This is to support two things:
1. Early switch to 2021 Edition, which is stable Oct 21; and
2. To make use of unstable const features.
The rationale is that switching to nightly does not really have any
significant downside for us, given that TAMER is used only by us and
the only risk is that unstable features may change a bit, which can be
mitigated with certain precautions.
The rationale for each unstable feature will be documented as they are used,
including documentation on what would be required to remove it and what
functionality would be lost / need to change in doing so.
This is enabled by default in nightly, and is not available at all in
stable. Considering the PITA that it will be to go back and rewrite docs to
use the new format, and how important of a feature this is, we will just
make use of it now.
Given that developers should be doing TDD and therefore running this target
frequently, this has the effect of providing immediate feedback when
formatting is needed and outputting a diff. Developers will then quickly
understand what changes need to be made to avoid future issues (and can run
`cargo fmt` to fix it), at which point they'll rarely ever encounter
formatting errors.
The original purpose was to ensure pipelines fail when the formatter has not
been run.
Mike Gerwitz