[Bug 2143149] Re: [FFE] Upgrade Rust to 1.92.0

[Jeremy Bícha]

I think the status of freeze exceptions is supposed to be New or
Confirmed until approved by the Release Team, when it would be set to
Triaged.

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Title:
  [FFE] Upgrade Rust to 1.92.0

Status in rust-defaults package in Ubuntu:
  In Progress
Status in rust-defaults source package in Resolute:
  In Progress

Bug description:
  ## FFE ##
  =========
  Rationale
  =========
  rust-defaults in Ubuntu currently points to Rust 1.88.0. Debian
  unstable uses Rust 1.91.1 as its Rust version, meaning that certain
  Ubuntu packages don't have sufficiently new Rust toolchains to build
  properly. Pointing rust-defaults to 1.92.0 ensures that we will be able
  to build everything we need.

  The FFe was necessitated in the first place by long RISC-V build times
  on an LLVM transition — rustc-1.92 itself has been in the archive for a
  while and has no known issues.

  Apart from giving users access to the most up-to-date Rust toolchain
  available, there are also the following notable changes specific to the
  Ubuntu package:
  - Rust 1.92 uses LLVM 21 instead of 20. This is important because the
    removal of LLVM 20 is planned for Debian and eventually Ubuntu.
  - Rust 1.92 on riscv64 is able to build for the RVA23 profile, allowing
    RISC-V Rust applications to take full advantage of the RVA23
    standards already required by this Ubuntu release.

  ================
  Proposed Changes
  ================
  - rust-defaults now points to rustc-1.92 instead of rustc-1.88, meaning
    that future Rust package builds will use Rust 1.92 instead of 1.88.
    * The upstream Rust team maintains a strict policy of backwards
      compatibility, meaning that any future rebuilds of packages are
      expected to work without issue.
    * Since Rust applications are statically linked, updating the default
      toolchain version won't trigger any rebuilds. It will only affect
      subsequent builds of Rust packages.
    * Rust 1.92 uses LLVM 21 instead of 20, so problems specific to LLVM
      21 could potentially cause issues, but none are anticipated.
  - A typo in the rust-gdbgui binary symlink was fixed.
    * Regression risk is minimal, as rust-gdbgui has been heretofore
      unusable, meaning we know that no packages in the archive currently
      rely on rust-gdbgui.
  - The dependencies of a particular autopkgtest have been moved to LLVM
    21.
    * This is purely a change to the autopkgtest to reflect the LLVM 21
      of the rest of the toolchain. It does not affect the distributed
      packages.

  ================
  seeded-in-ubuntu
  ================
  $ seeded-in-ubuntu rust-defaults
  cargo-doc (from rust-defaults) is seeded in:
    ubuntu-budgie: supported
  libstd-rust-dev (from rust-defaults) is seeded in:
    ubuntu-budgie: supported
    ubuntu: supported
  rust-doc (from rust-defaults) is seeded in:
    ubuntu-budgie: supported

  ==========
  PPA builds
  ==========
  The package builds for all architectures in a PPA:
  https://launchpad.net/~maxgmr/+archive/ubuntu/rust-defaults-1.92.0/+packages

  ============
  autopkgtests
  ============
  amd64: https://autopkgtest.ubuntu.com/results/autopkgtest-resolute-maxgmr-rust-defaults-1.92.0/resolute/amd64/r/rust-defaults/20260304_015947_6b936@/log.gz
  arm64: https://autopkgtest.ubuntu.com/results/autopkgtest-resolute-maxgmr-rust-defaults-1.92.0/resolute/arm64/r/rust-defaults/20260303_215342_4eae7@/log.gz
  armhf: https://autopkgtest.ubuntu.com/results/autopkgtest-resolute-maxgmr-rust-defaults-1.92.0/resolute/armhf/r/rust-defaults/20260303_213728_74928@/log.gz
  ppc64el: https://autopkgtest.ubuntu.com/results/autopkgtest-resolute-maxgmr-rust-defaults-1.92.0/resolute/ppc64el/r/rust-defaults/20260303_204604_7a9d3@/log.gz
  s390x: https://autopkgtest.ubuntu.com/results/autopkgtest-resolute-maxgmr-rust-defaults-1.92.0/resolute/s390x/r/rust-defaults/20260303_214508_fea23@/log.gz

  ==================
  Additional testing
  ==================
  In order to reduce the chances that a new Rust toolchain causes FTBFS on
  package rebuilds, I test-built, using rust-defaults 1.92.0, every
  package in main that Build-Depends on the Rust toolchain:
  https://launchpad.net/~maxgmr/+archive/ubuntu/rust-defaults-1.92.0-main-test/+packages

  ==============================
  Installation and local testing
  ==============================
  root at r:~# add-apt-repository -y ppa:maxgmr/rust-defaults-1.92.0
  [...]
  root at r:~# apt install -y rust-all
  [...]
  root at r:~# rust-gdbgui
  rust-gdbgui
  ===========
  gdbgui - https://gdbgui.com - is a graphical front-end to GDB
  that runs in a browser. This script invokes gdbgui with the Rust
  pretty printers loaded.
  [...]
  root at r:~# cargo new hello
      Creating binary (application) `hello` package
  note: see more `Cargo.toml` keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
  root at r:~# cd hello
  root at r:~/hello# cargo run --release
     Compiling hello v0.1.0 (/root/hello)
      Finished `release` profile [optimized] target(s) in 0.19s
       Running `target/release/hello`
  Hello, world!

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