[swift-evolution] [stdlib] CString.swift initializers

[Ben Rimmington]

The new initializers in "CString.swift" have a precondition:

> public init(cString: UnsafePointer<CChar>) {
>     _precondition(cString != nil, "cString must not be nil")
> }
> 
> public init?(validatingUTF8 cString: UnsafePointer<CChar>) {
>     _precondition(cString != nil, "cString must not be nil")
> }

<https://github.com/apple/swift/blob/swift-3-api-guidelines/stdlib/public/core/CString.swift>

The old `String.fromCString` function was convenient for wrapping inessential C function calls, if the `nil` coalescing operator can provide a fallback:

> let message =
>     String.fromCString(sqlite3_errmsg(handle)) ??
>     String.fromCString(sqlite3_errstr(result)) ?? ""

I also think the `validatingUTF8` argument label is misleading, because neither initializer allows ill-formed UTF-8 data to be copied.

If the precondition is removed, it should be possible to combine the initializers:

> /// Create a new `String` by copying the nul-terminated UTF-8 data
> /// referenced by a `cString`.
> ///
> /// Fails if the `cString` is `NULL`; or if it contains ill-formed code
> /// units and no repairing has been requested. Otherwise replaces
> /// ill-formed code units with replacement characters (U+FFFD).
> public init?(
>   cString: UnsafePointer<CChar>,
>   repairingInvalidCodeUnits isRepairing: Bool = true
> ) {
>   guard let (result, _) = String.decodeCString(
>     UnsafePointer(cString),
>       as: UTF8.self,
>       repairingInvalidCodeUnits: isRepairing) else {
>     return nil
>   }
>   self = result
> }

-- Ben