Permit DST types to unify like other types.

Also:
1. stop eagerly coercing from `[T, ..n]` to `[T]` only do so if requested.
2. permit error to be interact more freely.

Fixes rust-lang#17178.

Author: nikomatsakis

src/librustc/middle/typeck/infer/coercion.rs

@@ -72,7 +72,6 @@ use middle::typeck::infer::{CoerceResult, resolve_type, Coercion};
 use middle::typeck::infer::combine::{CombineFields, Combine};
 use middle::typeck::infer::sub::Sub;
 use middle::typeck::infer::resolve::try_resolve_tvar_shallow;
-use util::common::indenter;
 use util::ppaux;
 use util::ppaux::Repr;
 
@@ -93,7 +92,6 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         debug!("Coerce.tys({} => {})",
                a.repr(self.get_ref().infcx.tcx),
                b.repr(self.get_ref().infcx.tcx));
-        let _indent = indenter();
 
         // Special case: if the subtype is a sized array literal (`[T, ..n]`),
         // then it would get auto-borrowed to `&[T, ..n]` and then DST-ified
@@ -411,7 +409,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
 
         self.unpack_actual_value(ty_b, |sty_b|
             match (sty_a, sty_b) {
-                (&ty::ty_vec(t_a, Some(len)), _) => {
+                (&ty::ty_vec(t_a, Some(len)), &ty::ty_vec(_, None)) => {
                     let ty = ty::mk_vec(tcx, t_a, None);
                     Some((ty, ty::UnsizeLength(len)))
                 }

src/librustc/middle/typeck/infer/combine.rs

@@ -363,28 +363,6 @@ pub fn super_fn_sigs<'tcx, C: Combine<'tcx>>(this: &C,
 
 pub fn super_tys<'tcx, C: Combine<'tcx>>(this: &C, a: ty::t, b: ty::t) -> cres<ty::t> {
 
-    // This is a horrible hack - historically, [T] was not treated as a type,
-    // so, for example, &T and &[U] should not unify. In fact the only thing
-    // &[U] should unify with is &[T]. We preserve that behaviour with this
-    // check.
-    fn check_ptr_to_unsized<'tcx, C: Combine<'tcx>>(this: &C,
-                                                    a: ty::t,
-                                                    b: ty::t,
-                                                    a_inner: ty::t,
-                                                    b_inner: ty::t,
-                                                    result: ty::t) -> cres<ty::t> {
-        match (&ty::get(a_inner).sty, &ty::get(b_inner).sty) {
-            (&ty::ty_vec(_, None), &ty::ty_vec(_, None)) |
-            (&ty::ty_str, &ty::ty_str) |
-            (&ty::ty_trait(..), &ty::ty_trait(..)) => Ok(result),
-            (&ty::ty_vec(_, None), _) | (_, &ty::ty_vec(_, None)) |
-            (&ty::ty_str, _) | (_, &ty::ty_str) |
-            (&ty::ty_trait(..), _) | (_, &ty::ty_trait(..))
-                => Err(ty::terr_sorts(expected_found(this, a, b))),
-            _ => Ok(result),
-        }
-    }
-
     let tcx = this.infcx().tcx;
     let a_sty = &ty::get(a).sty;
     let b_sty = &ty::get(b).sty;
@@ -402,6 +380,10 @@ pub fn super_tys<'tcx, C: Combine<'tcx>>(this: &C, a: ty::t, b: ty::t) -> cres<t
                     b.repr(this.infcx().tcx)).as_slice());
       }
 
+      (&ty::ty_err, _) | (_, &ty::ty_err) => {
+          Ok(ty::mk_err())
+      }
+
         // Relate integral variables to other types
         (&ty::ty_infer(IntVar(a_id)), &ty::ty_infer(IntVar(b_id))) => {
             try!(this.infcx().simple_vars(this.a_is_expected(),
@@ -442,8 +424,7 @@ pub fn super_tys<'tcx, C: Combine<'tcx>>(this: &C, a: ty::t, b: ty::t) -> cres<t
       (&ty::ty_bool, _) |
       (&ty::ty_int(_), _) |
       (&ty::ty_uint(_), _) |
-      (&ty::ty_float(_), _) |
-      (&ty::ty_err, _) => {
+      (&ty::ty_float(_), _) => {
         if ty::get(a).sty == ty::get(b).sty {
             Ok(a)
         } else {
@@ -494,13 +475,13 @@ pub fn super_tys<'tcx, C: Combine<'tcx>>(this: &C, a: ty::t, b: ty::t) -> cres<t
       }
 
       (&ty::ty_uniq(a_inner), &ty::ty_uniq(b_inner)) => {
-            let typ = try!(this.tys(a_inner, b_inner));
-            check_ptr_to_unsized(this, a, b, a_inner, b_inner, ty::mk_uniq(tcx, typ))
+          let typ = try!(this.tys(a_inner, b_inner));
+          Ok(ty::mk_uniq(tcx, typ))
       }
 
       (&ty::ty_ptr(ref a_mt), &ty::ty_ptr(ref b_mt)) => {
-            let mt = try!(this.mts(a_mt, b_mt));
-            check_ptr_to_unsized(this, a, b, a_mt.ty, b_mt.ty, ty::mk_ptr(tcx, mt))
+          let mt = try!(this.mts(a_mt, b_mt));
+          Ok(ty::mk_ptr(tcx, mt))
       }
 
       (&ty::ty_rptr(a_r, ref a_mt), &ty::ty_rptr(b_r, ref b_mt)) => {
@@ -516,7 +497,7 @@ pub fn super_tys<'tcx, C: Combine<'tcx>>(this: &C, a: ty::t, b: ty::t) -> cres<t
                 }
                 _ => try!(this.mts(a_mt, b_mt))
             };
-            check_ptr_to_unsized(this, a, b, a_mt.ty, b_mt.ty, ty::mk_rptr(tcx, r, mt))
+            Ok(ty::mk_rptr(tcx, r, mt))
       }
 
       (&ty::ty_vec(a_t, sz_a), &ty::ty_vec(b_t, sz_b)) => {

src/librustc/middle/typeck/infer/sub.rs

@@ -139,30 +139,21 @@ impl<'f, 'tcx> Combine<'tcx> for Sub<'f, 'tcx> {
                     .relate_vars(a_id, SubtypeOf, b_id);
                 Ok(a)
             }
-            // The vec/str check here and below is so that we don't unify
-            // T with [T], this is necessary so we reflect subtyping of references
-            // (&T does not unify with &[T]) where that in turn is to reflect
-            // the historical non-typedness of [T].
-            (&ty::ty_infer(TyVar(_)), &ty::ty_str) |
-            (&ty::ty_infer(TyVar(_)), &ty::ty_vec(_, None)) => {
-                Err(ty::terr_sorts(expected_found(self, a, b)))
-            }
             (&ty::ty_infer(TyVar(a_id)), _) => {
                 try!(self.fields
                        .switch_expected()
                        .instantiate(b, SupertypeOf, a_id));
                 Ok(a)
             }
-
-            (&ty::ty_str, &ty::ty_infer(TyVar(_))) |
-            (&ty::ty_vec(_, None), &ty::ty_infer(TyVar(_))) => {
-                Err(ty::terr_sorts(expected_found(self, a, b)))
-            }
             (_, &ty::ty_infer(TyVar(b_id))) => {
                 try!(self.fields.instantiate(a, SubtypeOf, b_id));
                 Ok(a)
             }
 
+            (&ty::ty_err, _) | (_, &ty::ty_err) => {
+                Ok(ty::mk_err())
+            }
+
             (_, &ty::ty_bot) => {
                 Err(ty::terr_sorts(expected_found(self, a, b)))
             }