trimmingPrefixOrNil

Motivation

The current trimmingPrefx drops the information about whether the prefix was matched. I often do something like:

if x.starts(with: "foo") { doSomething(with: x.dropFirst("foo".count)) }

Proposed solution

if let y = x.trimmingPrefixOrNil("foo") { doSomething(with: y) }

Would be more expressive, less error prone, and more efficient. I don't love the name; maybe something better can be found. Frankly I think trimmingPrefix would have been a better name for the functionality I want. After all if "foo" can't be trimmed off the front of x, then x with the prefix "foo" trimmed is not a thing, and nil is an appropriate response.

Alternatives considered

No response

Additional information

No response

Start by locating the existing trimmingPrefx implementation and any tests covering prefix trimming. Compare the proposed optional-returning behavior and naming alternatives, then verify that matching prefixes return the trimmed value and non-matching prefixes return nil.