Build some crates with nightly compiler and some with stable -- automatically

[cbrune]

Hi,

First off -- thanks all for working on rules_rust. It is really helpful.

My question is how to support building some crates with nightly and some with stable rustc without using any bazel command line arguments. I know I can add --@rules_rust//rust/toolchain/channel=nightly to the command line to build a crate a with nightly.

For an example, say I have a WORKSPACE with two crates, crate-stable and and crate-nightly. crate-stable is intended to be built with the stable compiler and crate-nightly is intended to built with the nightly compiler.

I would like to build (and my users to build) these crates without knowing about specifying any command line args. So the following is the goal:

$ bazel build //demo:crate-stable     <--- uses stable compiler
$ bazel build //demo:crate-nightly    <--- uses nightly compiler

Question 1: Has anyone does this already?
Question 2: How to do it?

I gather it might involve writing a transition rule that transitions to use the nightly compiler. Any examples of transitions and rules_rust to look at?

Cheers,
Curt

[UebelAndre]

You'd need to write a rule that transitions on @rules_rust//rust/toolchain/channel. We do transitions for the unit testing of this build setting here but to get into the details of actually writing a rule that transitions I'd see:

• https://bazel.build/extending/config#defining-1-2-transitions
• https://bazel.build/rules/lib/builtins/transition

This will allow you to have top level targets that are explicitly built for stable or nightly compilers.

[cbrune]

Thanks! That help a lot. Sorted it out. Thanks again.