Using borrowing On String Causes Compiler Error

[thecoolwinter]

Description

When using the borrowing parameter on a String in an enum initializer the compiler reports the following error:

Copy of noncopyable typed value. This is a compiler bug. Please file a bug with a small example of the bug

I've attached a code sample demonstrating this behavior in the next section. This error occurs if the scenario is on the same enum with a static function borrowing a String or a regular function.

Steps to reproduce

Compiling the following code snippet will produce the error using Swift 5.9:

public enum LineEnding: String {
    /// The default unix `\n` character
    case lineFeed = "\n"
    /// MacOS line ending `\r` character
    case carriageReturn = "\r"
    /// Windows line ending sequence `\r\n`
    case carriageReturnLineFeed = "\r\n"

    /// Initialize a line ending from a line string.
    /// - Parameter line: The line to use
    public init?(line: borrowing String) {
        guard let lastChar = line.last, // ❌ Copy of noncopyable typed value. This is a compiler bug. Please file a bug with a small example of the bug
              let lineEnding = LineEnding(rawValue: String(lastChar)) else { return nil }
        self = lineEnding
    }

    // These also cause the same error:
    static func makeLineEnding(_ line: borrowing String) -> LineEnding? {
         guard let lastChar = line.last, // ❌ ...
         ...
    }

    func makeLineEnding(_ line: borrowing String) -> LineEnding? {
        guard let lastChar = line.last, // ❌ ...
         ...
    }
}

Expected behavior

The initializer, static function, or member function should be able to borrow a String value to ensure the value isn't copied.

Environment

• swift-driver version: 1.87.1 Apple Swift version 5.9 (swiftlang-5.9.0.128.108 clang-1500.0.40.1)
• Xcode 15.0, Build version 15A240d
• Target: arm64-apple-macosx13.0

[nate-chandler]

Regression test added in #74361 .