`comment_width = 80` counts the whole line, indentation included, which
wraps far more aggressively than intended; the latest nightly rustfmt
(rust-lang/rustfmt#6802) started enforcing it on comments it previously
left alone. Turn `wrap_comments` off rather than reformat everything.
`max_width = 100`, `comment_width = 80` and `use_small_heuristics =
"Default"` are rustfmt's defaults, so drop them too; only the two
nightly-only import options remain.
The page said encrypted bridges cannot work with MAS and that a solution
was being worked on. That solution is MSC4190, stable since Matrix 1.17
and enabled for every application service in Synapse 1.141.0, so the
limitation no longer exists.
Replace the page with a readiness item on the migration page, which is
where the original warning was aimed, and redirect the old URL there.
The `rust-toolchain` ecosystem updates the `channel` in `rust-toolchain.toml`, which is now the only place the Rust version is written down. Weekly with the usual 14-day cooldown, so the `.1` point release has usually landed by the time the PR opens.
The builder stage starts from `buildpack-deps` (what the `rust:` image is built on anyway), installs a pinned `rustup-init` with `--default-toolchain none`, copies only `rust-toolchain.toml` and lets rustup install the toolchain it names. The Rust version is now written in exactly one place, shared with CI, so a toolchain bump needs no Dockerfile edit.
The toolchain layer is cached on the content of `rust-toolchain.toml` and the base image, and it is the same bytes we pulled as `rust:` image layers before, so build times are unchanged.
The registry cache mount also moves to `/usr/local/cargo/registry`: the `rust:` image already set `CARGO_HOME=/usr/local/cargo`, so the previous `/root/.cargo/registry` mount never held anything.
Until now only the clippy CI job and the Dockerfile named a Rust version (kept in sync by hand), while every other CI job, the release binaries and the docs build ran on whatever `stable` happened to be that day. Rust 1.98.0 landing on 2026-08-20 broke `build-binaries` overnight because of that (#5935).
`rust-toolchain.toml` pins 1.96.0 with the `minimal` profile plus clippy, and every `rustup toolchain install stable ...` in CI becomes a bare `rustup toolchain install`, which reads the file. Starting at 1.96.0 (the version clippy is already clean against) keeps this change free of lint churn; catching up to 1.98.0 is a follow-up.
rustfmt stays on nightly because `.rustfmt.toml` uses nightly-only options, so that job now invokes `cargo +nightly fmt` explicitly instead of setting a rustup directory override, which would silently take precedence over the toolchain file.
The file does not list the linux cross-compilation targets on purpose: that would make every developer and CI job download `rust-std` they never use. The two consumers that cross-compile add the targets themselves.
Paul Chobert