Software update: Rest 1.52.1

Rust is a programming language invented by Graydon Hoare and originally developed by Mozilla. It is partly inspired by the programming language C, but has syntactic and semantic differences. It focuses on security and aims to use modern computer systems more efficiently. It will be deployed by Cloudflare, OVH, Mozilla, Deliveroo, Coursera, AppSignal and Threema, among others. Version 1.52.1 was released last month with the following announcement:

Announcing Rust 1.52.1

The Rust team has prepared a new release, 1.52.1, working around a bug in incremental compilation which was made into a compiler error in 1.52.0. We recommend all Rust users, including those currently using stable versions prior to 1.52.0, upgrade to 1.52.1 or disable incremental compilation. Guidance on how to do so is available below.

If you have a previous version of Rust installed via rustup, getting Rust 1.52.1 is as easy as:
rustup update stable
If you don’t have it already, you can get rustup from the appropriate page on our website.


This release works around broken builds on 1.52.0, which are caused by newly added verification. The bugs this verification detects are present in all Rust versions, and can trigger miscompilations in incremental builds, so downgrading to a prior stable version is not a fix.

Users are encouraged to upgrade to 1.52.1 or disable incremental in their local environment if on a prior version: please see the what you should do section for details on how to do so.

Incremental compilation is off by default for release builds, so few production builds should be affected (only for users who have opted in).

Miscompilations that can arise from the bugs in incremental compilation generate incorrect code in final artifacts, essentially producing malformed binaries, which means that in theory any behavior is possible. In practice we are currently only aware of one particular known miscompilation, but bugs due to incremental are notoriously hard to track down: users frequently simply rebuild after some light editing if they see unexpected results from their binaries, and this often causes sufficient recompilation to fix the bug (s).

What is the Rust project going to do to fix this

Short-term plan
We have issued 1.52.1 today which:

  • Disables incremental compilation in the Rust compiler (unless asked for by a new environment variable, RUSTC_FORCE_INCREMENTAL = 1).
  • Improves diagnostic output for the new verification if incremental compilation is enabled, indicating how to work around the bugs by purging incremental state or disabling incremental.

This is intended to be a mitigation that helps the majority of Rust users have an upgrade path to a safe Rust compiler which does not have the risk of miscompiling their code, but also provide the option for users willing to deal with the errors to do so .

We expect to continue to actively invest in fixing the bugs, and depending on our confidence in the fixes, may issue a 1.52.2 point release which backports those fixes to the stable channel. Users wishing to help us test can use the nightly channel, and report bugs to rust-lang / rust with any ICEs they are seeing.

We are also currently not planning to disable incremental on the beta channel, but this decision has not been firmly committed to. A number of fixes are available on 1.53 beta today, so users who wish to continue using incremental may want to switch to that. Nightly will always have the latest in fixes, of course.

Long-term plan
The long-term plan is to fix the bugs! Incremental compilation is the only realistic way for the Rust compiler to be able to provide a fast edit-compile-run cycle for all of its programmers, and so we need to address all of the issues that have been identified thus far via verify-ich . (There are 32 such issues as of this writing, though many are duplicates.)

We are actively investing in this, and a number of bugs have already been identified and fixed. Depending on the state of the fixes, future stable releases (1.53 and onwards) will likely re-enable incremental compilation.

The Rust teams will also be developing plans to ensure we have better tracking systems in place in the future for bugs, both to prevent situations like this from arising again, but also to further increase the stability of our releases by tracking bugs more accurately as they propagate across channels.

Version number 1.52.1
Release status Final
Website The Rust Programming Language Blog
License type Conditions (GNU / BSD / etc.)