[Pre-Pre-RFC] Reviving Refutable Let [Runoff]

language design
9

Details about $expr is $pat (or let $pat = $expr as a boolean expression) can be found in discussions of RFC #2260. One big problem of is is how bindings are made. Without name bindings it is not sufficient to replace let … else. 2260 compare is with several other languages (C#, Kotlin, TypeScript), but:

  • Swift’s $pat ~= $expr does not support name binding

  • In C# 7’s $expr is $pat, a declaration statement will be inserted before the current statement, i.e. the following two statements are equivalent:

    return f(c is float d);
// <=>

// declaration inserted
float d;
// when we evaluate `c is float d`, it will be expanded to
bool _result = c is float;
if (_result) {
    d = (float)c;
}
return f(_result);

    This is possible only because every type has a default value in C#. The variable d will be “leaked” after calling f, and has a value of 0 if c is not a float. This treatment is not possible in Rust.

  • Kotlin’s smart cast ($expr is $ty) and TypeScript’s type guards (e.g. $expr instanceof $ty) is not used for pattern matching (destructuring). They alter the type of $expr so that type-specific fields will become available.

There is an attempt to create a precise scoping rule in https://github.com/rust-lang/rfcs/pull/2260#issuecomment-354525996.

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