Output stream inconsistency on failed build command between build systems

[MaelRB]

Is it reproducible with SwiftPM command-line tools: swift build, swift test, swift package etc?

• Confirmed reproduction steps with SwiftPM CLI. The description text must include reproduction steps with either of command-line SwiftPM commands, swift build, swift test, swift package etc.

Description

When an error occurs during the build phase with the swift build command, swiftbuild build system uses stderr as output stream to report the error. xcode and native build systems use stdout.

Here is the PR that discussed about this issue: #8858

Moreover swift run command uses stderr as output stream for all build system, leading to more confusion.

Expected behavior

I expect a consistent output stream between build systems when an error occurs with swift build command.

Actual behavior

When running the swift build command on a package with syntax error here are the different outputs:

swift build --build-system swiftbuild > stdout-swiftbuild.txt 2> stderr-swiftbuild.txt

Stderr:

/Users/***/Desktop/Sample/Sources/Sample/Sample.swift:5:1: error: expected ')' in expression list

^
/Users/***/Desktop/Sample/Sources/Sample/Sample.swift:4:6: note: to match this opening '('
print("done"
     ^
error: /Users/***/Desktop/Sample/Sources/Sample/Sample.swift:5:1 expected ')' in expression list

^
error: Build failed

Stdout:

Building for debugging...
19%: 1 / 10

swift build --build-system xcode > stdout-xcode.txt 2> stderr-xcode.txt

Stderr:

Stdout:

0%: Compute target dependency graph
Building targets in dependency order
Target dependency graph (3 targets)
0%: Gather provisioning inputs
0%: Create build description
0%: Build stat cache for /Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX26.0.sdk
14%: Planning Swift module Sample (arm64)
14%: Planning Swift module Sample (x86_64)
expected ')' in expression list

^
to match this opening '('
print("done"
     ^
expected ')' in expression list

^
to match this opening '('
print("done"
     ^
Build failed

swift build --build-system native > stdout-native.txt 2> stderr-native.txt

Stderr:

Stdout:

[0/1] Planning build
Building for debugging...
[0/1] Write swift-version-39B54973F684ADAB.txt
error: emit-module command failed with exit code 1 (use -v to see invocation)
[2/3] Emitting module Sample
/Users/***/Desktop/Sample/Sources/Sample/Sample.swift:5:1: error: expected ')' in expression list
2 | // https://docs.swift.org/swift-book
3 | 
4 | print("done"
  |      `- note: to match this opening '('
5 | 
  | `- error: expected ')' in expression list
[3/3] Compiling Sample Sample.swift
/Users/***/Desktop/Sample/Sources/Sample/Sample.swift:5:1: error: expected ')' in expression list
2 | // https://docs.swift.org/swift-book
3 | 
4 | print("done"
  |      `- note: to match this opening '('
5 | 
  | `- error: expected ')' in expression list

Steps to reproduce

1. Create a new Package with swift package init
2. Update source files to contain a syntax error to trigger a build failure
3. Run: swift build --build-system swiftbuild > stdout-swiftbuild.txt 2> stderr-swiftbuild.txt
4. Run: swift build --build-system xcode > stdout-xcode.txt 2> stderr-xcode.txt
5. Run: swift build --build-system native > stdout-native.txt 2> stderr-native.txt

Swift Package Manager version/commit hash

Xcode 26 Beta 1 toolchain

Swift & OS version (output of swift --version ; uname -a)

swift-driver version: 1.127.4.2 Apple Swift version 6.2 (swiftlang-6.2.0.9.909 clang-1700.3.9.907)
Target: arm64-apple-macosx15.0
Darwin mac.home 24.5.0 Darwin Kernel Version 24.5.0: Tue Apr 22 19:53:27 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6041 arm64