Implement an overload of confirmation() that takes an unbounded range. (#598)

Unbounded ranges are not meaningful when used with `confirmation()`
because _any_ confirmation count matches. As well, the `...` operator
produces an instance of `UnboundedRange` which is a non-nominal type and
cannot conform to `RangeExpression`. It may be non-obvious to a
developer why `...` doesn't work as the `expectedCount` argument to that
function when other range operators work as expected.

This PR implements a stub overload of `confirmation()` that takes an
unbounded range. The stub overload is marked unavailable and cannot be
called.

Example usage:

```swift
await confirmation("Stuff happens", expectedCount: ...) { stuff in
  // ...
}
```

Generated diagnostic:

> 🛑 'confirmation(\_:expectedCount:sourceLocation:\_:)' is unavailable:
Unbounded range '...' has no effect when used with a confirmation.

As a reminder, using a range expression with `confirmation()` is an
experimental feature and has not been API-reviewed.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/Issues/Confirmation.swift

@@ -179,6 +179,22 @@ public func confirmation<R>(
   return try await body(confirmation)
 }
 
+/// An overload of ``confirmation(_:expectedCount:sourceLocation:_:)-9bfdc``
+/// that handles the unbounded range operator (`...`).
+///
+/// This overload is necessary because `UnboundedRange` does not conform to
+/// `RangeExpression`. It effectively always succeeds because any number of
+/// confirmations matches, so it is marked unavailable and is not implemented.
+@available(*, unavailable, message: "Unbounded range '...' has no effect when used with a confirmation.")
+public func confirmation<R>(
+  _ comment: Comment? = nil,
+  expectedCount: UnboundedRange,
+  sourceLocation: SourceLocation = #_sourceLocation,
+  _ body: (Confirmation) async throws -> R
+) async rethrows -> R {
+  fatalError("Unsupported")
+}
+
 @_spi(Experimental)
 extension Confirmation {
   /// A protocol that describes a range expression that can be used with

Sources/Testing/Support/Environment.swift

@@ -188,7 +188,7 @@ enum Environment {
               return nil
             case let errorCode:
               let error = Win32Error(rawValue: errorCode)
-              fatalError("unexpected error when getting environment variable '\(name)': \(error) (\(errorCode))")
+              fatalError("Unexpected error when getting environment variable '\(name)': \(error) (\(errorCode))")
             }
           } else if count > buffer.count {
             // Try again with the larger count.

Sources/TestingMacros/Support/AvailabilityGuards.swift

@@ -118,7 +118,7 @@ private func _createAvailabilityTraitExpr(
     return ".__unavailable(message: \(message), sourceLocation: \(sourceLocationExpr))"
 
   default:
-    fatalError("Unsupported keyword \(whenKeyword) passed to \(#function)")
+    fatalError("Unsupported keyword \(whenKeyword) passed to \(#function). Please file a bug report at https://github.com/swiftlang/swift-testing/issues/new")
   }
 }