compilation issue on nightly-2026-05-01

[funemy]

Hi!

Trying to bump the version of rustc_plugin and rustc_utils from 0.13 to 0.15.

Running into the compilation issue below:

error[E0464]: multiple candidates for `rmeta` dependency `smallvec` found
  --> /Users/xxx/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustc_utils-0.15.2-nightly-2026-05-01/src/lib.rs:58:1
   |
58 | extern crate smallvec;
   | ^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: candidate #1: /Users/xxx/.rustup/toolchains/nightly-2026-05-01-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libsmallvec-6d85e44af6dc7828.rmeta
   = note: candidate #2: /Users/xxx/.rustup/toolchains/nightly-2026-05-01-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libsmallvec-f1f1de05f1c8d8ac.rmeta

For more information about this error, try `rustc --explain E0464`.
error: could not compile `rustc_utils` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...

I investigated a bit as I saw flowistry is already using 0.15 and the ci was green.

As far as I can tell, this is an Intel Mac only issue.
Running the script below:

DATE=2026-05-01
for TARGET in x86_64-apple-darwin aarch64-apple-darwin x86_64-unknown-linux-gnu; do
  echo "=== $TARGET ==="
  curl -sL "https://static.rust-lang.org/dist/$DATE/rustc-dev-nightly-$TARGET.tar.xz" \
    | xz -d | tar -t 2>/dev/null | grep 'libsmallvec.*\.rmeta'
done

returns this

=== x86_64-apple-darwin ===
rustc-dev-nightly-x86_64-apple-darwin/rustc-dev/lib/rustlib/x86_64-apple-darwin/lib/libsmallvec-6d85e44af6dc7828.rmeta
rustc-dev-nightly-x86_64-apple-darwin/rustc-dev/lib/rustlib/x86_64-apple-darwin/lib/libsmallvec-f1f1de05f1c8d8ac.rmeta
=== aarch64-apple-darwin ===
rustc-dev-nightly-aarch64-apple-darwin/rustc-dev/lib/rustlib/aarch64-apple-darwin/lib/libsmallvec-b68aa81de4c19994.rmeta
=== x86_64-unknown-linux-gnu ===
rustc-dev-nightly-x86_64-unknown-linux-gnu/rustc-dev/lib/rustlib/x86_64-unknown-linux-gnu/lib/libsmallvec-7d77e97de99657ee.rmeta

Thought this is might be worth reporting :).