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.

9 files changed, 139 insertions, 22 deletions

diff --git a/tests/ui/parser/issues/issue-15980.rs b/tests/ui/parser/issues/issue-15980.rs
index 87faa7d5ff1..eb7b6ca8296 100644
--- a/tests/ui/parser/issues/issue-15980.rs
+++ b/tests/ui/parser/issues/issue-15980.rs
@@ -9,9 +9,6 @@ fn main(){
             //~^ ERROR expected identifier, found keyword `return`
             //~| NOTE expected identifier, found keyword
         }
-        //~^ NOTE expected one of `.`, `=>`, `?`, or an operator
         _ => {}
-        //~^ ERROR expected one of `.`, `=>`, `?`, or an operator, found reserved identifier `_`
-        //~| NOTE unexpected token
     }
 }
diff --git a/tests/ui/parser/issues/issue-15980.stderr b/tests/ui/parser/issues/issue-15980.stderr
index c59c811199e..cf8d0114787 100644
--- a/tests/ui/parser/issues/issue-15980.stderr
+++ b/tests/ui/parser/issues/issue-15980.stderr
@@ -11,15 +11,10 @@ help: escape `return` to use it as an identifier
    |
 LL |             r#return
    |             ++
-
-error: expected one of `.`, `=>`, `?`, or an operator, found reserved identifier `_`
-  --> $DIR/issue-15980.rs:13:9
+help: you might have meant to start a match arm after the match guard
    |
-LL |         }
-   |          - expected one of `.`, `=>`, `?`, or an operator
-LL |
-LL |         _ => {}
-   |         ^ unexpected token
+LL |         Err(ref e) if e.kind == io::EndOfFile => {
+   |                                               ++
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
diff --git a/tests/ui/parser/issues/issue-52496.stderr b/tests/ui/parser/issues/issue-52496.stderr
index 77335c64c21..78c81bf5b0d 100644
--- a/tests/ui/parser/issues/issue-52496.stderr
+++ b/tests/ui/parser/issues/issue-52496.stderr
@@ -8,10 +8,15 @@ error: expected one of `,`, `:`, or `}`, found `.`
   --> $DIR/issue-52496.rs:8:22
    |
 LL |     let _ = Foo { bar.into(), bat: -1, . };
-   |             ---   -  ^ expected one of `,`, `:`, or `}`
+   |             ---   ---^ expected one of `,`, `:`, or `}`
    |             |     |
-   |             |     help: try naming a field: `bar:`
+   |             |     while parsing this struct field
    |             while parsing this struct
+   |
+help: try naming a field
+   |
+LL |     let _ = Foo { bar: bar.into(), bat: -1, . };
+   |                   ++++
 
 error: expected identifier, found `.`
   --> $DIR/issue-52496.rs:8:40
diff --git a/tests/ui/parser/issues/issue-89396.fixed b/tests/ui/parser/issues/issue-89396.fixed
index 823ad8cd1f8..0c040ddea44 100644
--- a/tests/ui/parser/issues/issue-89396.fixed
+++ b/tests/ui/parser/issues/issue-89396.fixed
@@ -8,9 +8,9 @@ fn main() {
     let _ = match opt {
         Some(_) => true,
         //~^ ERROR: expected one of
-        //~| HELP: try using a fat arrow here
+        //~| HELP: use a fat arrow to start a match arm
         None => false,
         //~^ ERROR: expected one of
-        //~| HELP: try using a fat arrow here
+        //~| HELP: use a fat arrow to start a match arm
     };
 }
diff --git a/tests/ui/parser/issues/issue-89396.rs b/tests/ui/parser/issues/issue-89396.rs
index f1d9efa524f..d95f666d797 100644
--- a/tests/ui/parser/issues/issue-89396.rs
+++ b/tests/ui/parser/issues/issue-89396.rs
@@ -8,9 +8,9 @@ fn main() {
     let _ = match opt {
         Some(_) = true,
         //~^ ERROR: expected one of
-        //~| HELP: try using a fat arrow here
+        //~| HELP: use a fat arrow to start a match arm
         None -> false,
         //~^ ERROR: expected one of
-        //~| HELP: try using a fat arrow here
+        //~| HELP: use a fat arrow to start a match arm
     };
 }
diff --git a/tests/ui/parser/issues/issue-89396.stderr b/tests/ui/parser/issues/issue-89396.stderr
index 504420574e2..41ce0705074 100644
--- a/tests/ui/parser/issues/issue-89396.stderr
+++ b/tests/ui/parser/issues/issue-89396.stderr
@@ -5,7 +5,7 @@ LL |         Some(_) = true,
    |                 ^
    |                 |
    |                 expected one of `=>`, `if`, or `|`
-   |                 help: try using a fat arrow here: `=>`
+   |                 help: use a fat arrow to start a match arm: `=>`
 
 error: expected one of `=>`, `@`, `if`, or `|`, found `->`
   --> $DIR/issue-89396.rs:12:14
@@ -14,7 +14,7 @@ LL |         None -> false,
    |              ^^
    |              |
    |              expected one of `=>`, `@`, `if`, or `|`
-   |              help: try using a fat arrow here: `=>`
+   |              help: use a fat arrow to start a match arm: `=>`
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/parser/missing-fat-arrow.rs b/tests/ui/parser/missing-fat-arrow.rs
new file mode 100644
index 00000000000..fef16129cd0
--- /dev/null
+++ b/tests/ui/parser/missing-fat-arrow.rs
@@ -0,0 +1,41 @@
+fn main() {
+    let x = 1;
+    let y = 2;
+    let value = 3;
+
+    match value {
+        Some(x) if x == y {
+            self.next_token()?; //~ ERROR expected identifier, found keyword `self`
+            Ok(true)
+        },
+        _ => {
+            Ok(false)
+        }
+    }
+    let _: i32 = (); //~ ERROR mismatched types
+}
+
+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 `.`
+        },
+        _ => {}
+    }
+    let _: i32 = (); //~ ERROR mismatched types
+}
+
+fn bar(a: Option<&mut Foo>, b: usize) {
+    match a {
+        Some(a) if a.value == b {
+            a.value, //~ ERROR expected one of `,`, `:`, or `}`, found `.`
+        } => {
+        }
+        _ => {}
+    }
+    let _: i32 = (); //~ ERROR mismatched types
+}
diff --git a/tests/ui/parser/missing-fat-arrow.stderr b/tests/ui/parser/missing-fat-arrow.stderr
new file mode 100644
index 00000000000..abfed7a5683
--- /dev/null
+++ b/tests/ui/parser/missing-fat-arrow.stderr
@@ -0,0 +1,78 @@
+error: expected identifier, found keyword `self`
+  --> $DIR/missing-fat-arrow.rs:8:13
+   |
+LL |         Some(x) if x == y {
+   |                         - while parsing this struct
+LL |             self.next_token()?;
+   |             ^^^^ expected identifier, found keyword
+   |
+help: you might have meant to start a match arm after the match guard
+   |
+LL |         Some(x) if x == y => {
+   |                           ++
+
+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 => {
+   |                                 ++
+
+error: expected one of `,`, `:`, or `}`, found `.`
+  --> $DIR/missing-fat-arrow.rs:35:14
+   |
+LL |         Some(a) if a.value == b {
+   |                               - while parsing this struct
+LL |             a.value,
+   |             -^ expected one of `,`, `:`, or `}`
+   |             |
+   |             while parsing this struct field
+   |
+help: try naming a field
+   |
+LL |             a: a.value,
+   |             ++
+help: you might have meant to start a match arm after the match guard
+   |
+LL |         Some(a) if a.value == b => {
+   |                                 ++
+
+error[E0308]: mismatched types
+  --> $DIR/missing-fat-arrow.rs:15:18
+   |
+LL |     let _: i32 = ();
+   |            ---   ^^ expected `i32`, found `()`
+   |            |
+   |            expected due to this
+
+error[E0308]: mismatched types
+  --> $DIR/missing-fat-arrow.rs:29:18
+   |
+LL |     let _: i32 = ();
+   |            ---   ^^ expected `i32`, found `()`
+   |            |
+   |            expected due to this
+
+error[E0308]: mismatched types
+  --> $DIR/missing-fat-arrow.rs:40:18
+   |
+LL |     let _: i32 = ();
+   |            ---   ^^ expected `i32`, found `()`
+   |            |
+   |            expected due to this
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/tests/ui/parser/removed-syntax-with-2.stderr b/tests/ui/parser/removed-syntax-with-2.stderr
index c6ae1ce674f..e75c5bcd643 100644
--- a/tests/ui/parser/removed-syntax-with-2.stderr
+++ b/tests/ui/parser/removed-syntax-with-2.stderr
@@ -2,8 +2,9 @@ error: expected one of `,`, `:`, or `}`, found `a`
   --> $DIR/removed-syntax-with-2.rs:8:31
    |
 LL |     let b = S { foo: (), with a };
-   |             -                 ^ expected one of `,`, `:`, or `}`
-   |             |
+   |             -            ---- ^ expected one of `,`, `:`, or `}`
+   |             |            |
+   |             |            while parsing this struct field
    |             while parsing this struct
 
 error[E0063]: missing field `bar` in initializer of `S`