This does not (yet) upgrade ./rust-toolchain
The warnings:
* dead_code "field is never read"
* redundant_semicolons "unnecessary trailing semicolon"
* non_fmt_panic "panic message is not a string literal, this is no longer accepted in Rust 2021"
* unstable_name_collisions "a method with this name may be added to the standard library in the future"
* legacy_derive_helpers "derive helper attribute is used before it is introduced" https://github.com/rust-lang/rust/issues/79202
We still panic in a debug build, so that developers can notice when they
need to add a new static atom after modifying UA sheets.
We also add telemetry to note when this happens, add an app note to a
crash report, in case any crash later on occurs, and re-up the existing,
expired shared memory sheet telemetry probes so we can look at them
again.
Differential Revision: https://phabricator.services.mozilla.com/D73188
