As shown below, the build succeeds, but the program cannot be executed.
[omochi@omochi-mbp swift-subprocess-rpath-bug (main =)]$ uname -a
Darwin omochi-mbp.local 25.0.0 Darwin Kernel Version 25.0.0: Mon Aug 25 21:17:51 PDT 2025; root:xnu-12377.1.9~3/RELEASE_ARM64_T6020 arm64
[omochi@omochi-mbp swift-subprocess-rpath-bug (main =)]$ sw_vers
ProductName: macOS
ProductVersion: 26.0
BuildVersion: 25A354
[omochi@omochi-mbp swift-subprocess-rpath-bug (main =)]$ xcode-select -p
/Applications/Xcode26.1-beta.1.app/Contents/Developer
[omochi@omochi-mbp swift-subprocess-rpath-bug (main =)]$ swift --version
swift-driver version: 1.127.14.1 Apple Swift version 6.2.1 (swiftlang-6.2.1.1.1 clang-1700.4.1.1)
Target: arm64-apple-macosx26.0
[omochi@omochi-mbp swift-subprocess-rpath-bug (main =)]$ swift build
Building for debugging...
[1/1] Write swift-version-4CD727DE078F09A4.txt
Build complete! (0.12s)
[omochi@omochi-mbp swift-subprocess-rpath-bug (main =)]$ swift run
Building for debugging...
[1/1] Write swift-version-4CD727DE078F09A4.txt
Build of product 'Bug' complete! (0.12s)
dyld[48424]: Library not loaded: @rpath/libswiftCompatibilitySpan.dylib
Referenced from: <E771F9ED-AE48-355E-9D70-2B484AA3AC86> /Users/omochi/github/omochi/swift-subprocess-rpath-bug/.build/arm64-apple-macosx/debug/Bug
Reason: tried: '/Users/omochi/github/omochi/swift-subprocess-rpath-bug/.build/arm64-apple-macosx/debug/libswiftCompatibilitySpan.dylib' (no such file), '/Users/omochi/github/omochi/swift-subprocess-rpath-bug/.build/arm64-apple-macosx/debug/libswiftCompatibilitySpan.dylib' (no such file)
zsh: abort swift run
It seems that libswiftCompatibilitySpan.dylib cannot be found.
Curiously, it does run when executed from Xcode.
When I checked with otool, I found that when Xcode builds, /usr/lib/swift is set as an RPath, but when built with SwiftPM, that RPath is missing.
On Ubuntu 22.04 with Swift 6.1, there is an another bug where subprocess calls hang and never return in release builds. You can verify this with the following steps.
$ docker build . -t temp
$ docker run temp