Fix search for the Swift compiler on Windows

Sources/TSCBasic/Path.swift

@@ -397,6 +397,12 @@ private struct UNIXPath: Path {
         return name != "" && name != "." && name != ".." && !name.contains("/")
     }
 
+#if os(Windows)
+    static func isAbsolutePath(_ path: String) -> Bool {
+        return !path.withCString(encodedAs: UTF16.self, PathIsRelativeW)
+    }
+#endif
+
     var dirname: String {
 #if os(Windows)
         let dir = string.deletingLastPathComponent
@@ -421,7 +427,11 @@ private struct UNIXPath: Path {
     }
 
     var isAbsolute: Bool {
-        string.hasPrefix("/")
+#if os(Windows)
+        return UNIXPath.isAbsolutePath(string)
+#else
+        return string.hasPrefix("/")
+#endif
     }
 
     var basename: String {
@@ -608,7 +618,7 @@ private struct UNIXPath: Path {
         defer { fsr.deallocate() }
 
         let realpath = String(cString: fsr)
-        if realpath.withCString(encodedAs: UTF16.self, PathIsRelativeW) {
+        if !UNIXPath.isAbsolutePath(realpath) {
             throw PathValidationError.invalidAbsolutePath(path)
         }
         self.init(normalizingAbsolutePath: path)
@@ -630,7 +640,7 @@ private struct UNIXPath: Path {
         defer { fsr.deallocate() }
 
         let realpath: String = String(cString: fsr)
-        if !realpath.withCString(encodedAs: UTF16.self, PathIsRelativeW) {
+        if UNIXPath.isAbsolutePath(realpath) {
             throw PathValidationError.invalidRelativePath(path)
         }
         self.init(normalizingRelativePath: path)

Sources/TSCBasic/misc.swift

@@ -48,16 +48,16 @@ public func getEnvSearchPaths(
     currentWorkingDirectory: AbsolutePath?
 ) -> [AbsolutePath] {
     // Compute search paths from PATH variable.
-    return (pathString ?? "").split(separator: ":").map(String.init).compactMap({ pathString in
-        // If this is an absolute path, we're done.
-        if pathString.first == "/" {
-            return AbsolutePath(pathString)
-        }
-        // Otherwise convert it into absolute path relative to the working directory.
-        guard let cwd = currentWorkingDirectory else {
-            return nil
+#if os(Windows)
+    let pathSeparator: Character = ";"
+#else
+    let pathSeparator: Character = ":"
+#endif
+    return (pathString ?? "").split(separator: pathSeparator).map(String.init).compactMap({ pathString in
+        if let cwd = currentWorkingDirectory {
+            return AbsolutePath(pathString, relativeTo: cwd)
         }
-        return AbsolutePath(pathString, relativeTo: cwd)
+        return try? AbsolutePath(validating: pathString)
     })
 }