rustc 1.80 now complains about features not declared in Cargo.toml and cfg
keys/values not declared by build.rs to protect against typos or misuse (you
think you're using the right condition but you're not). See
rust-lang/cargo#10554 and rust-lang/rust#82450.
(We're not actually using TSAN under CI at this time, but I do want to re-enable
it at some point — especially if we get multithreaded execution going — using
the rust-native TSAN configuration.)
I'll be updating the `rsconf` crate and patching `build.rs` accordingly to also
handle the warnings about unknown cfg values, but tsan is a feature and not a
cfg and these can be dealt with in `Cargo.toml` directly.
As implied by the changelog.
Unfortunately it's not obvious how to access the RefCell value in spite
of a potential (albeit unlikely) present mutable borrow. We need to use a
different type to make it work in such cases, hopefully doing that in future.
In future we could even use panic=abort and use this style of cleanup for
panics (instead of RAII).
