Suppress useless clone suggestion

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -1442,9 +1442,14 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, '_, 'infcx, 'tcx> {
             // See `tests/ui/moves/needs-clone-through-deref.rs`
             return false;
         }
+        // We don't want to suggest `.clone()` in a move closure, since the value has already been captured.
         if self.in_move_closure(expr) {
             return false;
         }
+        // We also don't want to suggest cloning a closure itself, since the value has already been captured.
+        if let hir::ExprKind::Closure(_) = expr.kind {
+            return false;
+        }
         // Try to find predicates on *generic params* that would allow copying `ty`
         let mut suggestion =
             if let Some(symbol) = tcx.hir().maybe_get_struct_pattern_shorthand_field(expr) {

tests/ui/async-await/async-closures/without-precise-captures-we-are-powerless.stderr

@@ -56,11 +56,6 @@ LL |     outlives::<'a>(call_once(c));
 LL |
 LL |     let c = async move || { println!("{}", *x.0); };
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ move out of `x` occurs here
-   |
-help: consider cloning the value if the performance cost is acceptable
-   |
-LL |     let c = async || { println!("{}", *x.0); }.clone();
-   |                                               ++++++++
 
 error[E0597]: `c` does not live long enough
   --> $DIR/without-precise-captures-we-are-powerless.rs:33:20