box_syntax/pattern not supported

[csmoe]

let x: Box<i32> = box 1i32;
let box y = x;

cc rust-lang/rust#49733

[ecstatic-morse]

@csmoe @matklad

As noted in #1722, the following code causes a parse error on the latest master. It seems like this is not fully resolved.

#![feature(box_patterns)]

let x = Some(Box::new(5));
if let Some(box i) = x { // SYNTAX_ERROR
    dbg!(i);
}