Implemented a compiler diagnostic for move async mistake

Ran the tidy check

Following the diagnostic guide better

Diagnostic generation is now relegated to its own function in the diagnostics module.
Added tests

Fixed the ui test

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 {
+    fn parse_capture_clause(&mut self) -> PResult<'a, CaptureBy> {
-        if self.eat_keyword(kw::Move) { CaptureBy::Value } else { CaptureBy::Ref }
+        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.
@@ -2018,7 +2028,7 @@ 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
+