* ci: Update actions, replace actions-rs.
* `actions/checkout` is updated from `v1` to the current `v4`.
* `actions-rs/toolchain` is replaced by `dtolnay/rust-toolchain` as
the `actions-rs` actions haven't been maintained in a long time.
* clippy: Remove unnecessary call to `into_iter`.
The parameter takes `IntoIterator`, so we don't have to call
`into_iter` at the call site.
* clippy: Remove explicit lifetime that can be elided.
* clippy: tests: Fix useless conversion warnings.
* clippy: tests: Remove call to `format!`.
* Fix minimal-versions build.
Due to changes in the nightly compiler, using a recent nightly
requires proc-macro2 1.0.60 or later:
https://github.com/dtolnay/proc-macro2/issues/356
* ci: Use is-terminal 0.4.7 for MSRV builds.
is-terminal 0.4.8 updated its MSRV to 1.63, so we can't use it
with our MSRV of 1.56. Force usage of the older version which has
an older MSRV.
* Add more tests for diagnostic_source
* Use Borrow::borrow instead of AsRef
AsRef is not reflexive, meaning that it is not implemented as &T for all
T. Borrow is though, so we use that to make sure that we always get a
reference that is correct, even in the presence of smart pointers.
Bruce Mitchener
Will Crichton
Marcel Müller
Matthias Beyer