@switch annotatition

An annotation to be applied to a match expression. If present, the compiler will verify that the match has been compiled to a tableswitch or lookupswitch and issue an error if it instead compiles into a series of conditional expressions.

Example usage:

val Constant = 'Q'
def tokenMe(ch: Char) = (ch: @switch) match {
  case ' ' | '\t' | '\n'  => 1
  case 'A' | 'Z' | '$'    => 2
  case '5' | Constant     => 3  // a non-literal may prevent switch generation: this would not compile
  case _                  => 4
Note: for pattern matches with one or two cases, the compiler generates jump instructions. Annotating such a match with @switch does not issue any warning.
Scala Documetation

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