Improve diagnostic when passing arg to closure and missing borrow.

This checks the number of references for the given and expected type and
shows hints to the user if the numbers don't match.

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -1234,6 +1234,7 @@ fn report_selection_error(
                     _ => None,
                 };
 
+                let found_node = found_did.and_then(|did| self.tcx.hir().get_if_local(did));
                 let found_span = found_did.and_then(|did| self.tcx.hir().span_if_local(did));
 
                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
@@ -1287,6 +1288,7 @@ fn report_selection_error(
                         found_trait_ref,
                         expected_trait_ref,
                         obligation.cause.code(),
+                        found_node,
                     )
                 } else {
                     let (closure_span, closure_arg_span, found) = found_did

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -258,6 +258,7 @@ fn report_closure_arg_mismatch(
         found: ty::PolyTraitRef<'tcx>,
         expected: ty::PolyTraitRef<'tcx>,
         cause: &ObligationCauseCode<'tcx>,
+        found_node: Option<Node<'_>>,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
 
     fn note_conflicting_closure_bounds(
@@ -1695,6 +1696,7 @@ fn report_closure_arg_mismatch(
         found: ty::PolyTraitRef<'tcx>,
         expected: ty::PolyTraitRef<'tcx>,
         cause: &ObligationCauseCode<'tcx>,
+        found_node: Option<Node<'_>>,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         pub(crate) fn build_fn_sig_ty<'tcx>(
             infcx: &InferCtxt<'tcx>,
@@ -1756,6 +1758,75 @@ pub(crate) fn build_fn_sig_ty<'tcx>(
 
         self.note_conflicting_closure_bounds(cause, &mut err);
 
+        let found_args = match found.kind() {
+            ty::FnPtr(f) => f.inputs().skip_binder().iter(),
+            kind => {
+                span_bug!(span, "found was converted to a FnPtr above but is now {:?}", kind)
+            }
+        };
+        let expected_args = match expected.kind() {
+            ty::FnPtr(f) => f.inputs().skip_binder().iter(),
+            kind => {
+                span_bug!(span, "expected was converted to a FnPtr above but is now {:?}", kind)
+            }
+        };
+
+        if let Some(found_node) = found_node {
+            let fn_decl = match found_node {
+                Node::Expr(expr) => match &expr.kind {
+                    hir::ExprKind::Closure(hir::Closure { fn_decl, .. }) => fn_decl,
+                    kind => {
+                        span_bug!(found_span, "expression must be a closure but is {:?}", kind)
+                    }
+                },
+                Node::Item(item) => match &item.kind {
+                    hir::ItemKind::Fn(signature, _generics, _body) => signature.decl,
+                    kind => {
+                        span_bug!(found_span, "item must be a function but is {:?}", kind)
+                    }
+                },
+                node => {
+                    span_bug!(found_span, "node must be a expr or item but is {:?}", node)
+                }
+            };
+
+            let arg_spans = fn_decl.inputs.iter().map(|ty| ty.span);
+
+            fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, usize) {
+                let mut refs = 0;
+
+                while let ty::Ref(_, new_ty, _) = ty.kind() {
+                    ty = *new_ty;
+                    refs += 1;
+                }
+
+                (ty, refs)
+            }
+
+            for ((found_arg, expected_arg), arg_span) in
+                found_args.zip(expected_args).zip(arg_spans)
+            {
+                let (found_ty, found_refs) = get_deref_type_and_refs(*found_arg);
+                let (expected_ty, expected_refs) = get_deref_type_and_refs(*expected_arg);
+
+                if found_ty == expected_ty {
+                    let hint = if found_refs < expected_refs {
+                        "hint: consider borrowing here:"
+                    } else if found_refs == expected_refs {
+                        continue;
+                    } else {
+                        "hint: consider removing the borrow:"
+                    };
+                    err.span_suggestion_verbose(
+                        arg_span,
+                        hint,
+                        expected_arg.to_string(),
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+            }
+        }
+
         err
     }