`elem` applied to a known string should compile to a case expression.

Motivation

Writing foo x = x elem "abc" should compile to a case expression like

case x of
  'a' -> True
  'b' -> True
  'c' -> True
  _ -> False

Case expression are in general either O(log(n)) or O(1) whereas a traversal based elem is O(n) so this can be a big performance win.

Code of this sort is particularly common for parser/lexer like code. And ghcide at some point saw double digit % reductions in runtime when doing this transformation by hand.

Proposal

Ideally we would have a rule for this transformation which precedence over more general rules applying to elem.

However there is currently no easy way to achieve this.