Struct literals are forbidden in MatchExpressions (feature(if_let_guard))

[pushkine]

Bug Report

🕗 Version & Regression Information

nightly

🧪 Features

• if_let_guard

💻 Code

Playground Link

#![feature(if_let_guard)]

struct Bar {}
struct Foo { a: u32 }

fn main() {
    match 0 {
        _ if let _ = Bar {} => {}
        _ if let _ = Foo { a: 0 } => {}
        _ => {}
    }
}

🙁 Actual behavior

  _ if let _ = Bar {} => {}
//                 ^ error: expected one of `!`, `.`, `::`, `=>`, `?`, or an operator

  _ if let _ = Foo { a: 0 } => {}
//             ^^^^^^^^^^^^ error: struct literals are not allowed here
//                           help: surround the struct literal with parentheses

🙂 Expected behavior

No error

[scottmcm]

This is probably using the same parsing rules as normal if let expressions, where this behaviour is essential for avoiding ambiguity. I don't know how to weigh the tradeoffs between "it's the same rules" and "we can be more flexible here".

(Nice to see the help for it, at least!)

[joshtriplett]

I'm also not sure we should be more flexible here, given the potential difficulty of reading it in more complex cases.

[pushkine]

given the potential difficulty of reading it in more complex cases

How so? _ if Bar {} => {} is already valid, the function reading the let expr is just hardcoded to not parse a struct expr