Rollup merge of rust-lang#80160 - diondokter:move_async_fix, r=davidtwco

Implemented a compiler diagnostic for move async mistake

Fixes rust-lang#79694

First time contributing, so I hope I'm doing everything right.
(If not, please correct me!)

This code performs a check when a move capture clause is parsed. The check is to detect if the user has reversed the async move keywords and to provide a diagnostic with a suggestion to fix it.

Checked code:
```rust
fn main() {
    move async { };
}
```

Previous output:
```txt
PS C:\Repos\move_async_test> cargo build
   Compiling move_async_test v0.1.0 (C:\Repos\move_async_test)
error: expected one of `|` or `||`, found keyword `async`
 --> src\main.rs:2:10
  |
2 |     move async { };
  |          ^^^^^ expected one of `|` or `||`

error: aborting due to previous error

error: could not compile `move_async_test`
```

New output:
```txt
PS C:\Repos\move_async_test> cargo +dev build
   Compiling move_async_test v0.1.0 (C:\Repos\move_async_test)
error: the order of `move` and `async` is incorrect
 --> src\main.rs:2:13
  |
2 |     let _ = move async { };
  |             ^^^^^^^^^^
  |
help: try switching the order
  |
2 |     let _ = async move { };
  |             ^^^^^^^^^^

error: aborting due to previous error

error: could not compile `move_async_test`
```

Is there a file/module where these kind of things are tested?
Would love some feedback 😄

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -1912,4 +1912,22 @@ impl<'a> Parser<'a> {
         *self = snapshot;
         Err(err)
     }
+
+    /// Get the diagnostics for the cases where `move async` is found.
+    ///
+    /// `move_async_span` starts at the 'm' of the move keyword and ends with the 'c' of the async keyword
+    pub(super) fn incorrect_move_async_order_found(
+        &self,
+        move_async_span: Span,
+    ) -> DiagnosticBuilder<'a> {
+        let mut err =
+            self.struct_span_err(move_async_span, "the order of `move` and `async` is incorrect");
+        err.span_suggestion_verbose(
+            move_async_span,
+            "try switching the order",
+            "async move".to_owned(),
+            Applicability::MaybeIncorrect,
+        );
+        err
+    }
 }

compiler/rustc_parse/src/parser/expr.rs

@@ -1603,7 +1603,7 @@ impl<'a> Parser<'a> {
             self.sess.gated_spans.gate(sym::async_closure, span);
         }
 
-        let capture_clause = self.parse_capture_clause();
+        let capture_clause = self.parse_capture_clause()?;
         let decl = self.parse_fn_block_decl()?;
         let decl_hi = self.prev_token.span;
         let body = match decl.output {
@@ -1626,8 +1626,18 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses an optional `move` prefix to a closure-like construct.
-    fn parse_capture_clause(&mut self) -> CaptureBy {
-        if self.eat_keyword(kw::Move) { CaptureBy::Value } else { CaptureBy::Ref }
+    fn parse_capture_clause(&mut self) -> PResult<'a, CaptureBy> {
+        if self.eat_keyword(kw::Move) {
+            // Check for `move async` and recover
+            if self.check_keyword(kw::Async) {
+                let move_async_span = self.token.span.with_lo(self.prev_token.span.data().lo);
+                Err(self.incorrect_move_async_order_found(move_async_span))
+            } else {
+                Ok(CaptureBy::Value)
+            }
+        } else {
+            Ok(CaptureBy::Ref)
+        }
     }
 
     /// Parses the `|arg, arg|` header of a closure.
@@ -2019,7 +2029,7 @@ impl<'a> Parser<'a> {
     fn parse_async_block(&mut self, mut attrs: AttrVec) -> PResult<'a, P<Expr>> {
         let lo = self.token.span;
         self.expect_keyword(kw::Async)?;
-        let capture_clause = self.parse_capture_clause();
+        let capture_clause = self.parse_capture_clause()?;
         let (iattrs, body) = self.parse_inner_attrs_and_block()?;
         attrs.extend(iattrs);
         let kind = ExprKind::Async(capture_clause, DUMMY_NODE_ID, body);

src/test/ui/parser/incorrect-move-async-order-issue-79694.fixed

@@ -0,0 +1,8 @@
+// run-rustfix
+// edition:2018
+
+// Regression test for issue 79694
+
+fn main() {
+    let _ = async move { }; //~ ERROR 7:13: 7:23: the order of `move` and `async` is incorrect
+}

src/test/ui/parser/incorrect-move-async-order-issue-79694.rs

@@ -0,0 +1,8 @@
+// run-rustfix
+// edition:2018
+
+// Regression test for issue 79694
+
+fn main() {
+    let _ = move async { }; //~ ERROR 7:13: 7:23: the order of `move` and `async` is incorrect
+}

src/test/ui/parser/incorrect-move-async-order-issue-79694.stderr

@@ -0,0 +1,13 @@
+error: the order of `move` and `async` is incorrect
+  --> $DIR/incorrect-move-async-order-issue-79694.rs:7:13
+   |
+LL |     let _ = move async { };
+   |             ^^^^^^^^^^
+   |
+help: try switching the order
+   |
+LL |     let _ = async move { };
+   |             ^^^^^^^^^^
+
+error: aborting due to previous error
+