Fix map_clone with deref and clone

Author: camsteffen

clippy_lints/src/map_clone.rs

@@ -1,6 +1,7 @@
 use crate::utils::paths;
 use crate::utils::{
-    is_copy, is_type_diagnostic_item, match_trait_method, remove_blocks, snippet_with_applicability, span_lint_and_sugg,
+    implements_trait, is_copy, is_type_diagnostic_item, match_trait_method, remove_blocks, snippet_with_applicability,
+    span_lint_and_sugg,
 };
 use if_chain::if_chain;
 use rustc_errors::Applicability;
@@ -83,7 +84,9 @@ impl<'tcx> LateLintPass<'tcx> for MapClone {
                                     if let ty::Ref(_, ty, _) = obj_ty.kind() {
                                         let copy = is_copy(cx, ty);
                                         lint(cx, e.span, args[0].span, copy);
-                                    } else {
+                                    } else if cx.tcx.lang_items().clone_trait()
+                                        .map_or(false, |clone_trait| implements_trait(cx, obj_ty, clone_trait, &[]))
+                                    {
                                         lint_needless_cloning(cx, e.span, args[0].span);
                                     }
                                 }

tests/ui/map_clone.fixed

@@ -44,4 +44,11 @@ fn main() {
         let v = vec![&mut d];
         let _: Vec<u32> = v.into_iter().map(|&mut x| x).collect();
     }
+
+    // Issue #6239 deref coercion and clone deref
+    {
+        use std::cell::RefCell;
+
+        let _ = Some(RefCell::new(String::new()).borrow()).map(|s| s.clone());
+    }
 }

tests/ui/map_clone.rs

@@ -44,4 +44,11 @@ fn main() {
         let v = vec![&mut d];
         let _: Vec<u32> = v.into_iter().map(|&mut x| x).collect();
     }
+
+    // Issue #6239 deref coercion and clone deref
+    {
+        use std::cell::RefCell;
+
+        let _ = Some(RefCell::new(String::new()).borrow()).map(|s| s.clone());
+    }
 }