feat(inside-rust): announce rustup 1.29.0 beta CFT

Author: rami3l

content/inside-rust/rustup-1.29.0-beta-cft.md

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+path = "inside-rust/2025/12/20/rustup-1.29.0-beta-cft"
+title = "Rustup 1.29.0 beta: Call for Testing!"
+authors = ["rami3l"]
+
+[extra]
+team = "The Rustup Team"
+team_url = "https://rust-lang.org/governance/teams/dev-tools/#team-rustup"
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+
+We are excited to announce that rustup 1.29.0 beta is now available for testing and we are currently looking for testers.
+
+## What's new
+
+The headline of this release is that `rustup` has been enabled to **download components concurrently** and **unpack during downloads** in operations such as `rustup update` or `rustup toolchain` and to concurrently check for updates in `rustup check`, thanks to the GSoC 2025 event. This is by no means a trivial change so a long tail of issues might occur, please report them if you have found any! [pr#4388] [pr#4426] [pr#4436] [pr#4455] [pr#4471] [pr#4605]
+
+As usual, we would be happy to receive regression/breakage reports of any kind, especially regarding the installation and/or usage on the following environments:
+
+* New host platforms:
+  - `sparcv9-sun-solaris` [pr#4380]
+  - `x86_64-pc-solaris` [pr#4380]
+
+* New shells:
+  - `tcsh` [pr#4459]
+  - `xonsh` [pr#4626]
+
+* Environments where you would like to bring your own `rust-analyzer` binary (such as r-a developers and certain Neovim/Helix users): rustup will now consider the `rust-analyzer` binary from `PATH` when the rustup-managed one is not found.
+  [pr#4324]
+
+- Environments where you would like to override an environment variable back to the default: rustup now treats empty values as unset.
+  [pr#4422]
+
+[pr#4324]: https://github.com/rust-lang/rustup/pull/4324
+[pr#4380]: https://github.com/rust-lang/rustup/pull/4380
+[pr#4388]: https://github.com/rust-lang/rustup/pull/4388
+[pr#4422]: https://github.com/rust-lang/rustup/pull/4422
+[pr#4426]: https://github.com/rust-lang/rustup/pull/4426
+[pr#4436]: https://github.com/rust-lang/rustup/pull/4436
+[pr#4455]: https://github.com/rust-lang/rustup/pull/4455
+[pr#4459]: https://github.com/rust-lang/rustup/pull/4459
+[pr#4471]: https://github.com/rust-lang/rustup/pull/4471
+[pr#4605]: https://github.com/rust-lang/rustup/pull/4605
+[pr#4626]: https://github.com/rust-lang/rustup/pull/4626
+
+## How to test
+
+To begin testing this new version, all you need to do is simply switching to the dev environment by setting the following environment variable and update (or install) rustup:
+
+```sh
+RUSTUP_UPDATE_ROOT=https://dev-static.rust-lang.org/rustup
+```
+
+To switch out of the dev environment, just remove that environment variable and do a `rustup self update`.
+
+Finally, please don't forget to check out the corresponding section in our [CHANGELOG.md](https://github.com/rust-lang/rustup/blob/master/CHANGELOG.md#1290---) for the complete list of changes included in this version.
+
+## Acknowledgements
+
+A big thank you to:
+- [@djc] for continuously polishing the codebase to get rid of a whole lot of historical burden and carefully shaping the final form of preliminary concurrency support we have today;
+- [@ChrisDenton] for the careful handling of IO and Windows-related complexities;
+- [@FranciscoTGouveia] for joining me in the GSoC event to investigate the possibilities and lay the foundation of concurrency in rustup;
+- [@Kobzol] for enabling and organizing rustup's participation in GSoC 2025 and improving our CLI's cold start performance;
+- ... and many other contributors who have made this new release possible!
+
+Many thanks for everyone's continued support! Wishing you a magical holiday season surrounded by love, peace, and laughter 🍀
+
+[@djc]: https://github.com/djc
+[@ChrisDenton]: https://github.com/chrisdenton
+[@FranciscoTGouveia]: https://github.com/FranciscoTGouveia
+[@Kobzol]: https://github.com/kobzol