add blog post for rustup 1.25.1

Author: pietroalbini

posts/2022-07-12-Rustup-1.25.1.md

@@ -0,0 +1,86 @@
+---
+layout: post
+title: "Announcing Rustup 1.25.1"
+author: The Rustup Working Group
+---
+
+The rustup working group is announcing the release of rustup version 1.25.1.
+[Rustup][install] is the recommended tool to install [Rust][rust], a
+programming language that is empowering everyone to build reliable and
+efficient software.
+
+If you have a previous version of rustup installed, getting rustup 1.25.1 is as
+easy as stopping any programs which may be using Rustup (e.g. closing your IDE)
+and running:
+
+```
+rustup self update
+```
+
+Rustup will also automatically update itself at the end of a normal toolchain
+update:
+
+```
+rustup update
+```
+
+If you don't have it already, you can [get rustup][install] from the
+appropriate page on our website.
+
+[rust]: https://www.rust-lang.org
+[install]: https://rustup.rs
+
+## What's new in rustup 1.25.1
+
+This version of rustup fixes a regression introduced in the previous release
+(1.25.0), which caused some workflows to fail.
+
+### Regression in nested Cargo invocations with different toolchains
+
+When you invoke Rust or Cargo installed by rustup, you're not running them
+directly. Instead, you run [rustup "proxy" binaries][proxies], whose job is to
+detect the right toolchain (parsing the `+channel` CLI argument or using one of
+the defaults) and run it.
+
+Running these proxies is not instantaneous though, and for example a `cargo
+build` invocation might execute several of them (the initial `cargo` invocation
+plus one `rustc` for every dependency), slowing down the build.
+
+To improve performance, rustup 1.25.0 changed the proxies code to set the
+[`RUSTC` and `RUSTDOC` environment variables][cargo-env] when missing, which
+instructed Cargo to skip the proxies and invoke the binaries defined in those
+variables directly. This provided a performance gain when building crates with
+lots of dependencies.
+
+Unfortunately this change broke some users of rustup, who did something like:
+
+* The first Cargo invocation (for example an extension, an alias or an
+  integration test) uses toolchain `foo`, setting the `RUSTC` and `RUSTDOC`
+  environment variables pointing to that toolchain.
+
+* The first invocation calls Cargo again, but this time using toolchain `bar`
+  (for example `cargo +bar build`). This does **not** set the `RUSTC` and
+  `RUSTDOC` environment variables pointing to `bar`, as those variables are
+  already present.
+
+* The second invocation of Cargo then invokes rustc by using the `RUSTC`
+  environment variable and skipping the proxy, which results in the `foo`
+  toolchain being invoked. Previous versions of rustup invoked the proxy
+  instead, which would correctly detect and use the `bar` toolchain.
+
+Rustup 1.25.1 fixes this regression by reverting the change. The rustup working
+group is discussing in [issue #3035][issue] plans to re-introduce the change in
+a future release while avoiding breakage.
+
+[proxies]: https://rust-lang.github.io/rustup/concepts/proxies.html
+[cargo-env]: https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-reads
+[issue]: https://github.com/rust-lang/rustup/issues/3035
+
+## Thanks
+
+Thanks again to all the contributors who made rustup 1.25.1 possible!
+
+- Daniel Silverstone (kinnison)
+- Robert Collins (rbtcollins)
+- Joshua Nelson (jyn514)
+- Pietro Albini (pietroalbini)