    -d, --force-date    Whether date-stamped toolchains like stable-2019-04-25 should be used instead of version numbers
                        for stable releases.
    -h, --help          Prints help information
    -V, --version       Prints version information

OPTIONS:
    -c, --channel <channel>    Release channel to use. [default: stable]
    -a, --max-age <max_age>    Number of days back to search for viable builds. This is relative to the latest release
                               of the channel. [default: 90]
    -p, --profile <profile>    Which package profile to use. [default: default]  [possible values: complete, default,
                               minimal]
    -t, --targets <targets>    Which set of targets to filter by, either all Tier-1 targets or only the current target.
                               [default: all]  [possible values: all, current]

When you run the tool, it will output a toolchain name that can be used in your rust-toolchain file:

$ rust-latest
1.34.1
$ rust-latest --force-date
stable-2019-04-25
$ rust-latest -c nightly
nightly-2019-05-04

Note that the tool requires an internet connection and can take a while to complete, as it has to download release manifests that can be serveral hundred kilobytes each. The farther back in time it has to search for a viable release, the longer it will take.

Contributing

If you have any problems using this tool or ideas for improvement, please create an issue and I'll respond as soon as I can!