Enable strict concurrency (#82)

Motivation:

To catch potential data races at build time.

Modifications:

- Enable strict concurrency in Package.swift.
- Update CI flags.

Result:

Strict concurrency adoption.

.github/workflows/main.yml

@@ -13,9 +13,9 @@ jobs:
     with:
       linux_5_9_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
       linux_5_10_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-      linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-      linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error"
-      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
+      linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --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"
+      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       windows_6_0_enabled: true
       windows_nightly_6_0_enabled: true
       windows_nightly_main_enabled: true

.github/workflows/pull_request.yml

@@ -17,9 +17,9 @@ jobs:
     with:
       linux_5_9_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
       linux_5_10_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-      linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-      linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error"
-      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
+      linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --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"
+      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       windows_6_0_enabled: true
       windows_nightly_6_0_enabled: true
       windows_nightly_main_enabled: true

Package.swift

@@ -31,6 +31,12 @@ let package = Package(
     ]
 )
 
+for target in package.targets {
+    var settings = target.swiftSettings ?? []
+    settings.append(.enableExperimentalFeature("StrictConcurrency=complete"))
+    target.swiftSettings = settings
+}
+
 // ---    STANDARD CROSS-REPO SETTINGS DO NOT EDIT   --- //
 for target in package.targets {
     if target.type != .plugin {