Fix let_and_return bad suggestion

Add a cast to the suggestion when the return expression has adjustments.
These adjustments are lost when the suggestion is applied.

This is similar to the problem in issue #4437.

Closes #5729

clippy_lints/src/returns.rs

@@ -99,7 +99,10 @@ fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'_>) {
                     |err| {
                         err.span_label(local.span, "unnecessary `let` binding");
 
-                        if let Some(snippet) = snippet_opt(cx, initexpr.span) {
+                        if let Some(mut snippet) = snippet_opt(cx, initexpr.span) {
+                            if !cx.typeck_results().expr_adjustments(&retexpr).is_empty() {
+                                snippet.push_str(" as _");
+                            }
                             err.multipart_suggestion(
                                 "return the expression directly",
                                 vec![

tests/ui/let_and_return.rs

@@ -135,4 +135,25 @@ fn test() -> i32 {
     }
 }
 
+mod issue_5729 {
+    use std::sync::Arc;
+
+    trait Foo {}
+
+    trait FooStorage {
+        fn foo_cloned(&self) -> Arc<dyn Foo>;
+    }
+
+    struct FooStorageImpl<T: Foo> {
+        foo: Arc<T>,
+    }
+
+    impl<T: Foo + 'static> FooStorage for FooStorageImpl<T> {
+        fn foo_cloned(&self) -> Arc<dyn Foo> {
+            let clone = Arc::clone(&self.foo);
+            clone
+        }
+    }
+}
+
 fn main() {}

tests/ui/let_and_return.stderr

@@ -27,5 +27,19 @@ LL |
 LL |         5
    |
 
-error: aborting due to 2 previous errors
+error: returning the result of a `let` binding from a block
+  --> $DIR/let_and_return.rs:154:13
+   |
+LL |             let clone = Arc::clone(&self.foo);
+   |             ---------------------------------- unnecessary `let` binding
+LL |             clone
+   |             ^^^^^
+   |
+help: return the expression directly
+   |
+LL |             
+LL |             Arc::clone(&self.foo) as _
+   |
+
+error: aborting due to 3 previous errors