The same pattern can also be useful in Rust for early returning Result<_,_> errors (you cannot `let x = foo()?` inside of a normal block like that). let co

The same pattern can also be useful in Rust for early returning Result<_,_> errors (you cannot `let x = foo()?` inside of a normal block like that). let config: Result<i32, i32> = { Ok( "1234".parse::<i32>().map_err(|_| -1)? ) }; would fail to compile, or worse: would return out of the entire method if surrounding method would have return type Result<_,i32>. On the other hand, let config: Result<i32, i32> = (||{ Ok( "1234".parse::<i32>().map_err(|_| -1)? ) })(); runs just fine. Hopefully try blocks will allow using ? inside of expression blocks in the future, though.