rollup merge of rust-lang#23638: pnkfelix/fsk-reject-specialized-drops

Reject specialized Drop impls.

See Issue rust-lang#8142 for discussion.

This makes it illegal for a Drop impl to be more specialized than the original item.

So for example, all of the following are now rejected (when they would have been blindly accepted before):

```rust
struct S<A> { ... };
impl Drop for S<i8> { ... } // error: specialized to concrete type

struct T<'a> { ... };
impl Drop for T<'static> { ... } // error: specialized to concrete region

struct U<A> { ... };
impl<A:Clone> Drop for U<A> { ... } // error: added extra type requirement

struct V<'a,'b>;
impl<'a,'b:a> Drop for V<'a,'b> { ... } // error: added extra region requirement
```

Due to examples like the above, this is a [breaking-change].

(The fix is to either remove the specialization from the `Drop` impl, or to transcribe the requirements into the struct/enum definition; examples of both are shown in the PR's fixed to `libstd`.)

----

This is likely to be the last thing blocking the removal of the `#[unsafe_destructor]` attribute.

Fix rust-lang#8142
Fix rust-lang#23584

Author: alexcrichton

src/doc/trpl/unsafe.md

@@ -197,15 +197,16 @@ use std::ptr;
 
 // Define a wrapper around the handle returned by the foreign code.
 // Unique<T> has the same semantics as Box<T>
-pub struct Unique<T> {
+//
+// NB: For simplicity and correctness, we require that T has kind Send
+// (owned boxes relax this restriction).
+pub struct Unique<T: Send> {
     // It contains a single raw, mutable pointer to the object in question.
     ptr: *mut T
 }
 
 // Implement methods for creating and using the values in the box.
 
-// NB: For simplicity and correctness, we require that T has kind Send
-// (owned boxes relax this restriction).
 impl<T: Send> Unique<T> {
     pub fn new(value: T) -> Unique<T> {
         unsafe {
@@ -239,11 +240,11 @@ impl<T: Send> Unique<T> {
 // Unique<T>, making the struct manage the raw pointer: when the
 // struct goes out of scope, it will automatically free the raw pointer.
 //
-// NB: This is an unsafe destructor, because rustc will not normally
-// allow destructors to be associated with parameterized types, due to
-// bad interaction with managed boxes. (With the Send restriction,
-// we don't have this problem.) Note that the `#[unsafe_destructor]`
-// feature gate is required to use unsafe destructors.
+// NB: This is an unsafe destructor; rustc will not normally allow
+// destructors to be associated with parameterized types (due to
+// historically failing to check them soundly).  Note that the
+// `#[unsafe_destructor]` feature gate is currently required to use
+// unsafe destructors.
 #[unsafe_destructor]
 impl<T: Send> Drop for Unique<T> {
     fn drop(&mut self) {

src/liballoc/arc.rs

@@ -321,7 +321,7 @@ impl<T: Send + Sync + Clone> Arc<T> {
 
 #[unsafe_destructor]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Sync + Send> Drop for Arc<T> {
+impl<T> Drop for Arc<T> {
     /// Drops the `Arc<T>`.
     ///
     /// This will decrement the strong reference count. If the strong reference
@@ -388,7 +388,7 @@ impl<T: Sync + Send> Drop for Arc<T> {
 
 #[unstable(feature = "alloc",
            reason = "Weak pointers may not belong in this module.")]
-impl<T: Sync + Send> Weak<T> {
+impl<T> Weak<T> {
     /// Upgrades a weak reference to a strong reference.
     ///
     /// Upgrades the `Weak<T>` reference to an `Arc<T>`, if possible.
@@ -454,7 +454,7 @@ impl<T: Sync + Send> Clone for Weak<T> {
 
 #[unsafe_destructor]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Sync + Send> Drop for Weak<T> {
+impl<T> Drop for Weak<T> {
     /// Drops the `Weak<T>`.
     ///
     /// This will decrement the weak reference count.

src/librustc/middle/infer/higher_ranked/mod.rs

@@ -14,6 +14,7 @@
 use super::{CombinedSnapshot, cres, InferCtxt, HigherRankedType, SkolemizationMap};
 use super::combine::{Combine, Combineable};
 
+use middle::subst;
 use middle::ty::{self, Binder};
 use middle::ty_fold::{self, TypeFoldable};
 use syntax::codemap::Span;
@@ -455,6 +456,63 @@ impl<'a,'tcx> InferCtxtExt for InferCtxt<'a,'tcx> {
     }
 }
 
+/// Constructs and returns a substitution that, for a given type
+/// scheme parameterized by `generics`, will replace every generic
+/// parmeter in the type with a skolemized type/region (which one can
+/// think of as a "fresh constant", except at the type/region level of
+/// reasoning).
+///
+/// Since we currently represent bound/free type parameters in the
+/// same way, this only has an effect on regions.
+///
+/// (Note that unlike a substitution from `ty::construct_free_substs`,
+/// this inserts skolemized regions rather than free regions; this
+/// allows one to use `fn leak_check` to catch attmepts to unify the
+/// skolemized regions with e.g. the `'static` lifetime)
+pub fn construct_skolemized_substs<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
+                                            generics: &ty::Generics<'tcx>,
+                                            snapshot: &CombinedSnapshot)
+                                            -> (subst::Substs<'tcx>, SkolemizationMap)
+{
+    let mut map = FnvHashMap();
+
+    // map T => T
+    let mut types = subst::VecPerParamSpace::empty();
+    push_types_from_defs(infcx.tcx, &mut types, generics.types.as_slice());
+
+    // map early- or late-bound 'a => fresh 'a
+    let mut regions = subst::VecPerParamSpace::empty();
+    push_region_params(infcx, &mut map, &mut regions, generics.regions.as_slice(), snapshot);
+
+    let substs = subst::Substs { types: types,
+                                 regions: subst::NonerasedRegions(regions) };
+    return (substs, map);
+
+    fn push_region_params<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
+                                   map: &mut SkolemizationMap,
+                                   regions: &mut subst::VecPerParamSpace<ty::Region>,
+                                   region_params: &[ty::RegionParameterDef],
+                                   snapshot: &CombinedSnapshot)
+    {
+        for r in region_params {
+            let br = r.to_bound_region();
+            let skol_var = infcx.region_vars.new_skolemized(br, &snapshot.region_vars_snapshot);
+            let sanity_check = map.insert(br, skol_var);
+            assert!(sanity_check.is_none());
+            regions.push(r.space, skol_var);
+        }
+    }
+
+    fn push_types_from_defs<'tcx>(tcx: &ty::ctxt<'tcx>,
+                                  types: &mut subst::VecPerParamSpace<ty::Ty<'tcx>>,
+                                  defs: &[ty::TypeParameterDef<'tcx>]) {
+        for def in defs {
+            let ty = ty::mk_param_from_def(tcx, def);
+            types.push(def.space, ty);
+        }
+    }
+}
+
 pub fn skolemize_late_bound_regions<'a,'tcx,T>(infcx: &InferCtxt<'a,'tcx>,
                                                binder: &ty::Binder<T>,
                                                snapshot: &CombinedSnapshot)

src/librustc/middle/infer/mod.rs

@@ -726,6 +726,15 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         })
     }
 
+    pub fn construct_skolemized_subst(&self,
+                                      generics: &ty::Generics<'tcx>,
+                                      snapshot: &CombinedSnapshot)
+                                      -> (subst::Substs<'tcx>, SkolemizationMap) {
+        /*! See `higher_ranked::construct_skolemized_subst` */
+
+        higher_ranked::construct_skolemized_substs(self, generics, snapshot)
+    }
+
     pub fn skolemize_late_bound_regions<T>(&self,
                                            value: &ty::Binder<T>,
                                            snapshot: &CombinedSnapshot)

src/librustc/middle/ty.rs

@@ -1793,6 +1793,9 @@ impl RegionParameterDef {
     pub fn to_early_bound_region(&self) -> ty::Region {
         ty::ReEarlyBound(self.def_id.node, self.space, self.index, self.name)
     }
+    pub fn to_bound_region(&self) -> ty::BoundRegion {
+        ty::BoundRegion::BrNamed(self.def_id, self.name)
+    }
 }
 
 /// Information about the formal type/lifetime parameters associated