Auto merge of #39655 - durka:recursion-limit-suggestion, r=nikomatsakis

suggest doubling recursion limit in more situations

Fixes #38852.

r? @bluss

src/librustc_typeck/check/autoderef.rs

@@ -54,12 +54,16 @@ impl<'a, 'gcx, 'tcx> Iterator for Autoderef<'a, 'gcx, 'tcx> {
 
         if self.steps.len() == tcx.sess.recursion_limit.get() {
             // We've reached the recursion limit, error gracefully.
+            let suggested_limit = tcx.sess.recursion_limit.get() * 2;
             struct_span_err!(tcx.sess,
                              self.span,
                              E0055,
                              "reached the recursion limit while auto-dereferencing {:?}",
                              self.cur_ty)
                 .span_label(self.span, &format!("deref recursion limit reached"))
+                .help(&format!(
+                        "consider adding a `#[recursion_limit=\"{}\"]` attribute to your crate",
+                        suggested_limit))
                 .emit();
             return None;
         }