PSA: Incoming breakage in 1.100. Test your code on beta Rust.

TL;DR: Rust 1.100 is releasing on November 12 (in a month). I recommend testing your code with cargo +beta test at least once before then.

The upcoming release of Rust 1.100 has some big features. However, there are some details being changed in how the language works in order to support these new features. The Rust project has tried to minimize the amount of breakage that happens, but the amount of breakage won't be zero. Every rust release has some amount of minor breakage like this, but 1.100 is expected to have relatively larger amounts of breakage.

So, if you plan to ever upgrade to 1.100, or if you write code meant for use by other people that might upgrade to 1.100, then I highly recommend testing your code with cargo +beta test to see if you're affected. And even if you think you don't write code that's affected, some code in your dependencies might be affected.

There will be two main sources of breakage in Rust 1.100:

1. The stabilization of the never type. Some details of how types are inferred will be changed, causing types to infer as ! instead of the previous type of

()

1. . Also, some code was previously using some unsupported hacks to refer to this never type on stable rust, and will be broken by the fact that the Infallible type will become an alias for the never type. For more info, see Waffle's talk on the topic, and the pull request where stabilization happened.
2. A change in the layout of Cargo's target directory. Some tools and some build scripts were expecting certain files to be at certain places, and will start to fail in Rust 1.100. For more info, see the call for testing blog post,and the pull request where stabilization happened.