Confusing parse error for missing `=>` after match guard

[camelid]

Can you spot the mistake?

fn main() {
    let x = 1;
    let y = 2;
    let value = 3;

    match value {
        Some(x) if x == y {
            self.next_token()?;
            Ok(true)
        },
        _ => {
            Ok(false)
        }
    }
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error: expected identifier, found keyword `self`
 --> src/main.rs:8:13
  |
7 |         Some(x) if x == y {
  |                         - while parsing this struct
8 |             self.next_token()?;
  |             ^^^^ expected identifier, found keyword

error: expected one of `.`, `=>`, `?`, or an operator, found `,`
  --> src/main.rs:10:10
   |
10 |         },
   |          ^ expected one of `.`, `=>`, `?`, or an operator

error: aborting due to 2 previous errors

error: could not compile `playground`.

To learn more, run the command again with --verbose.

[msrd0]

Just spent 10 minutes trying to figure out why rust wants me to put a : here. Turns out I was missing a => as well.

struct Foo {
    value: usize
}

fn foo(a: Option<&mut Foo>, b: usize) {
    match a {
        Some(a) if a.value == b {
            a.value = 1;
        },
        _ => {}
    }
}

error: expected one of `,`, `:`, or `}`, found `.`
 --> src/lib.rs:8:14
  |
7 |         Some(a) if a.value == b {
  |                               - while parsing this struct
8 |             a.value = 1;
  |             -^ expected one of `,`, `:`, or `}`
  |             |
  |             help: try naming a field: `a:`

error: expected one of `.`, `=>`, `?`, or an operator, found `,`
 --> src/lib.rs:9:10
  |
9 |         },
  |          ^ expected one of `.`, `=>`, `?`, or an operator

error: could not compile `playground` due to 2 previous errors