Auto merge of #24906 - pnkfelix:fsk-fix-24895, r=alexcrichton

dropck: Remove `Copy` from special-cased traits

Fix #24895.

[breaking-change]

What does this break?  Basically, code that implements `Drop` and is
using `T:Copy` for one of its type parameters and is relying on the
Drop Check rule not applying to it.

Here is an example:

```rust
#![allow(dead_code,unused_variables,unused_assignments)]
struct D<T:Copy>(T);
impl<T:Copy> Drop for D<T> { fn drop(&mut self) { } }

trait UserT { fn c(&self) { } }
impl<T:Copy> UserT for T { }
struct E<T:UserT>(T);
impl<T:UserT> Drop for E<T> { fn drop(&mut self) { } }

// This one will start breaking.
fn foo() { let (d2, d1); d1 = D(34); d2 = D(&d1); }

#[cfg(this_one_does_and_should_always_break)]
fn bar() { let (e2, e1); e1 = E(34); e2 = E(&e1); }

fn main() {
    foo();
}
```

Author: bors

src/librustc_typeck/check/dropck.rs

@@ -464,9 +464,9 @@ fn iterate_over_potentially_unsafe_regions_in_type<'a, 'tcx>(
                     ty::Predicate::Trait(ty::Binder(ref t_pred)) => {
                         let def_id = t_pred.trait_ref.def_id;
                         match rcx.tcx().lang_items.to_builtin_kind(def_id) {
+                            // Issue 24895: deliberately do not include `BoundCopy` here.
                             Some(ty::BoundSend) |
                             Some(ty::BoundSized) |
-                            Some(ty::BoundCopy) |
                             Some(ty::BoundSync) => false,
                             _ => true,
                         }

src/test/compile-fail/issue-24895-copy-clone-dropck.rs

@@ -0,0 +1,38 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Check that one cannot subvert Drop Check rule via a user-defined
+// Clone implementation.
+
+#![allow(unused_variables, unused_assignments)]
+
+struct D<T:Copy>(T, &'static str);
+
+#[derive(Copy)]
+struct S<'a>(&'a D<i32>, &'static str);
+impl<'a> Clone for S<'a> {
+    fn clone(&self) -> S<'a> {
+        println!("cloning `S(_, {})` and thus accessing: {}", self.1, (self.0).0);
+        S(self.0, self.1)
+    }
+}
+
+impl<T:Copy> Drop for D<T> {
+    fn drop(&mut self) {
+        println!("calling Drop for {}", self.1);
+        let _call = self.0.clone();
+    }
+}
+
+fn main() {
+    let (d2, d1);
+    d1 = D(34, "d1");
+    d2 = D(S(&d1, "inner"), "d2"); //~ ERROR `d1` does not live long enough
+}