E0077

Warning name: lexmatch_longest_match

Using lexmatch with longest-match semantics is deprecated.

This warning is emitted for lexmatch ... with longest. Regex match expressions do not provide longest-match semantics; use lexscan ... with longest for lexer-style longest matching.

Erroneous example

///|
fn classify(input : String) -> String {
  lexmatch input with longest {
    ("if|[a-z]*" as token) => token.to_owned()
    _ => "other"
  }
}

///|
test {
  inspect(classify("iff"), content="iff")
}

Suggestion

Convert each legacy string pattern to a regex literal. Anchor it at ^, and at $ as well when the case must consume the whole input. Replace a trailing rest binder with after=. Longest-match lexscan does not support leading rest binders or case guards, so those cases require manual refactoring.

///|
fn classify(input : String) -> String {
  lexscan input with longest {
    re"^(if|[a-z]*)$" as token => token.to_owned()
    _ => "other"
  }
}

///|
test {
  inspect(classify("iff"), content="iff")
}