Remove code that supports < Swift 6.0 (#8348)

`swift-build` is a dependency of swift-package-manager that requires >=
Swift 6.0 to build. SPM also requires swift-testing for tests, which
requires >= Swift 6.0.

Clean up legacy code that skipped tests on 5.10 and below, as well as
conditional compilation directives on imports.

Author: plemarquand

IntegrationTests/Tests/IntegrationTests/BasicTests.swift

@@ -19,7 +19,6 @@ final class BasicTests: XCTestCase {
 
     func testExamplePackageDealer() throws {
         try XCTSkipIf(isSelfHosted, "These packages don't use the latest runtime library, which doesn't work with self-hosted builds.")
-        try skipUnlessAtLeastSwift6()
 
         try withTemporaryDirectory { tempDir in
             let packagePath = tempDir.appending(component: "dealer")
@@ -94,8 +93,6 @@ final class BasicTests: XCTestCase {
     }
 
     func testSwiftPackageInitExec() throws {
-        try skipUnlessAtLeastSwift6()
-
         try withTemporaryDirectory { tempDir in
             // Create a new package with an executable target.
             let packagePath = tempDir.appending(component: "Project")
@@ -121,8 +118,6 @@ final class BasicTests: XCTestCase {
     }
 
     func testSwiftPackageInitExecTests() throws {
-        try skipUnlessAtLeastSwift6()
-
         try XCTSkip("FIXME: swift-test invocations are timing out in Xcode and self-hosted CI")
 
         try withTemporaryDirectory { tempDir in
@@ -146,8 +141,6 @@ final class BasicTests: XCTestCase {
     }
 
     func testSwiftPackageInitLib() throws {
-        try skipUnlessAtLeastSwift6()
-
         try withTemporaryDirectory { tempDir in
             // Create a new package with an executable target.
             let packagePath = tempDir.appending(component: "Project")
@@ -166,8 +159,6 @@ final class BasicTests: XCTestCase {
     }
 
     func testSwiftPackageLibsTests() throws {
-        try skipUnlessAtLeastSwift6()
-
         try XCTSkip("FIXME: swift-test invocations are timing out in Xcode and self-hosted CI")
 
         try withTemporaryDirectory { tempDir in
@@ -226,8 +217,6 @@ final class BasicTests: XCTestCase {
     }
 
     func testSwiftRun() throws {
-        try skipUnlessAtLeastSwift6()
-
         try withTemporaryDirectory { tempDir in
             let packagePath = tempDir.appending(component: "secho")
             try localFileSystem.createDirectory(packagePath)
@@ -255,8 +244,6 @@ final class BasicTests: XCTestCase {
     }
 
     func testSwiftTest() throws {
-        try skipUnlessAtLeastSwift6()
-
         try XCTSkip("FIXME: swift-test invocations are timing out in Xcode and self-hosted CI")
 
         try withTemporaryDirectory { tempDir in
@@ -378,9 +365,3 @@ private extension Character {
         }
     }
 }
-
-private func skipUnlessAtLeastSwift6() throws {
-    #if compiler(<6.0)
-    try XCTSkipIf(true, "Skipping because test requires at least Swift 6.0")
-    #endif
-}

Sources/Basics/OSSignpost.swift

@@ -11,11 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #if canImport(os)
-#if compiler(>=6.0)
 import os
-#else
-@preconcurrency import os
-#endif
 
 extension os.OSLog {
     @usableFromInline

Sources/Build/LLBuildCommands.swift

@@ -240,11 +240,6 @@ final class TestEntryPointCommand: CustomLLBuildCommand, TestBuildCommand {
         if buildParameters.triple.isLinux() {
             // FIXME: work around crash on Amazon Linux 2 when main function is async (rdar://128303921)
             needsAsyncMainWorkaround = true
-        } else if buildParameters.triple.isDarwin() {
-#if compiler(<5.10)
-            // FIXME: work around duplicate async_Main symbols (SEE https://github.com/swiftlang/swift/pull/69113)
-            needsAsyncMainWorkaround = true
-#endif
         }
 
         stream.send(

Sources/Commands/PackageCommands/Init.swift

@@ -85,8 +85,4 @@ extension SwiftPackageCommand {
     }
 }
 
-#if compiler(<6.0)
-extension InitPackage.PackageType: ExpressibleByArgument {}
-#else
 extension InitPackage.PackageType: @retroactive ExpressibleByArgument {}
-#endif

Sources/Commands/Utilities/APIDigester.swift

@@ -331,8 +331,4 @@ extension SerializedDiagnostics.SourceLocation {
     }
 }
 
-#if compiler(<6.0)
-extension SerializedDiagnostics.SourceLocation: DiagnosticLocation {}
-#else
 extension SerializedDiagnostics.SourceLocation: @retroactive DiagnosticLocation {}
-#endif

Sources/CoreCommands/Options.swift

@@ -780,16 +780,6 @@ extension URL {
     }
 }
 
-#if compiler(<6.0)
-extension BuildConfiguration: ExpressibleByArgument, CaseIterable {}
-extension AbsolutePath: ExpressibleByArgument {}
-extension WorkspaceConfiguration.CheckingMode: ExpressibleByArgument {}
-extension Sanitizer: ExpressibleByArgument {}
-extension BuildSystemProvider.Kind: ExpressibleByArgument, CaseIterable {}
-extension Version: ExpressibleByArgument {}
-extension PackageIdentity: ExpressibleByArgument {}
-extension URL: ExpressibleByArgument {}
-#else
 extension BuildConfiguration: @retroactive ExpressibleByArgument, CaseIterable {}
 extension AbsolutePath: @retroactive ExpressibleByArgument {}
 extension WorkspaceConfiguration.CheckingMode: @retroactive ExpressibleByArgument {}
@@ -798,4 +788,3 @@ extension BuildSystemProvider.Kind: @retroactive ExpressibleByArgument, CaseIter
 extension Version: @retroactive ExpressibleByArgument {}
 extension PackageIdentity: @retroactive ExpressibleByArgument {}
 extension URL: @retroactive ExpressibleByArgument {}
-#endif

Sources/PackageRegistryCommand/PackageRegistryCommand+Publish.swift

@@ -247,11 +247,7 @@ extension SignatureFormat {
     }
 }
 
-#if compiler(<6.0)
-extension SignatureFormat: ExpressibleByArgument {}
-#else
 extension SignatureFormat: @retroactive ExpressibleByArgument {}
-#endif
 
 enum MetadataLocation {
     case sourceTree(AbsolutePath)

Sources/Workspace/Diagnostics.swift

@@ -255,8 +255,4 @@ extension FileSystemError {
     }
 }
 
-#if compiler(<6.0)
-extension FileSystemError: CustomStringConvertible {}
-#else
 extension FileSystemError: @retroactive CustomStringConvertible {}
-#endif

Sources/Workspace/Workspace.swift

@@ -1519,11 +1519,7 @@ private func warnToStderr(_ message: String) {
 }
 
 // used for manifest validation
-#if compiler(<6.0)
-extension RepositoryManager: ManifestSourceControlValidator {}
-#else
 extension RepositoryManager: @retroactive ManifestSourceControlValidator {}
-#endif
 
 extension ContainerUpdateStrategy {
     var repositoryUpdateStrategy: RepositoryUpdateStrategy {