Ignore not really redundant clones of ManuallyDrop

"Redundant" clones of `ManuallyDrop` are sometimes used for the side effect of
invoking the clone, without running the drop implementation of the inner type.
In other words, they aren't really redundant. For example, futures-rs crate:

```rust
#[allow(clippy::redundant_clone)] // The clone here isn't actually redundant.
unsafe fn increase_refcount<T: ArcWake>(data: *const ()) {
    // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
    let arc = mem::ManuallyDrop::new(Arc::<T>::from_raw(data as *const T));
    // Now increase refcount, but don't drop new refcount either
    let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
}
```

Ignore redundant clone lint for ManuallyDrop.

clippy_lints/src/redundant_clone.rs

@@ -124,6 +124,12 @@ impl<'tcx> LateLintPass<'tcx> for RedundantClone {
                 continue;
             }
 
+            if let ty::Adt(ref def, _) = arg_ty.kind {
+                if match_def_path(cx, def.did, &paths::MEM_MANUALLY_DROP) {
+                    continue;
+                }
+            }
+
             // `{ cloned = &arg; clone(move cloned); }` or `{ cloned = &arg; to_path_buf(cloned); }`
             let (cloned, cannot_move_out) = unwrap_or_continue!(find_stmt_assigns_to(cx, mir, arg, from_borrow, bb));
 

clippy_lints/src/utils/paths.rs

@@ -59,6 +59,7 @@ pub const LINKED_LIST: [&str; 4] = ["alloc", "collections", "linked_list", "Link
 pub const LINT: [&str; 3] = ["rustc_session", "lint", "Lint"];
 pub const MEM_DISCRIMINANT: [&str; 3] = ["core", "mem", "discriminant"];
 pub const MEM_FORGET: [&str; 3] = ["core", "mem", "forget"];
+pub const MEM_MANUALLY_DROP: [&str; 4] = ["core", "mem", "manually_drop", "ManuallyDrop"];
 pub const MEM_MAYBEUNINIT: [&str; 4] = ["core", "mem", "maybe_uninit", "MaybeUninit"];
 pub const MEM_MAYBEUNINIT_UNINIT: [&str; 5] = ["core", "mem", "maybe_uninit", "MaybeUninit", "uninit"];
 pub const MEM_REPLACE: [&str; 3] = ["core", "mem", "replace"];

tests/ui/redundant_clone.fixed

@@ -52,6 +52,7 @@ fn main() {
     borrower_propagation();
     not_consumed();
     issue_5405();
+    manually_drop();
 }
 
 #[derive(Clone)]
@@ -170,3 +171,17 @@ fn issue_5405() {
     let c: [usize; 2] = [2, 3];
     let _d: usize = c[1].clone();
 }
+
+fn manually_drop() {
+    use std::mem::ManuallyDrop;
+    use std::sync::Arc;
+
+    let a = ManuallyDrop::new(Arc::new("Hello!".to_owned()));
+    let _ = a.clone(); // OK
+
+    let p: *const String = Arc::into_raw(ManuallyDrop::into_inner(a));
+    unsafe {
+        Arc::from_raw(p);
+        Arc::from_raw(p);
+    }
+}

tests/ui/redundant_clone.rs

@@ -52,6 +52,7 @@ fn main() {
     borrower_propagation();
     not_consumed();
     issue_5405();
+    manually_drop();
 }
 
 #[derive(Clone)]
@@ -170,3 +171,17 @@ fn issue_5405() {
     let c: [usize; 2] = [2, 3];
     let _d: usize = c[1].clone();
 }
+
+fn manually_drop() {
+    use std::mem::ManuallyDrop;
+    use std::sync::Arc;
+
+    let a = ManuallyDrop::new(Arc::new("Hello!".to_owned()));
+    let _ = a.clone(); // OK
+
+    let p: *const String = Arc::into_raw(ManuallyDrop::into_inner(a));
+    unsafe {
+        Arc::from_raw(p);
+        Arc::from_raw(p);
+    }
+}

tests/ui/redundant_clone.stderr

@@ -108,61 +108,61 @@ LL |     let _t = tup.0.clone();
    |              ^^^^^
 
 error: redundant clone
-  --> $DIR/redundant_clone.rs:61:22
+  --> $DIR/redundant_clone.rs:62:22
    |
 LL |         (a.clone(), a.clone())
    |                      ^^^^^^^^ help: remove this
    |
 note: this value is dropped without further use
-  --> $DIR/redundant_clone.rs:61:21
+  --> $DIR/redundant_clone.rs:62:21
    |
 LL |         (a.clone(), a.clone())
    |                     ^
 
-error: redundant clone
-  --> $DIR/redundant_clone.rs:121:15
-   |
-LL |     let _s = s.clone();
-   |               ^^^^^^^^ help: remove this
-   |
-note: this value is dropped without further use
-  --> $DIR/redundant_clone.rs:121:14
-   |
-LL |     let _s = s.clone();
-   |              ^
-
 error: redundant clone
   --> $DIR/redundant_clone.rs:122:15
    |
-LL |     let _t = t.clone();
+LL |     let _s = s.clone();
    |               ^^^^^^^^ help: remove this
    |
 note: this value is dropped without further use
   --> $DIR/redundant_clone.rs:122:14
    |
+LL |     let _s = s.clone();
+   |              ^
+
+error: redundant clone
+  --> $DIR/redundant_clone.rs:123:15
+   |
+LL |     let _t = t.clone();
+   |               ^^^^^^^^ help: remove this
+   |
+note: this value is dropped without further use
+  --> $DIR/redundant_clone.rs:123:14
+   |
 LL |     let _t = t.clone();
    |              ^
 
 error: redundant clone
-  --> $DIR/redundant_clone.rs:132:19
+  --> $DIR/redundant_clone.rs:133:19
    |
 LL |         let _f = f.clone();
    |                   ^^^^^^^^ help: remove this
    |
 note: this value is dropped without further use
-  --> $DIR/redundant_clone.rs:132:18
+  --> $DIR/redundant_clone.rs:133:18
    |
 LL |         let _f = f.clone();
    |                  ^
 
 error: redundant clone
-  --> $DIR/redundant_clone.rs:144:14
+  --> $DIR/redundant_clone.rs:145:14
    |
 LL |     let y = x.clone().join("matthias");
    |              ^^^^^^^^ help: remove this
    |
 note: cloned value is neither consumed nor mutated
-  --> $DIR/redundant_clone.rs:144:13
+  --> $DIR/redundant_clone.rs:145:13
    |
 LL |     let y = x.clone().join("matthias");
    |             ^^^^^^^^^