feat: Add Swift WebAssembly compilation support to swift-distributed-tracing (#174)

# Summary

This PR adds support for compiling swift-distributed-tracing to wasm
using the [Swift SDK for
WebAssembly](https://www.swift.org/documentation/articles/wasm-getting-started.html).

This PR is [part of a larger
effort](PassiveLogic/swift-web-examples#1) by
a company called PassiveLogic to enable broad support for Swift
WebAssembly compilation.

# Details

There are three changes required to enable wasm compilation for
swift-distributed-tracing.

- Removed unused `import Dispatch` imports. The Swift SDK for
WebAssembly doesn't currently include GCD. Luckily, all usage of GCD was
unused.
- Added some missing wasi-libc and pthread imports
- Add shim to allow wrapped access to CLOCK_REALTIME struct from
WASILibc. This is a critical piece of distributed tracing to acquire
timestamps that needed some manual shim code to map to the matching API
in WASI.

# Testing done

- [x] Cleaned up swiftformat lint on modified lines of change.
- [x] Verified unit tests still pass with these changes
- [x] Verified `swift build` completes without errors
- [x] Verified no new compiler warnings are added with these changes
- [x] Verified `swift build --swift-sdk wasm32-unknown-wasi` completes
without errors
- [x] Verified `swift build --swift-sdk wasm32-unknown-wasip1-threads`
completes without errors
- [x] Verified a third-party executable can build this library as part
of a larger wasm executable using the command `swift package --swift-sdk
wasm32-unknown-wasip1-threads js --use-cdn`

# Impact Risk

Realistically, this change should not cause any impact or risk. Outside
of wasm targets, the changes in this PR are additive.

Author: scottmarchant

.github/workflows/pull_request.yml

@@ -17,8 +17,24 @@ jobs:
     name: Unit tests
     uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
     with:
-      linux_5_9_arguments_override: "--explicit-target-dependency-import-check error"
-      linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
+
+      # NOTE: Aug 5, 2025. We've determined SPM has bugs for `condition: .when(platforms: [.somePlatform])`
+      # that incorrectly ignore the condition and import anyways for unintended platforms.
+      #
+      # See https://github.com/apple/swift-distributed-tracing/actions/runs/16578688393/job/46920025183?pr=174
+      # for an example of the resulting build failure.
+      #
+      # This is documented as a bug in Swift 5.10, resolved in Swift 6.
+      # See FB14859516.
+      # See also https://forums.swift.org/t/do-swiftpm-conditional-target-dependencies-work/74047
+      #
+      # Given this, the explicit target dependency check is disabled for Swift versions before Swift 6.
+
+      # Intentionally disabled:
+      #
+      # linux_5_9_arguments_override: "--explicit-target-dependency-import-check error"
+      # linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
+
       linux_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_6_1_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"

Package.swift

@@ -35,6 +35,7 @@ let package = Package(
             dependencies: [
                 .product(name: "ServiceContextModule", package: "swift-service-context"),
                 .target(name: "Instrumentation"),
+                .target(name: "_CWASI", condition: .when(platforms: [.wasi])),
             ]
         ),
         .testTarget(
@@ -43,6 +44,15 @@ let package = Package(
                 .target(name: "Tracing")
             ]
         ),
+
+        // ==== --------------------------------------------------------------------------------------------------------
+        // MARK: Wasm Support
+
+        // Provides C shims for compiling to wasm
+        .target(
+            name: "_CWASI",
+            dependencies: []
+        ),
     ]
 )
 

Sources/Instrumentation/Locks.swift

@@ -34,6 +34,11 @@ import Glibc
 import Android
 #elseif canImport(Musl)
 import Musl
+#elseif canImport(WASILibc)
+import WASILibc
+#if canImport(wasi_pthread)
+import wasi_pthread
+#endif
 #else
 #error("Unsupported runtime")
 #endif

Sources/Tracing/NoOpTracer.swift

@@ -12,7 +12,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-import Dispatch
 @_exported import Instrumentation
 @_exported import ServiceContextModule
 

Sources/Tracing/Tracer.swift

@@ -12,7 +12,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-import Dispatch
 @_exported import Instrumentation
 @_exported import ServiceContextModule
 

Sources/Tracing/TracerProtocol+Legacy.swift

@@ -12,7 +12,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-import Dispatch
 @_exported import Instrumentation
 @_exported import ServiceContextModule
 

Sources/Tracing/TracingTime.swift

@@ -23,10 +23,16 @@ import Glibc
 import Android
 #elseif canImport(Musl)
 import Musl
+#elseif canImport(WASILibc)
+import WASILibc
 #else
 #error("Unsupported runtime")
 #endif
 
+#if os(WASI)
+import _CWASI
+#endif
+
 public protocol TracerInstant: Comparable, Hashable, Sendable {
     /// Representation of this instant as the number of nanoseconds since UNIX Epoch (January 1st 1970)
     var nanosecondsSinceEpoch: UInt64 { get }
@@ -84,7 +90,11 @@ public struct DefaultTracerClock {
 
     public var now: Self.Instant {
         var ts = timespec()
+        #if os(WASI)
+        CWASI_clock_gettime_realtime(&ts)
+        #else
         clock_gettime(CLOCK_REALTIME, &ts)
+        #endif
         /// We use unsafe arithmetic here because `UInt64.max` nanoseconds is more than 580 years,
         /// and the odds that this code will still be running 530 years from now is very, very low,
         /// so as a practical matter this will never overflow.

Sources/_CWASI/_CWASI.c

@@ -0,0 +1,13 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Distributed Tracing open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift Distributed Tracing project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift Distributed Tracing project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//

Sources/_CWASI/include/_CWASI.h

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Distributed Tracing open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift Distributed Tracing project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift Distributed Tracing project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+#pragma once
+
+#if __wasi__
+
+#include <fcntl.h>
+#include <time.h>
+
+static inline void CWASI_clock_gettime_realtime(struct timespec *tv) {
+    // ClangImporter doesn't support `CLOCK_REALTIME` declaration in WASILibc, thus we have to define a bridge manually
+    clock_gettime(CLOCK_REALTIME, tv);
+}
+
+#endif // __wasi__