BUILD FAILED when installing Ruby 3.4.7 (macOS 26.1, Xcode 26.1)

[simba909]

I've looked around and couldn't find a similar issue. I'm trying to install Ruby 3.4.7:

rbenv install 3.4.7

Which yields:

*BUILD FAILED* (macOS 26.1 on arm64 using ruby-build 20251023)

Looking at the logs, the error seems to be:

[...]
linking miniruby
Undefined symbols for architecture arm64:
  "_coroutine_transfer", referenced from:
      _fiber_setcontext in cont.o
      _thread_sched_wait_running_turn in thread.o
      _thread_sched_wait_running_turn in thread.o
      _nt_start in thread.o
      _co_start in thread.o
      _co_start in thread.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [miniruby] Error 1
external command failed with status 2

brew doctor output says "Your system is ready to brew.", the output of brew config says:

HOMEBREW_VERSION: 4.6.20
ORIGIN: https://github.com/Homebrew/brew
HEAD: 71358caec4959d8c06b9d469ebc09fa7aa7a210e
Last commit: 10 days ago
Branch: stable
Core tap JSON: 11 Nov 18:03 UTC
Core cask tap JSON: 11 Nov 18:03 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_EDITOR: vim
HOMEBREW_FORBID_PACKAGES_FROM_PATHS: set
HOMEBREW_MAKE_JOBS: 14
Homebrew Ruby: 3.4.5 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.4.5/bin/ruby
CPU: 14-core 64-bit arm_brava
Clang: 17.0.0 build 1700
Git: 2.50.1 => /Applications/Xcode-26.1.0.app/Contents/Developer/usr/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 26.1-arm64
CLT: 26.1.0.0.1.1761104275
Xcode: 26.1 => /Applications/Xcode-26.1.0.app/Contents/Developer
Rosetta 2: false

[simba909]

Answering this myself; the issue was that I had Swiftly installed and on my $PATH. For some reason this interfered with the install. If you have version 1.1.0 (or later) of the tool, you can temporarily unlink it:

swiftly unlink

Then, after installing Ruby, you can re-link it:

swiftly link