What is this issue?

If you're a crate author who's been linked here, this issue tracks removing a backwards-compatibility hack in Rust.

Rust has a longstanding issue #43081, which causes procedural macros to lose location and hygiene information (known as a "Span") under certain circumstances. Recently, pull request #73084 was merged, which makes progress towards resolving #43081.

Unfortunately, older versions of certain procedural macro crates (such as proc-macro-hack v0.5.15 and js-sys v0.3.39) cannot handle the changes in input caused by the Rust bugfix. To allow these crates to continue to compile, a backward-compatibility hack was added to adjust the input passed to proc-macro-hack and js-sys specifically.

Eventually, we would like to remove this backwards-compatibility hack, since the compiler should not have hard-coded exceptions for certain crates. However, removing this hack will break any crates that depend on affected versions of proc-macro-hack or js-sys.

To ensure that your crate continues to work, you'll want to ensure that your Cargo.lock references proc-macro-hack v0.5.16 or above, and js-sys v0.3.40 or above. This can be done by running cargo update -p proc-macro-hack and cargo update -p js-sys. If you maintain a library crate (without a Cargo.lock, no action is needed on your part).

Internal compiler details

In #73084 (comment), I added a hack to change the behavior of NtIdents passed to certain proc-macros. This was done by special-casing certain identifiers, and should be eventually be removed in favor of a proper solution.

If we decide to always wrap single identifiers in None-delimited groups, then we will need to wait until enough of the ecosystem has bumped proc-macro-hack and wasm-bindgen, to avoid breaking a large number of crates.

Crater run: https://crater-reports.s3.amazonaws.com/pr-73084-1/index.html
Triage: https://hackmd.io/O7icbSylRP6uVZyAQ9EDeA