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.

1 files changed, 11 insertions, 0 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 7c75e440aaa..aeb4fd0a304 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -431,6 +431,17 @@ pub(crate) struct ExpectedElseBlock {
 }
 
 #[derive(Diagnostic)]
+#[diag(parse_expected_struct_field)]
+pub(crate) struct ExpectedStructField {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub token: Token,
+    #[label(parse_ident_label)]
+    pub ident_span: Span,
+}
+
+#[derive(Diagnostic)]
 #[diag(parse_outer_attribute_not_allowed_on_if_else)]
 pub(crate) struct OuterAttributeNotAllowedOnIfElse {
     #[primary_span]