Auto merge of #23938 - nikomatsakis:invariant, r=pnkfelix

There are still some remnants we could remove from the compiler (e.g. references to "subtraitrefs"; traits still have variance entries in the variance table), but this removes all user-visible bits I believe.

r? @pnkfelix 

Fixes #22806 (since such traits would no longer exist)

src/libarena/lib.rs

@@ -192,13 +192,16 @@ struct TyDesc {
     align: usize
 }
 
+trait AllTypes { fn dummy(&self) { } }
+impl<T:?Sized> AllTypes for T { }
+
 unsafe fn get_tydesc<T>() -> *const TyDesc {
     use std::raw::TraitObject;
 
     let ptr = &*(1 as *const T);
 
     // Can use any trait that is implemented for all types.
-    let obj = mem::transmute::<&marker::MarkerTrait, TraitObject>(ptr);
+    let obj = mem::transmute::<&AllTypes, TraitObject>(ptr);
     obj.vtable as *const TyDesc
 }
 

src/libcore/marker.rs

@@ -35,6 +35,7 @@ use hash::Hasher;
 #[stable(feature = "rust1", since = "1.0.0")]
 #[lang="send"]
 #[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"]
+#[allow(deprecated)]
 pub unsafe trait Send : MarkerTrait {
     // empty.
 }
@@ -50,6 +51,7 @@ impl !Send for Managed { }
 #[lang="sized"]
 #[rustc_on_unimplemented = "`{Self}` does not have a constant size known at compile-time"]
 #[fundamental] // for Default, for example, which requires that `[T]: !Default` be evaluatable
+#[allow(deprecated)]
 pub trait Sized : MarkerTrait {
     // Empty.
 }
@@ -203,6 +205,7 @@ pub trait Copy : Clone {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[lang="sync"]
 #[rustc_on_unimplemented = "`{Self}` cannot be shared between threads safely"]
+#[allow(deprecated)]
 pub unsafe trait Sync : MarkerTrait {
     // Empty
 }
@@ -269,84 +272,41 @@ macro_rules! impls{
         )
 }
 
-/// `MarkerTrait` is intended to be used as the supertrait for traits
-/// that don't have any methods but instead serve just to designate
-/// categories of types. An example would be the `Send` trait, which
-/// indicates types that are sendable: `Send` does not itself offer
-/// any methods, but instead is used to gate access to data.
-///
-/// FIXME. Better documentation needed here!
-#[stable(feature = "rust1", since = "1.0.0")]
+/// `MarkerTrait` is deprecated and no longer needed.
+#[unstable(feature = "core", reason = "deprecated")]
+#[deprecated(since = "1.0.0", reason = "No longer needed")]
+#[allow(deprecated)]
+#[cfg(stage0)]
 pub trait MarkerTrait : PhantomFn<Self,Self> { }
-//                                    ~~~~~ <-- FIXME(#22806)?
-//
-// Marker trait has been made invariant so as to avoid inf recursion,
-// but we should ideally solve the underlying problem. That's a bit
-// complicated.
 
+/// `MarkerTrait` is deprecated and no longer needed.
+#[unstable(feature = "core", reason = "deprecated")]
+#[deprecated(since = "1.0.0", reason = "No longer needed")]
+#[allow(deprecated)]
+#[cfg(not(stage0))]
+pub trait MarkerTrait { }
+
+#[allow(deprecated)]
 impl<T:?Sized> MarkerTrait for T { }
 
-/// `PhantomFn` is a marker trait for use with traits that contain
-/// type or lifetime parameters that do not appear in any of their
-/// methods. In that case, you can either remove those parameters, or
-/// add a `PhantomFn` supertrait that reflects the signature of
-/// methods that compiler should "pretend" exists. This most commonly
-/// occurs for traits with no methods: in that particular case, you
-/// can extend `MarkerTrait`, which is equivalent to
-/// `PhantomFn<Self>`.
-///
-/// # Examples
-///
-/// As an example, consider a trait with no methods like `Even`, meant
-/// to represent types that are "even":
-///
-/// ```rust,ignore
-/// trait Even { }
-/// ```
-///
-/// In this case, because the implicit parameter `Self` is unused, the
-/// compiler will issue an error. The only purpose of this trait is to
-/// categorize types (and hence instances of those types) as "even" or
-/// not, so if we *were* going to have a method, it might look like:
-///
-/// ```rust,ignore
-/// trait Even {
-///     fn is_even(self) -> bool { true }
-/// }
-/// ```
-///
-/// Therefore, we can model a method like this as follows:
-///
-/// ```
-/// use std::marker::PhantomFn;
-/// trait Even : PhantomFn<Self> { }
-/// ```
-///
-/// Another equivalent, but clearer, option would be to use
-/// `MarkerTrait`:
-///
-/// ```
-/// # #![feature(core)]
-/// use std::marker::MarkerTrait;
-/// trait Even : MarkerTrait { }
-/// ```
-///
-/// # Parameters
-///
-/// - `A` represents the type of the method's argument. You can use a
-///   tuple to represent "multiple" arguments. Any types appearing here
-///   will be considered "contravariant".
-/// - `R`, if supplied, represents the method's return type. This defaults
-///   to `()` as it is rarely needed.
-///
-/// # Additional reading
-///
-/// More details and background can be found in [RFC 738][738].
-///
-/// [738]: https://github.com/rust-lang/rfcs/blob/master/text/0738-variance.md
+/// `PhantomFn` is a deprecated marker trait that is no longer needed.
 #[lang="phantom_fn"]
-#[stable(feature = "rust1", since = "1.0.0")]
-pub trait PhantomFn<A:?Sized,R:?Sized=()> { }
+#[unstable(feature = "core", reason = "deprecated")]
+#[deprecated(since = "1.0.0", reason = "No longer needed")]
+#[cfg(stage0)]
+pub trait PhantomFn<A:?Sized,R:?Sized=()> {
+}
+
+/// `PhantomFn` is a deprecated marker trait that is no longer needed.
+#[unstable(feature = "core", reason = "deprecated")]
+#[deprecated(since = "1.0.0", reason = "No longer needed")]
+#[cfg(not(stage0))]
+pub trait PhantomFn<A:?Sized,R:?Sized=()> {
+}
+
+#[allow(deprecated)]
+#[cfg(not(stage0))]
+impl<A:?Sized,R:?Sized,T:?Sized> PhantomFn<A,R> for T { }
 
 /// `PhantomData<T>` allows you to describe that a type acts as if it stores a value of type `T`,
 /// even though it does not. This allows you to inform the compiler about certain safety properties
@@ -444,6 +404,7 @@ mod impls {
 /// [1]: http://en.wikipedia.org/wiki/Parametricity
 #[rustc_reflect_like]
 #[unstable(feature = "core", reason = "requires RFC and more experience")]
+#[allow(deprecated)]
 pub trait Reflect : MarkerTrait {
 }
 

src/libcore/nonzero.rs

@@ -14,6 +14,7 @@ use marker::{Sized, MarkerTrait};
 use ops::Deref;
 
 /// Unsafe trait to indicate what types are usable with the NonZero struct
+#[allow(deprecated)]
 pub unsafe trait Zeroable : MarkerTrait {}
 
 unsafe impl<T:?Sized> Zeroable for *const T {}

src/librustc/middle/lang_items.rs

@@ -321,7 +321,6 @@ lets_do_this! {
     ExchangeHeapLangItem,            "exchange_heap",           exchange_heap;
     OwnedBoxLangItem,                "owned_box",               owned_box;
 
-    PhantomFnItem,                   "phantom_fn",              phantom_fn;
     PhantomDataItem,                 "phantom_data",            phantom_data;
 
     // Deprecated:

src/librustc/middle/traits/object_safety.rs

@@ -138,11 +138,10 @@ fn supertraits_reference_self<'tcx>(tcx: &ty::ctxt<'tcx>,
             match predicate {
                 ty::Predicate::Trait(ref data) => {
                     // In the case of a trait predicate, we can skip the "self" type.
-                    Some(data.def_id()) != tcx.lang_items.phantom_fn() &&
-                        data.0.trait_ref.substs.types.get_slice(TypeSpace)
-                                                     .iter()
-                                                     .cloned()
-                                                     .any(is_self)
+                    data.0.trait_ref.substs.types.get_slice(TypeSpace)
+                                                 .iter()
+                                                 .cloned()
+                                                 .any(is_self)
                 }
                 ty::Predicate::Projection(..) |
                 ty::Predicate::TypeOutlives(..) |

src/librustc/middle/traits/select.rs

@@ -836,14 +836,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             ambiguous: false
         };
 
-        // Check for the `PhantomFn` trait. This is really just a
-        // special annotation that is *always* considered to match, no
-        // matter what the type parameters are etc.
-        if self.tcx().lang_items.phantom_fn() == Some(obligation.predicate.def_id()) {
-            candidates.vec.push(PhantomFnCandidate);
-            return Ok(candidates);
-        }
-
         // Other bounds. Consider both in-scope bounds from fn decl
         // and applicable impls. There is a certain set of precedence rules here.
 

src/librustc/middle/ty_relate/mod.rs

@@ -122,11 +122,11 @@ fn relate_item_substs<'a,'tcx:'a,R>(relation: &mut R,
     relate_substs(relation, opt_variances, a_subst, b_subst)
 }
 
-fn relate_substs<'a,'tcx,R>(relation: &mut R,
-                            variances: Option<&ty::ItemVariances>,
-                            a_subst: &Substs<'tcx>,
-                            b_subst: &Substs<'tcx>)
-                            -> RelateResult<'tcx, Substs<'tcx>>
+fn relate_substs<'a,'tcx:'a,R>(relation: &mut R,
+                               variances: Option<&ty::ItemVariances>,
+                               a_subst: &Substs<'tcx>,
+                               b_subst: &Substs<'tcx>)
+                               -> RelateResult<'tcx, Substs<'tcx>>
     where R: TypeRelation<'a,'tcx>
 {
     let mut substs = Substs::empty();
@@ -161,11 +161,11 @@ fn relate_substs<'a,'tcx,R>(relation: &mut R,
     Ok(substs)
 }
 
-fn relate_type_params<'a,'tcx,R>(relation: &mut R,
-                                 variances: Option<&[ty::Variance]>,
-                                 a_tys: &[Ty<'tcx>],
-                                 b_tys: &[Ty<'tcx>])
-                                 -> RelateResult<'tcx, Vec<Ty<'tcx>>>
+fn relate_type_params<'a,'tcx:'a,R>(relation: &mut R,
+                                    variances: Option<&[ty::Variance]>,
+                                    a_tys: &[Ty<'tcx>],
+                                    b_tys: &[Ty<'tcx>])
+                                    -> RelateResult<'tcx, Vec<Ty<'tcx>>>
     where R: TypeRelation<'a,'tcx>
 {
     if a_tys.len() != b_tys.len() {
@@ -264,10 +264,10 @@ impl<'a,'tcx:'a> Relate<'a,'tcx> for ty::FnSig<'tcx> {
     }
 }
 
-fn relate_arg_vecs<'a,'tcx,R>(relation: &mut R,
-                              a_args: &[Ty<'tcx>],
-                              b_args: &[Ty<'tcx>])
-                              -> RelateResult<'tcx, Vec<Ty<'tcx>>>
+fn relate_arg_vecs<'a,'tcx:'a,R>(relation: &mut R,
+                                 a_args: &[Ty<'tcx>],
+                                 b_args: &[Ty<'tcx>])
+                                 -> RelateResult<'tcx, Vec<Ty<'tcx>>>
     where R: TypeRelation<'a,'tcx>
 {
     if a_args.len() != b_args.len() {
@@ -629,10 +629,10 @@ impl<'a,'tcx:'a,T> Relate<'a,'tcx> for Box<T>
 ///////////////////////////////////////////////////////////////////////////
 // Error handling
 
-pub fn expected_found<'a,'tcx,R,T>(relation: &mut R,
-                                   a: &T,
-                                   b: &T)
-                                   -> ty::expected_found<T>
+pub fn expected_found<'a,'tcx:'a,R,T>(relation: &mut R,
+                                      a: &T,
+                                      b: &T)
+                                      -> ty::expected_found<T>
     where R: TypeRelation<'a,'tcx>, T: Clone
 {
     expected_found_bool(relation.a_is_expected(), a, b)

src/librustc_typeck/check/wf.rs

@@ -117,15 +117,10 @@ impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
 
                 self.check_variances_for_type_defn(item, ast_generics);
             }
-            ast::ItemTrait(_, ref ast_generics, _, ref items) => {
+            ast::ItemTrait(_, _, _, ref items) => {
                 let trait_predicates =
                     ty::lookup_predicates(ccx.tcx, local_def(item.id));
-                reject_non_type_param_bounds(
-                    ccx.tcx,
-                    item.span,
-                    &trait_predicates);
-                self.check_variances(item, ast_generics, &trait_predicates,
-                                     self.tcx().lang_items.phantom_fn());
+                reject_non_type_param_bounds(ccx.tcx, item.span, &trait_predicates);
                 if ty::trait_has_default_impl(ccx.tcx, local_def(item.id)) {
                     if !items.is_empty() {
                         ccx.tcx.sess.span_err(
@@ -287,30 +282,7 @@ impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
                                      ast_generics: &ast::Generics)
     {
         let item_def_id = local_def(item.id);
-        let predicates = ty::lookup_predicates(self.tcx(), item_def_id);
-        self.check_variances(item,
-                             ast_generics,
-                             &predicates,
-                             self.tcx().lang_items.phantom_data());
-    }
-
-    fn check_variances(&self,
-                       item: &ast::Item,
-                       ast_generics: &ast::Generics,
-                       ty_predicates: &ty::GenericPredicates<'tcx>,
-                       suggested_marker_id: Option<ast::DefId>)
-    {
-        let variance_lang_items = &[
-            self.tcx().lang_items.phantom_fn(),
-            self.tcx().lang_items.phantom_data(),
-        ];
-
-        let item_def_id = local_def(item.id);
-        let is_lang_item = variance_lang_items.iter().any(|n| *n == Some(item_def_id));
-        if is_lang_item {
-            return;
-        }
-
+        let ty_predicates = ty::lookup_predicates(self.tcx(), item_def_id);
         let variances = ty::item_variances(self.tcx(), item_def_id);
 
         let mut constrained_parameters: HashSet<_> =
@@ -331,7 +303,7 @@ impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
                 continue;
             }
             let span = self.ty_param_span(ast_generics, item, space, index);
-            self.report_bivariance(span, param_ty.name, suggested_marker_id);
+            self.report_bivariance(span, param_ty.name);
         }
 
         for (space, index, &variance) in variances.regions.iter_enumerated() {
@@ -342,7 +314,7 @@ impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
             assert_eq!(space, TypeSpace);
             let span = ast_generics.lifetimes[index].lifetime.span;
             let name = ast_generics.lifetimes[index].lifetime.name;
-            self.report_bivariance(span, name, suggested_marker_id);
+            self.report_bivariance(span, name);
         }
     }
 
@@ -377,14 +349,14 @@ impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
 
     fn report_bivariance(&self,
                          span: Span,
-                         param_name: ast::Name,
-                         suggested_marker_id: Option<ast::DefId>)
+                         param_name: ast::Name)
     {
         self.tcx().sess.span_err(
             span,
             &format!("parameter `{}` is never used",
                      param_name.user_string(self.tcx())));
 
+        let suggested_marker_id = self.tcx().lang_items.phantom_data();
         match suggested_marker_id {
             Some(def_id) => {
                 self.tcx().sess.fileline_help(