Rollup merge of rust-lang#40319 - eddyb:it's-"unsize"-not-"unsound", r=nikomatsakis

Disallow subtyping between T and U in T: Unsize<U>.

Because `&mut T` can be coerced to `&mut U`, `T` and `U` must be unified invariantly. Fixes rust-lang#40288.
E.g. coercing `&mut [&'a X; N]` to `&mut [&'b X]` must require `'a` be equal to `'b`, otherwise you can convert between `&'a X` and `&'b X` (in either direction), potentially unsoundly lengthening lifetimes.

Subtyping here was introduced with `Unsize` in rust-lang#24619 (landed in 1.1, original PR is rust-lang#23785).

Author: Ariel Ben-Yehuda

src/librustc/traits/select.rs

@@ -2461,7 +2461,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 let new_trait = tcx.mk_dynamic(
                     ty::Binder(tcx.mk_existential_predicates(iter)), r_b);
                 let InferOk { obligations, .. } =
-                    self.infcx.sub_types(false, &obligation.cause, new_trait, target)
+                    self.infcx.eq_types(false, &obligation.cause, new_trait, target)
                     .map_err(|_| Unimplemented)?;
                 self.inferred_obligations.extend(obligations);
 
@@ -2520,7 +2520,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             // [T; n] -> [T].
             (&ty::TyArray(a, _), &ty::TySlice(b)) => {
                 let InferOk { obligations, .. } =
-                    self.infcx.sub_types(false, &obligation.cause, a, b)
+                    self.infcx.eq_types(false, &obligation.cause, a, b)
                     .map_err(|_| Unimplemented)?;
                 self.inferred_obligations.extend(obligations);
             }
@@ -2583,7 +2583,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 });
                 let new_struct = tcx.mk_adt(def, tcx.mk_substs(params));
                 let InferOk { obligations, .. } =
-                    self.infcx.sub_types(false, &obligation.cause, new_struct, target)
+                    self.infcx.eq_types(false, &obligation.cause, new_struct, target)
                     .map_err(|_| Unimplemented)?;
                 self.inferred_obligations.extend(obligations);
 

src/librustc_typeck/check/autoderef.rs

@@ -12,6 +12,7 @@ use astconv::AstConv;
 
 use super::FnCtxt;
 
+use rustc::infer::InferOk;
 use rustc::traits;
 use rustc::ty::{self, Ty, TraitRef};
 use rustc::ty::{ToPredicate, TypeFoldable};
@@ -149,6 +150,14 @@ impl<'a, 'gcx, 'tcx> Autoderef<'a, 'gcx, 'tcx> {
 
     pub fn finalize<'b, I>(self, pref: LvaluePreference, exprs: I)
         where I: IntoIterator<Item = &'b hir::Expr>
+    {
+        let fcx = self.fcx;
+        fcx.register_infer_ok_obligations(self.finalize_as_infer_ok(pref, exprs));
+    }
+
+    pub fn finalize_as_infer_ok<'b, I>(self, pref: LvaluePreference, exprs: I)
+                                       -> InferOk<'tcx, ()>
+        where I: IntoIterator<Item = &'b hir::Expr>
     {
         let methods: Vec<_> = self.steps
             .iter()
@@ -176,8 +185,9 @@ impl<'a, 'gcx, 'tcx> Autoderef<'a, 'gcx, 'tcx> {
             }
         }
 
-        for obligation in self.obligations {
-            self.fcx.register_predicate(obligation);
+        InferOk {
+            value: (),
+            obligations: self.obligations
         }
     }
 }