Problem

There is no way to "unset"/reset to "default, the number of build jobs specified to Cargo.

Consider your team has a .cargo/config.toml located in the root of your repository. You want to set the following:

[build]
jobs = -1

So that on all developer's machines, one core will be left available when running cargo for your computer to do other tasks.

But your CI infrastructure also runs cargo, and you want CI to use all possible cores to makes builds as fast as possible. You can set the CARGO_BUILD_JOBS environment variable, but via the env variable (or --jobs cli option) but there is no way to override the number of jobs back to "default"/num cpus.

Proposed Solution

A solution is to define a new value that represents the default/num_cpus state. Two proposals would be:

1. Reinterpret 0 jobs as meaning "default"
   a. Today specifying 0 as the number of jobs returns an error, so changing this shouldn't(?) break any flows
2. Add a new supported value like "d", "default", or "num_cpus" that when specified we use the default value

Personally I'd prefer option 1 as it seems like the easiest, but I'm not sure if there are any flows that rely on cargo returning an error.

Notes

No response