[Bug] “DependenciesTestObserver” fails to build with the Swift 6.0 musl-based static SDK

[Andrewangeta]

Description

Overview

When building swift-dependencies on a musl-based Linux environment (i.e., using the Swift Static Linux SDK for full static linking), we encounter build failures around DependenciesTestObserver and XCTest. The new static SDK includes a musl variant (--swift-sdk x86_64-swift-linux-musl), but references to XCTest (and dynamic linking logic) in DependenciesTestObserver cause compile errors:
no such module 'XCTest' and

import Glibc
28.17     |          `- error: no such module 'Glibc'

Generally the guides suggest adding the import conditional vs os check

#if os(macOS) || os(iOS)
import Darwin
#elseif canImport(Glibc)
import Glibc
#elseif canImport(Musl)
import Musl
#elseif os(Windows)
import ucrt
#else
#error(Unknown platform)
#endif

But I'm unclear of all the context of the runtime checks in those utilities and the package manifest conditionals as well.

Checklist

• I have determined whether this bug is also reproducible in a vanilla SwiftUI project.
• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue or discussion.

Expected behavior

Successful compilation

Actual behavior

Failed compilation

Steps to reproduce

1. Create a Dockerfile (for example) that uses Swift 6.0.3’s Static Linux SDK and musl target. Here’s a minimal snippet:

FROM swiftlang/swift:6.0-noble AS build

ARG STATIC_SDK_URL="https://download.swift.org/swift-6.0.3-release/static-sdk/swift-6.0.3-RELEASE/swift-6.0.3-RELEASE_static-linux-0.0.1.artifactbundle.tar.gz"
ARG STATIC_SDK_CHECKSUM="67765e0030e661a745967e4877cfe088db45f7177d5a98a0e26fd661cdd0bd"

# Install the musl-based static SDK
RUN swift sdk install $STATIC_SDK_URL --checksum $STATIC_SDK_CHECKSUM

WORKDIR /app
COPY . .

# Attempt to build just the "Dependencies" product:
RUN swift build \
  --configuration release \
  --swift-sdk x86_64-swift-linux-musl \
  --product Dependencies

# ...

2. Build and observe build failure output. docker build -t dependencies-musl-test .

Dependencies version information

1.6.3

Destination operating system

linux

Xcode version information

16.2

Swift Compiler version information

See Dockerfile

[stephencelis]

@Andrewangeta Can you attach a project that reproduces the issue for easier debugging?

[Andrewangeta]

@stephencelis For some reason GitHub won't let me upload the zip file no matter what I do.

[Andrewangeta]

@stephencelis
Oops, I compressed the build folder so it was a large attachment.
Just run docker build -t musl-demo . in the root.
musl-demo.zip

[stephencelis]

@Andrewangeta Thanks for the repro! I've troubleshot a bit with a musl branch: https://github.com/pointfreeco/swift-dependencies/compare/musl

I get past the import errors, but get more general linker errors:

ld.lld: error: relocation R_X86_64_PC32 cannot be used against symbol 'vtable for std::bad_alloc'; recompile with -fPIC
…

I'm going to convert this to a discussion for now, and we're open to solutions to these issues if it's possible to solve in our repo, but it's unclear if this is a bug with Dependencies or something else.