Rust installed via Homebrew lacks rust-src, preventing embedded (no_std) builds

[luofang34]

brew gist-logs <formula> link OR brew config AND brew doctor output

$brew config
HOMEBREW_VERSION: 4.4.19
ORIGIN: https://github.com/Homebrew/brew
HEAD: 23facb219df8615680636ef12103bb3313f44e2a
Last commit: 2 days ago
Branch: stable
Core tap JSON: 29 Jan 06:51 UTC
Core cask tap JSON: 29 Jan 06:51 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_DISPLAY: /private/tmp/com.apple.launchd.LqH8qgQdBZ/org.xquartz:0
HOMEBREW_MAKE_JOBS: 16
Homebrew Ruby: 3.3.7 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.3.7/bin/ruby
CPU: 16-core 64-bit arm_palma
Clang: 16.0.0 build 1600
Git: 2.47.1 => /opt/homebrew/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 15.3-arm64
CLT: 16.2.0.0.1.1733547573
Xcode: 16.2
Rosetta 2: false

$brew doctor
Your system is ready to brew.

Verification

• My brew doctor output says Your system is ready to brew. and am still able to reproduce my issue.
• I ran brew update and am still able to reproduce my issue.
• I have resolved all warnings from brew doctor and that did not fix my problem.
• I searched for recent similar issues at https://github.com/Homebrew/homebrew-core/issues?q=is%3Aissue and found no duplicates.

What were you trying to do (and why)?

When installing Rust via Homebrew (brew install rust), the rust-src component is missing by default. This prevents compilation of embedded (no_std) projects, as the core and alloc libraries are unavailable for targets like thumbv7em-none-eabihf.

What happened (include all command output)?

Attempting to compile an embedded Rust project leads to:

error[E0463]: can't find crate for `core`
= note: the `thumbv7em-none-eabihf` target may not be installed

and running

ls $(rustc --print sysroot)/lib/rustlib/thumbv7em-none-eabihf/lib/

returns no libcore.rlib

What did you expect to happen?

The Homebrew Rust package could include rust-src by default.
Alternatively, brew info rust could warn users that embedded Rust requires rust-src

Step-by-step reproduction instructions (by running brew commands)

brew install rust
rustup target add thumbv7em-none-eabihf
cargo new test-project && cd test-project
echo '#![no_std] #![no_main]' > src/main.rs
cargo build --release --target thumbv7em-none-eabihf