But Rust lacks `guard let x = a else { return }` which is one of the nicest additions of Swift. I know there were attempts to add it to Rust, but somehow the community really doesn't like it because a `let x = match a { Some(a) => a, None => return; }` is too similar. However I feel that guard also expresses the intent.
I frequently define a `guard!` macro in Rust that does exactly that. Seems clean enough to solve at the macro level -- doesn't necessarily need to be built into the language itself.
This kind of stuff should be on the stdlib if it's meant to be idiomatic.
Rust errs on the side of not putting things in the stdlib. Even things like random number generation or regexes are not in stdlib.
The excellent package manager (Cargo) makes this a non-issue.