[swift-evolution] Proposal: Add StaticString.UnicodeScalarView

[Kevin Ballard]

There's no way to create a substring of `StaticString` that's still
typed as `StaticString`. This is occasionally desirable, for example if
you want to extract the filename from `__FILE__` to pass to another API
that requires a `StaticString`.

I believe the best solution to this is to add a type
`StaticString.UnicodeScalarView`, similar to `String.UnicodeScalarView`.
`StaticString` would also be extended with a property `unicodeScalars`
and two initializers to construct a `StaticString` from a
`String.UnicodeScalarView`. The full proposed API looks like:

extension StaticString {  /// The value of `self` as a collection of
[Unicode scalar values]
(http://www.unicode.org/glossary/#unicode_scalar_value).  public var
unicodeScalars: UnicodeScalarView

/// Construct the `StaticString` corresponding to the given  ///
`UnicodeScalarView`.  public init(_: UnicodeScalarView)

/// Construct the `StaticString` corresponding to the given  ///
`UnicodeScalarView` slice.  public init(_: Slice<UnicodeScalarView>)

/// A collection of [Unicode scalar values]
(http://www.unicode.org/glossary/#unicode_scalar_value) that  /// encode
a `StaticString`.  public struct UnicodeScalarView : CollectionType {

init(_: StaticString)

/// A position in a `StaticString.UnicodeScalarView`.    public struct
Index : BidirectionalIndexType, Comparable {      /// Returns the next
consecutive value after `self`.      ///      /// - Requires: The next
value is representable.      @warn_unused_result      public func
successor() -> Index

/// Returns the previous consecutive value before `self`.      ///
/// - Requires: The previous value is representable.
@warn_unused_result      public func predecessor() -> Index    }

/// The position of the first `UnicodeScalar` if the `StaticString` is
/// non-empty; identical to `endIndex` otherwise.    public var
startIndex: Index

/// The "past the end" position.    ///    /// `endIndex` is not a valid
argument to `subscript`, and is always    /// reachable from
`startIndex` by zero or more applications of    /// `successor()`.
public var endIndex: Index

/// Returns `true` iff `self` is empty.    public var isEmpty: Bool

public subscript(position: Index) -> UnicodeScalar  } }

An alternative would be to make StaticString itself conform to
CollectionType, but this is a bad idea for the same reasons that String
doesn't conform to CollectionType.

-Kevin Ballard
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