Rust 1.99.0, released October 1, 2026, turns one old warning into a hard error: a generic function marked `#[no_mangle]` no longer compiles.
Cargo now switches off incremental compilation whenever the `CI` environment variable is set, which shrank our test `target/` folder from 340 MB to 129 MB.
The same change made a one-line rebuild 5.6 times slower in our test, from 0.79 seconds to 4.43 seconds, unless you set `CARGO_INCREMENTAL=1`.
Most teams can upgrade today. Wait only if a dependency uses generic `#[no_mangle]` items or you cache `target/` between runs.