[6.0] Properly handle swift-testing installations in toolchain/SDK (#7856)

- Explanation:

On macOS SwiftPM should prefer swift-testing installed into a custom
toolchain when used. On Windows we need special logic to discover
swift-testing location.

- Add special swift compiler "extra" flags to favor swift-testing
installed in a toolchain.
- Inject `-I`, `-L` on Windows that point to where swift-testing is
installed in SDKROOT.
- Inject a path to testing on `PATH` environment variable on Windows to
make sure that the library is always discoverable.

- Main Branch PR:
#7840

- Resolves: rdar://132828246

- Risk: Medium (Although changes are only viable with toolchains have
certain directories in them and test we could do for testing was manual
validation).

- Reviewed By: @MaxDesiatov @rintaro 

- Testing: Existing tests and manual validation using new toolchain
(which is currently in development) on Windows and a custom toolchain
plus CommandLine tools on macOS.

Author: xedin

Sources/Commands/Utilities/TestingSupport.swift

@@ -176,8 +176,11 @@ enum TestingSupport {
         }
         #if !os(macOS)
         #if os(Windows)
-        if let location = toolchain.xctestPath {
-            env.prependPath(key: .path, value: location.pathString)
+        if let xctestLocation = toolchain.xctestPath {
+            env.prependPath(key: .path, value: xctestLocation.pathString)
+        }
+        if let swiftTestingLocation = toolchain.swiftTestingPathOnWindows {
+            env.prependPath(key: .path, value: swiftTestingLocation.pathString)
         }
         #endif
         return env

Sources/PackageModel/Toolchain.swift

@@ -71,20 +71,24 @@ extension Toolchain {
 
     public var hostLibDir: AbsolutePath {
         get throws {
-            return try toolchainLibDir.appending(components: ["swift", "host"])
+            try Self.toolchainLibDir(swiftCompilerPath: self.swiftCompilerPath).appending(
+                components: ["swift", "host"]
+            )
         }
     }
 
     public var macosSwiftStdlib: AbsolutePath {
         get throws {
-            return try AbsolutePath(validating: "../../lib/swift/macosx", relativeTo: resolveSymlinks(swiftCompilerPath))
+            try Self.toolchainLibDir(swiftCompilerPath: self.swiftCompilerPath).appending(
+                components: ["swift", "macosx"]
+            )
         }
     }
 
     public var toolchainLibDir: AbsolutePath {
         get throws {
             // FIXME: Not sure if it's better to base this off of Swift compiler or our own binary.
-            return try AbsolutePath(validating: "../../lib", relativeTo: resolveSymlinks(swiftCompilerPath))
+            try Self.toolchainLibDir(swiftCompilerPath: self.swiftCompilerPath)
         }
     }
 
@@ -107,4 +111,8 @@ extension Toolchain {
     public var extraSwiftCFlags: [String] {
         extraFlags.swiftCompilerFlags
     }
+
+    package static func toolchainLibDir(swiftCompilerPath: AbsolutePath) throws -> AbsolutePath {
+        try AbsolutePath(validating: "../../lib", relativeTo: resolveSymlinks(swiftCompilerPath))
+    }
 }

Sources/PackageModel/ToolchainConfiguration.swift

@@ -42,6 +42,10 @@ public struct ToolchainConfiguration {
     /// This is optional for example on macOS w/o Xcode.
     public var xctestPath: AbsolutePath?
 
+    /// Path to the swift-testing utility.
+    /// Currently computed only for Windows.
+    public var swiftTestingPath: AbsolutePath?
+
     /// Creates the set of manifest resources associated with a `swiftc` executable.
     ///
     /// - Parameters:
@@ -59,7 +63,8 @@ public struct ToolchainConfiguration {
         swiftCompilerEnvironment: Environment = .current,
         swiftPMLibrariesLocation: SwiftPMLibrariesLocation? = nil,
         sdkRootPath: AbsolutePath? = nil,
-        xctestPath: AbsolutePath? = nil
+        xctestPath: AbsolutePath? = nil,
+        swiftTestingPath: AbsolutePath? = nil
     ) {
         let swiftPMLibrariesLocation = swiftPMLibrariesLocation ?? {
             return .init(swiftCompilerPath: swiftCompilerPath)
@@ -72,6 +77,7 @@ public struct ToolchainConfiguration {
         self.swiftPMLibrariesLocation = swiftPMLibrariesLocation
         self.sdkRootPath = sdkRootPath
         self.xctestPath = xctestPath
+        self.swiftTestingPath = swiftTestingPath
     }
 }