[Windows] TSCBasic/Path Asserts with long paths > 260 in length

[kcieplak]

2025-05-01T15:00:18.097Z] TSCBasic/Path.swift:969: Assertion failed
[2025-05-01T15:00:18.097Z]
[2025-05-01T15:00:18.097Z] Current stack trace:
[2025-05-01T15:00:18.097Z]

When creating an absolute path by appending a relative path the appending method is used for Windows.

/// Initializes the AbsolutePath by concatenating a relative path to an
/// existing absolute path, and renormalizing if necessary.
public init(_ absPath: AbsolutePath, _ relPath: RelativePath) {
    self.init(absPath._impl.appending(relativePath: relPath._impl))
}

This in turn calls a windows API that will canonicalize the path adding the \?\ prefix for long path support

func appending(relativePath: Self) -> Self {
        var result: PWSTR?
        _ = self.string.withCString(encodedAs: UTF16.self) { root in
            relativePath.string.withCString(encodedAs: UTF16.self) { path in
                PathAllocCombine(root, path, ULONG(PATHCCH_ALLOW_LONG_PATHS.rawValue), &result)
            }
        }
        defer { LocalFree(result) }
        return Self(string: String(decodingCString: result!, as: UTF16.self))
    }

The Assert compares the path to the non prefixed path and will fail.

[kcieplak]

Is triggered by swiftlang/swift-package-manager#8602