Rollup merge of #116400 - estebank:issue-78585, r=WaffleLapkin

Detect missing `=>` after match guard during parsing

```
error: expected one of `,`, `:`, or `}`, found `.`
  --> $DIR/missing-fat-arrow.rs:25:14
   |
LL |         Some(a) if a.value == b {
   |                               - while parsing this struct
LL |             a.value = 1;
   |             -^ expected one of `,`, `:`, or `}`
   |             |
   |             while parsing this struct field
   |
help: try naming a field
   |
LL |             a: a.value = 1;
   |             ++
help: you might have meant to start a match arm after the match guard
   |
LL |         Some(a) if a.value == b => {
   |                                 ++
```

Fix #78585.

Author: workingjubilee