Fix orphan checking (cc rust-lang#19470). (This is not a complete fix of rust-lang#19470 because of the backwards compatibility feature gate.)

This is a [breaking-change]. The new rules require that, for an impl of a trait defined
in some other crate, two conditions must hold:

1. Some type must be local.
2. Every type parameter must appear "under" some local type.

Here are some examples that are legal:

```rust
struct MyStruct<T> { ... }

// Here `T` appears "under' `MyStruct`.
impl<T> Clone for MyStruct<T> { }

// Here `T` appears "under' `MyStruct` as well. Note that it also appears
// elsewhere.
impl<T> Iterator<T> for MyStruct<T> { }
```

Here is an illegal example:

```rust
// Here `U` does not appear "under" `MyStruct` or any other local type.
// We call `U` "uncovered".
impl<T,U> Iterator<U> for MyStruct<T> { }
```

There are a couple of ways to rewrite this last example so that it is
legal:

1. In some cases, the uncovered type parameter (here, `U`) should be converted
   into an associated type. This is however a non-local change that requires access
   to the original trait. Also, associated types are not fully baked.
2. Add `U` as a type parameter of `MyStruct`:
   ```rust
   struct MyStruct<T,U> { ... }
   impl<T,U> Iterator<U> for MyStruct<T,U> { }
   ```
3. Create a newtype wrapper for `U`
   ```rust
   impl<T,U> Iterator<Wrapper<U>> for MyStruct<T,U> { }
   ```

Because associated types are not fully baked, which in the case of the
`Hash` trait makes adhering to this rule impossible, you can
temporarily disable this rule in your crate by using
`#![feature(old_orphan_check)]`. Note that the `old_orphan_check`
feature will be removed before 1.0 is released.

Author: nikomatsakis

src/liballoc/lib.rs

@@ -64,7 +64,8 @@
        html_root_url = "http://doc.rust-lang.org/nightly/")]
 
 #![no_std]
-#![feature(lang_items, phase, unsafe_destructor, default_type_params)]
+#![allow(unknown_features)]
+#![feature(lang_items, phase, unsafe_destructor, default_type_params, old_orphan_check)]
 
 #[phase(plugin, link)]
 extern crate core;

src/libcollections/lib.rs

@@ -25,6 +25,7 @@
 #![feature(macro_rules, default_type_params, phase, globs)]
 #![feature(unsafe_destructor, slicing_syntax)]
 #![feature(unboxed_closures)]
+#![feature(old_orphan_check)]
 #![no_std]
 
 #[phase(plugin, link)] extern crate core;

src/libgraphviz/maybe_owned_vec.rs

@@ -96,9 +96,9 @@ impl<'a, T: Ord> Ord for MaybeOwnedVector<'a, T> {
 }
 
 #[allow(deprecated)]
-impl<'a, T: PartialEq, Sized? V: AsSlice<T>> Equiv<V> for MaybeOwnedVector<'a, T> {
-    fn equiv(&self, other: &V) -> bool {
-        self.as_slice() == other.as_slice()
+impl<'a, T: PartialEq> Equiv<[T]> for MaybeOwnedVector<'a, T> {
+    fn equiv(&self, other: &[T]) -> bool {
+        self.as_slice() == other
     }
 }
 

src/librustc/lib.rs

@@ -22,10 +22,12 @@
       html_favicon_url = "http://www.rust-lang.org/favicon.ico",
       html_root_url = "http://doc.rust-lang.org/nightly/")]
 
+#![allow(unknown_features)]
 #![feature(default_type_params, globs, macro_rules, phase, quote)]
 #![feature(slicing_syntax, unsafe_destructor)]
 #![feature(rustc_diagnostic_macros)]
 #![feature(unboxed_closures)]
+#![feature(old_orphan_check)]
 
 extern crate arena;
 extern crate flate;

src/librustc/middle/traits/coherence.rs

@@ -14,10 +14,10 @@ use super::SelectionContext;
 use super::{Obligation, ObligationCause};
 use super::util;
 
-use middle::subst;
 use middle::subst::Subst;
 use middle::ty::{mod, Ty};
 use middle::infer::InferCtxt;
+use std::collections::HashSet;
 use std::rc::Rc;
 use syntax::ast;
 use syntax::codemap::DUMMY_SP;
@@ -52,9 +52,21 @@ pub fn impl_can_satisfy(infcx: &InferCtxt,
     selcx.evaluate_impl(impl2_def_id, &obligation)
 }
 
-pub fn impl_is_local(tcx: &ty::ctxt,
-                     impl_def_id: ast::DefId)
-                     -> bool
+#[allow(missing_copy_implementations)]
+pub enum OrphanCheckErr {
+    NoLocalInputType,
+    UncoveredTypeParameter(ty::ParamTy),
+}
+
+/// Checks the coherence orphan rules. `impl_def_id` should be the
+/// def-id of a trait impl. To pass, either the trait must be local, or else
+/// two conditions must be satisfied:
+///
+/// 1. At least one of the input types must involve a local type.
+/// 2. All type parameters must be covered by a local type.
+pub fn orphan_check(tcx: &ty::ctxt,
+                    impl_def_id: ast::DefId)
+                    -> Result<(), OrphanCheckErr>
 {
     debug!("impl_is_local({})", impl_def_id.repr(tcx));
 
@@ -63,99 +75,74 @@ pub fn impl_is_local(tcx: &ty::ctxt,
     let trait_ref = ty::impl_trait_ref(tcx, impl_def_id).unwrap();
     debug!("trait_ref={}", trait_ref.repr(tcx));
 
-    // If the trait is local to the crate, ok.
+    // If the *trait* is local to the crate, ok.
     if trait_ref.def_id.krate == ast::LOCAL_CRATE {
         debug!("trait {} is local to current crate",
                trait_ref.def_id.repr(tcx));
-        return true;
+        return Ok(());
     }
 
-    // Otherwise, at least one of the input types must be local to the
-    // crate.
-    trait_ref.input_types().iter().any(|&t| ty_is_local(tcx, t))
+    // Check condition 1: at least one type must be local.
+    if !trait_ref.input_types().iter().any(|&t| ty_reaches_local(tcx, t)) {
+        return Err(OrphanCheckErr::NoLocalInputType);
+    }
+
+    // Check condition 2: type parameters must be "covered" by a local type.
+    let covered_params: HashSet<_> =
+        trait_ref.input_types().iter()
+                               .flat_map(|&t| type_parameters_covered_by_ty(tcx, t).into_iter())
+                               .collect();
+    let all_params: HashSet<_> =
+        trait_ref.input_types().iter()
+                               .flat_map(|&t| type_parameters_reachable_from_ty(t).into_iter())
+                               .collect();
+    for &param in all_params.difference(&covered_params) {
+        return Err(OrphanCheckErr::UncoveredTypeParameter(param));
+    }
+
+    return Ok(());
+}
+
+fn ty_reaches_local<'tcx>(tcx: &ty::ctxt<'tcx>, ty: Ty<'tcx>) -> bool {
+    ty.walk().any(|t| ty_is_local_constructor(tcx, t))
 }
 
-pub fn ty_is_local<'tcx>(tcx: &ty::ctxt<'tcx>, ty: Ty<'tcx>) -> bool {
-    debug!("ty_is_local({})", ty.repr(tcx));
+fn ty_is_local_constructor<'tcx>(tcx: &ty::ctxt<'tcx>, ty: Ty<'tcx>) -> bool {
+    debug!("ty_is_local_constructor({})", ty.repr(tcx));
 
     match ty.sty {
         ty::ty_bool |
         ty::ty_char |
         ty::ty_int(..) |
         ty::ty_uint(..) |
         ty::ty_float(..) |
-        ty::ty_str(..) => {
-            false
-        }
-
-        ty::ty_unboxed_closure(..) => {
-            // This routine is invoked on types specified by users as
-            // part of an impl and hence an unboxed closure type
-            // cannot appear.
-            tcx.sess.bug("ty_is_local applied to unboxed closure type")
-        }
-
+        ty::ty_str(..) |
         ty::ty_bare_fn(..) |
-        ty::ty_closure(..) => {
+        ty::ty_closure(..) |
+        ty::ty_vec(..) |
+        ty::ty_ptr(..) |
+        ty::ty_rptr(..) |
+        ty::ty_tup(..) |
+        ty::ty_param(..) |
+        ty::ty_projection(..) => {
             false
         }
 
-        ty::ty_uniq(t) => {
-            let krate = tcx.lang_items.owned_box().map(|d| d.krate);
-            krate == Some(ast::LOCAL_CRATE) || ty_is_local(tcx, t)
-        }
-
-        ty::ty_vec(t, _) |
-        ty::ty_ptr(ty::mt { ty: t, .. }) |
-        ty::ty_rptr(_, ty::mt { ty: t, .. }) => {
-            ty_is_local(tcx, t)
-        }
-
-        ty::ty_tup(ref ts) => {
-            ts.iter().any(|&t| ty_is_local(tcx, t))
+        ty::ty_enum(def_id, _) |
+        ty::ty_struct(def_id, _) => {
+            def_id.krate == ast::LOCAL_CRATE
         }
 
-        ty::ty_enum(def_id, ref substs) |
-        ty::ty_struct(def_id, ref substs) => {
-            def_id.krate == ast::LOCAL_CRATE || {
-                let variances = ty::item_variances(tcx, def_id);
-                subst::ParamSpace::all().iter().any(|&space| {
-                    substs.types.get_slice(space).iter().enumerate().any(
-                        |(i, &t)| {
-                            match *variances.types.get(space, i) {
-                                ty::Bivariant => {
-                                    // If Foo<T> is bivariant with respect to
-                                    // T, then it doesn't matter whether T is
-                                    // local or not, because `Foo<U>` for any
-                                    // U will be a subtype of T.
-                                    false
-                                }
-                                ty::Contravariant |
-                                ty::Covariant |
-                                ty::Invariant => {
-                                    ty_is_local(tcx, t)
-                                }
-                            }
-                        })
-                })
-            }
+        ty::ty_uniq(_) => { // treat ~T like Box<T>
+            let krate = tcx.lang_items.owned_box().map(|d| d.krate);
+            krate == Some(ast::LOCAL_CRATE)
         }
 
         ty::ty_trait(ref tt) => {
             tt.principal_def_id().krate == ast::LOCAL_CRATE
         }
 
-        // Type parameters may be bound to types that are not local to
-        // the crate.
-        ty::ty_param(..) => {
-            false
-        }
-
-        // Associated types could be anything, I guess.
-        ty::ty_projection(..) => {
-            false
-        }
-
+        ty::ty_unboxed_closure(..) |
         ty::ty_infer(..) |
         ty::ty_open(..) |
         ty::ty_err => {
@@ -165,3 +152,27 @@ pub fn ty_is_local<'tcx>(tcx: &ty::ctxt<'tcx>, ty: Ty<'tcx>) -> bool {
         }
     }
 }
+
+fn type_parameters_covered_by_ty<'tcx>(tcx: &ty::ctxt<'tcx>,
+                                 ty: Ty<'tcx>)
+                                 -> HashSet<ty::ParamTy>
+{
+    if ty_is_local_constructor(tcx, ty) {
+        type_parameters_reachable_from_ty(ty)
+    } else {
+        ty.walk_children().flat_map(|t| type_parameters_covered_by_ty(tcx, t).into_iter()).collect()
+    }
+}
+
+/// All type parameters reachable from `ty`
+fn type_parameters_reachable_from_ty<'tcx>(ty: Ty<'tcx>) -> HashSet<ty::ParamTy> {
+    ty.walk()
+        .filter_map(|t| {
+            match t.sty {
+                ty::ty_param(ref param_ty) => Some(param_ty.clone()),
+                _ => None,
+            }
+        })
+        .collect()
+}
+

src/librustc/middle/traits/mod.rs

@@ -25,6 +25,8 @@ use syntax::codemap::{Span, DUMMY_SP};
 use util::ppaux::Repr;
 
 pub use self::error_reporting::report_fulfillment_errors;
+pub use self::coherence::orphan_check;
+pub use self::coherence::OrphanCheckErr;
 pub use self::fulfill::{FulfillmentContext, RegionObligation};
 pub use self::project::MismatchedProjectionTypes;
 pub use self::project::normalize;
@@ -245,15 +247,6 @@ pub struct VtableBuiltinData<N> {
     pub nested: subst::VecPerParamSpace<N>
 }
 
-/// True if neither the trait nor self type is local. Note that `impl_def_id` must refer to an impl
-/// of a trait, not an inherent impl.
-pub fn is_orphan_impl(tcx: &ty::ctxt,
-                      impl_def_id: ast::DefId)
-                      -> bool
-{
-    !coherence::impl_is_local(tcx, impl_def_id)
-}
-
 /// True if there exist types that satisfy both of the two given impls.
 pub fn overlapping_impls(infcx: &InferCtxt,
                          impl1_def_id: ast::DefId,

src/librustc_back/lib.rs

@@ -32,6 +32,7 @@
 #![allow(unknown_features)]
 #![feature(globs, phase, macro_rules, slicing_syntax)]
 #![feature(unboxed_closures)]
+#![feature(old_orphan_check)]
 
 #[phase(plugin, link)]
 extern crate log;

src/librustc_borrowck/lib.rs

@@ -16,10 +16,12 @@
       html_favicon_url = "http://www.rust-lang.org/favicon.ico",
       html_root_url = "http://doc.rust-lang.org/nightly/")]
 
+#![allow(unknown_features)]
 #![feature(default_type_params, globs, macro_rules, phase, quote)]
 #![feature(slicing_syntax, unsafe_destructor)]
 #![feature(rustc_diagnostic_macros)]
 #![feature(unboxed_closures)]
+#![feature(old_orphan_check)]
 #![allow(non_camel_case_types)]
 
 #[phase(plugin, link)] extern crate log;

src/librustc_trans/lib.rs

@@ -22,10 +22,12 @@
       html_favicon_url = "http://www.rust-lang.org/favicon.ico",
       html_root_url = "http://doc.rust-lang.org/nightly/")]
 
+#![allow(unknown_features)]
 #![feature(default_type_params, globs, macro_rules, phase, quote)]
 #![feature(slicing_syntax, unsafe_destructor)]
 #![feature(rustc_diagnostic_macros)]
 #![feature(unboxed_closures)]
+#![feature(old_orphan_check)]
 
 extern crate arena;
 extern crate flate;

src/librustc_typeck/coherence/orphan.rs

@@ -18,7 +18,7 @@ use syntax::ast;
 use syntax::ast_util;
 use syntax::codemap::Span;
 use syntax::visit;
-use util::ppaux::Repr;
+use util::ppaux::{Repr, UserString};
 
 pub fn check(tcx: &ty::ctxt) {
     let mut orphan = OrphanChecker { tcx: tcx };
@@ -67,10 +67,27 @@ impl<'cx, 'tcx,'v> visit::Visitor<'v> for OrphanChecker<'cx, 'tcx> {
             ast::ItemImpl(_, _, Some(_), _, _) => {
                 // "Trait" impl
                 debug!("coherence2::orphan check: trait impl {}", item.repr(self.tcx));
-                if traits::is_orphan_impl(self.tcx, def_id) {
-                    span_err!(self.tcx.sess, item.span, E0117,
-                              "cannot provide an extension implementation \
-                               where both trait and type are not defined in this crate");
+                match traits::orphan_check(self.tcx, def_id) {
+                    Ok(()) => { }
+                    Err(traits::OrphanCheckErr::NoLocalInputType) => {
+                        span_err!(self.tcx.sess, item.span, E0117,
+                                  "cannot provide an extension implementation \
+                                   where both trait and type are not defined in this crate");
+                    }
+                    Err(traits::OrphanCheckErr::UncoveredTypeParameter(param_ty)) => {
+                        if !self.tcx.sess.features.borrow().old_orphan_check {
+                            self.tcx.sess.span_err(
+                                item.span,
+                                format!("type parameter `{}` must also appear as a type parameter \
+                                         of some type defined within this crate",
+                                        param_ty.user_string(self.tcx)).as_slice());
+                            self.tcx.sess.span_note(
+                                item.span,
+                                format!("for a limited time, you can add \
+                                         `#![feature(old_orphan_check)]` to your crate \
+                                         to disable this rule").as_slice());
+                        }
+                    }
                 }
             }
             _ => {