The tracking issue for this feature is: #44495

This feature changes the way that "irrefutable patterns" are handled in the if let and while let forms. An irrefutable pattern is one that cannot fail to match -- for example, the _ pattern matches any value, and hence it is "irrefutable". Without this feature, using an irrefutable pattern in an if let gives a hard error (since often this indicates programmer error). But when the feature is enabled, the error becomes a lint (since in some cases irrefutable patterns are expected). This means you can use #[allow] to silence the lint:


fn main() {
    // These two examples used to be errors, but now they
    // trigger a lint (that is allowed):
    if let _ = 5 {}
    while let _ = 5 { break; }