Forbid items with the same name being defined in overlapping inherent

impl blocks.

For example, the following is now correctly illegal:

```rust
struct Foo;

impl Foo {
    fn id() {}
}

impl Foo {
    fn id() {}
}
```

"Overlapping" here is determined the same way it is for traits (and in
fact shares the same code path): roughly, there must be some way of
substituting any generic types to unify the impls, such that none of the
`where` clauses are provably unsatisfiable under such a unification.

Closes #22889

9 files changed, 113 insertions, 45 deletions

diff --git a/src/librustc/dep_graph/mod.rs b/src/librustc/dep_graph/mod.rs
index 2fad161652f..f4df6994e04 100644
--- a/src/librustc/dep_graph/mod.rs
+++ b/src/librustc/dep_graph/mod.rs
@@ -58,6 +58,7 @@ pub enum DepNode {
     CoherenceCheckImpl(DefId),
     CoherenceOverlapCheck(DefId),
     CoherenceOverlapCheckSpecial(DefId),
+    CoherenceOverlapInherentCheck(DefId),
     CoherenceOrphanCheck(DefId),
     Variance,
     WfCheck(DefId),
diff --git a/src/librustc/middle/infer/mod.rs b/src/librustc/middle/infer/mod.rs
index 7cc7f599e14..3eca4624bc1 100644
--- a/src/librustc/middle/infer/mod.rs
+++ b/src/librustc/middle/infer/mod.rs
@@ -482,14 +482,14 @@ pub fn mk_sub_poly_trait_refs<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>,
 pub fn mk_eq_impl_headers<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>,
                                     a_is_expected: bool,
                                     origin: TypeOrigin,
-                                    a: ty::ImplHeader<'tcx>,
-                                    b: ty::ImplHeader<'tcx>)
+                                    a: &ty::ImplHeader<'tcx>,
+                                    b: &ty::ImplHeader<'tcx>)
                                     -> UnitResult<'tcx>
 {
     debug!("mk_eq_impl_header({:?} = {:?})", a, b);
     match (a.trait_ref, b.trait_ref) {
-        (Some(a_ref), Some(b_ref)) => mk_eq_trait_refs(cx, a_is_expected, a_ref, b_ref),
-        (None, None) => mk_eqty(cx, a_is_expected, a.self_ty, b.self_ty),
+        (Some(a_ref), Some(b_ref)) => mk_eq_trait_refs(cx, a_is_expected, origin, a_ref, b_ref),
+        (None, None) => mk_eqty(cx, a_is_expected, origin, a.self_ty, b.self_ty),
         _ => cx.tcx.sess.bug("mk_eq_impl_headers given mismatched impl kinds"),
     }
 }
diff --git a/src/librustc/middle/traits/coherence.rs b/src/librustc/middle/traits/coherence.rs
index b59fb5c7e25..6005d36ff4e 100644
--- a/src/librustc/middle/traits/coherence.rs
+++ b/src/librustc/middle/traits/coherence.rs
@@ -10,18 +10,15 @@
 
 //! See `README.md` for high-level documentation
 
-use super::{Normalized, SelectionContext};
-use super::{Obligation, ObligationCause, PredicateObligation};
-use super::project;
-use super::util;
+use super::{SelectionContext};
+use super::{Obligation, ObligationCause};
 
 use middle::cstore::LOCAL_CRATE;
 use middle::def_id::DefId;
-use middle::subst::{Subst, Substs, TypeSpace};
+use middle::subst::TypeSpace;
 use middle::ty::{self, Ty, TyCtxt};
-use middle::ty::error::TypeError;
 use middle::infer::{self, InferCtxt, TypeOrigin};
-use syntax::codemap::{DUMMY_SP, Span};
+use syntax::codemap::DUMMY_SP;
 
 #[derive(Copy, Clone)]
 struct InferIsLocal(bool);
@@ -31,7 +28,7 @@ struct InferIsLocal(bool);
 pub fn overlapping_impls<'cx, 'tcx>(infcx: &InferCtxt<'cx, 'tcx>,
                                     impl1_def_id: DefId,
                                     impl2_def_id: DefId)
-                                    -> Option<ImplTy<'tcx>>
+                                    -> Option<ty::ImplHeader<'tcx>>
 {
     debug!("impl_can_satisfy(\
            impl1_def_id={:?}, \
@@ -48,7 +45,7 @@ pub fn overlapping_impls<'cx, 'tcx>(infcx: &InferCtxt<'cx, 'tcx>,
 fn overlap<'cx, 'tcx>(selcx: &mut SelectionContext<'cx, 'tcx>,
                       a_def_id: DefId,
                       b_def_id: DefId)
-                      -> Option<ImplHeader<'tcx>>
+                      -> Option<ty::ImplHeader<'tcx>>
 {
     debug!("overlap(a_def_id={:?}, b_def_id={:?})",
            a_def_id,
@@ -64,8 +61,8 @@ fn overlap<'cx, 'tcx>(selcx: &mut SelectionContext<'cx, 'tcx>,
     if let Err(_) = infer::mk_eq_impl_headers(selcx.infcx(),
                                               true,
                                               TypeOrigin::Misc(DUMMY_SP),
-                                              a_impl_header,
-                                              b_impl_header) {
+                                              &a_impl_header,
+                                              &b_impl_header) {
         return None;
     }
 
@@ -74,7 +71,7 @@ fn overlap<'cx, 'tcx>(selcx: &mut SelectionContext<'cx, 'tcx>,
     // Are any of the obligations unsatisfiable? If so, no overlap.
     let infcx = selcx.infcx();
     let opt_failing_obligation =
-        a_impl_header.prediates
+        a_impl_header.predicates
                      .iter()
                      .chain(&b_impl_header.predicates)
                      .map(|p| infcx.resolve_type_vars_if_possible(p))
diff --git a/src/librustc/middle/ty/mod.rs b/src/librustc/middle/ty/mod.rs
index 2405f661ab3..b54b0b73ef2 100644
--- a/src/librustc/middle/ty/mod.rs
+++ b/src/librustc/middle/ty/mod.rs
@@ -164,9 +164,9 @@ pub struct ImplHeader<'tcx> {
 }
 
 impl<'tcx> ImplHeader<'tcx> {
-    pub fn with_fresh_ty_vars<'a,'tcx>(selcx: &mut traits::SelectionContext<'a,'tcx>,
-                                       impl_def_id: DefId)
-                                       -> ImplHeader<'tcx>
+    pub fn with_fresh_ty_vars<'a>(selcx: &mut traits::SelectionContext<'a, 'tcx>,
+                                  impl_def_id: DefId)
+                                  -> ImplHeader<'tcx>
     {
         let tcx = selcx.tcx();
         let impl_generics = tcx.lookup_item_type(impl_def_id).generics;
@@ -174,13 +174,13 @@ impl<'tcx> ImplHeader<'tcx> {
 
         let header = ImplHeader {
             impl_def_id: impl_def_id,
-            self_ty: tcx.lookup_item_type(impl_def_id),
+            self_ty: tcx.lookup_item_type(impl_def_id).ty,
             trait_ref: tcx.impl_trait_ref(impl_def_id),
-            predicates: tcx.lookup_predicates(impl_def_id),
-        }.subst(tcx, impl_substs);
+            predicates: tcx.lookup_predicates(impl_def_id).predicates.into_vec(),
+        }.subst(tcx, &impl_substs);
 
-        let Normalized { value: mut header, obligations: obligations } =
-            proect::normalize(selcx, ObligationCause::dummy(), &header);
+        let traits::Normalized { value: mut header, obligations } =
+            traits::normalize(selcx, traits::ObligationCause::dummy(), &header);
 
         header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
         header
diff --git a/src/librustc/middle/ty/structural_impls.rs b/src/librustc/middle/ty/structural_impls.rs
index 645e709f512..57cfdd0d8b8 100644
--- a/src/librustc/middle/ty/structural_impls.rs
+++ b/src/librustc/middle/ty/structural_impls.rs
@@ -452,8 +452,7 @@ impl<'tcx> TypeFoldable<'tcx> for ty::ImplHeader<'tcx> {
             impl_def_id: self.impl_def_id,
             self_ty: self.self_ty.fold_with(folder),
             trait_ref: self.trait_ref.map(|t| t.fold_with(folder)),
-            predicates: self.predicates.into_iter().map(|p| p.fold_with(folder)).collect(),
-            polarity: self.polarity,
+            predicates: self.predicates.iter().map(|p| p.fold_with(folder)).collect(),
         }
     }
 
diff --git a/src/librustc_typeck/coherence/mod.rs b/src/librustc_typeck/coherence/mod.rs
index cad321c0b23..9dc8d7ae943 100644
--- a/src/librustc_typeck/coherence/mod.rs
+++ b/src/librustc_typeck/coherence/mod.rs
@@ -35,7 +35,9 @@ use CrateCtxt;
 use middle::infer::{self, InferCtxt, TypeOrigin, new_infer_ctxt};
 use std::cell::RefCell;
 use std::rc::Rc;
+use syntax::ast;
 use syntax::codemap::Span;
+use syntax::errors::DiagnosticBuilder;
 use util::nodemap::{DefIdMap, FnvHashMap};
 use rustc::dep_graph::DepNode;
 use rustc::front::map as hir_map;
@@ -519,6 +521,13 @@ fn enforce_trait_manually_implementable(tcx: &TyCtxt, sp: Span, trait_def_id: De
     err.emit();
 }
 
+// Factored out into helper because the error cannot be defined in multiple locations.
+pub fn report_duplicate_item<'tcx>(tcx: &TyCtxt<'tcx>, sp: Span, name: ast::Name)
+                                   -> DiagnosticBuilder<'tcx>
+{
+    struct_span_err!(tcx.sess, sp, E0201, "duplicate definitions with name `{}`:", name)
+}
+
 pub fn check_coherence(crate_context: &CrateCtxt) {
     let _task = crate_context.tcx.dep_graph.in_task(DepNode::Coherence);
     let infcx = new_infer_ctxt(crate_context.tcx, &crate_context.tcx.tables, None);
diff --git a/src/librustc_typeck/coherence/overlap.rs b/src/librustc_typeck/coherence/overlap.rs
index 9ec42b6d3f8..80430076f19 100644
--- a/src/librustc_typeck/coherence/overlap.rs
+++ b/src/librustc_typeck/coherence/overlap.rs
@@ -9,7 +9,8 @@
 // except according to those terms.
 
 //! Overlap: No two impls for the same trait are implemented for the
-//! same type.
+//! same type. Likewise, no two inherent impls for a given type
+//! constructor provide a method with the same name.
 
 use middle::cstore::{CrateStore, LOCAL_CRATE};
 use middle::def_id::DefId;
@@ -115,7 +116,6 @@ impl<'cx, 'tcx> OverlapChecker<'cx, 'tcx> {
         }
     }
 
-
     fn check_if_impls_overlap(&self,
                               impl1_def_id: DefId,
                               impl2_def_id: DefId)
@@ -128,8 +128,8 @@ impl<'cx, 'tcx> OverlapChecker<'cx, 'tcx> {
                    impl2_def_id);
 
             let infcx = infer::new_infer_ctxt(self.tcx, &self.tcx.tables, None);
-            if let Some(trait_ref) = traits::overlapping_impls(&infcx, impl1_def_id, impl2_def_id) {
-                self.report_overlap_error(impl1_def_id, impl2_def_id, trait_ref);
+            if let Some(header) = traits::overlapping_impls(&infcx, impl1_def_id, impl2_def_id) {
+                self.report_overlap_error(impl1_def_id, impl2_def_id, header.trait_ref.unwrap());
             }
         }
     }
@@ -150,13 +150,13 @@ impl<'cx, 'tcx> OverlapChecker<'cx, 'tcx> {
             }).unwrap_or(String::new())
         };
 
-        let mut err = struct_span_err!(self.tcx.sess, self.span_of_impl(impl1), E0119,
+        let mut err = struct_span_err!(self.tcx.sess, self.span_of_def_id(impl1), E0119,
                                        "conflicting implementations of trait `{}`{}:",
                                        trait_ref,
                                        self_type);
 
         if impl2.is_local() {
-            span_note!(&mut err, self.span_of_impl(impl2),
+            span_note!(&mut err, self.span_of_def_id(impl2),
                        "conflicting implementation is here:");
         } else {
             let cname = self.tcx.sess.cstore.crate_name(impl2.krate);
@@ -165,10 +165,61 @@ impl<'cx, 'tcx> OverlapChecker<'cx, 'tcx> {
         err.emit();
     }
 
-    fn span_of_impl(&self, impl_did: DefId) -> Span {
-        let node_id = self.tcx.map.as_local_node_id(impl_did).unwrap();
+    fn span_of_def_id(&self, did: DefId) -> Span {
+        let node_id = self.tcx.map.as_local_node_id(did).unwrap();
         self.tcx.map.span(node_id)
     }
+
+    fn check_for_common_items_in_impls(&self, impl1: DefId, impl2: DefId) {
+        #[derive(Copy, Clone, PartialEq)]
+        enum Namespace { Type, Value }
+
+        fn name_and_namespace(tcx: &TyCtxt, item: &ty::ImplOrTraitItemId)
+                              -> (ast::Name, Namespace)
+        {
+            let name = tcx.impl_or_trait_item(item.def_id()).name();
+            (name, match *item {
+                ty::TypeTraitItemId(..) => Namespace::Type,
+                ty::ConstTraitItemId(..) => Namespace::Value,
+                ty::MethodTraitItemId(..) => Namespace::Value,
+            })
+        }
+
+        let impl_items = self.tcx.impl_items.borrow();
+
+        for item1 in &impl_items[&impl1] {
+            let (name, namespace) = name_and_namespace(&self.tcx, item1);
+
+            for item2 in &impl_items[&impl2] {
+                if (name, namespace) == name_and_namespace(&self.tcx, item2) {
+                    let mut err = super::report_duplicate_item(
+                        &self.tcx, self.span_of_def_id(item1.def_id()), name);
+                    span_note!(&mut err, self.span_of_def_id(item2.def_id()),
+                               "conflicting definition is here:");
+                    err.emit();
+                }
+            }
+        }
+    }
+
+    fn check_for_overlapping_inherent_impls(&self, ty_def_id: DefId) {
+        let _task = self.tcx.dep_graph.in_task(DepNode::CoherenceOverlapInherentCheck(ty_def_id));
+
+        let inherent_impls = self.tcx.inherent_impls.borrow();
+        let impls = match inherent_impls.get(&ty_def_id) {
+            Some(impls) => impls,
+            None => return
+        };
+
+        for (i, &impl1_def_id) in impls.iter().enumerate() {
+            for &impl2_def_id in &impls[(i+1)..] {
+                let infcx = infer::new_infer_ctxt(self.tcx, &self.tcx.tables, None);
+                if traits::overlapping_impls(&infcx, impl1_def_id, impl2_def_id).is_some() {
+                    self.check_for_common_items_in_impls(impl1_def_id, impl2_def_id)
+                }
+            }
+        }
+    }
 }
 
 
@@ -180,6 +231,11 @@ impl<'cx, 'tcx,'v> intravisit::Visitor<'v> for OverlapChecker<'cx, 'tcx> {
                 self.check_for_overlapping_impls_of_trait(trait_def_id);
             }
 
+            hir::ItemEnum(..) | hir::ItemStruct(..) => {
+                let type_def_id = self.tcx.map.local_def_id(item.id);
+                self.check_for_overlapping_inherent_impls(type_def_id);
+            }
+
             hir::ItemDefaultImpl(..) => {
                 // look for another default impl; note that due to the
                 // general orphan/coherence rules, it must always be
diff --git a/src/librustc_typeck/collect.rs b/src/librustc_typeck/collect.rs
index 3ce03e24578..b493b64a45f 100644
--- a/src/librustc_typeck/collect.rs
+++ b/src/librustc_typeck/collect.rs
@@ -63,6 +63,7 @@ use lint;
 use middle::def::Def;
 use middle::def_id::DefId;
 use constrained_type_params as ctp;
+use coherence;
 use middle::lang_items::SizedTraitLangItem;
 use middle::resolve_lifetime;
 use middle::const_eval::{self, ConstVal};
@@ -750,17 +751,7 @@ fn convert_item(ccx: &CrateCtxt, it: &hir::Item) {
                     _                    => &mut seen_value_items,
                 };
                 if !seen_items.insert(impl_item.name) {
-                    let desc = match impl_item.node {
-                        hir::ImplItemKind::Const(_, _) => "associated constant",
-                        hir::ImplItemKind::Type(_) => "associated type",
-                        hir::ImplItemKind::Method(ref sig, _) =>
-                            match sig.explicit_self.node {
-                                hir::SelfStatic => "associated function",
-                                _ => "method",
-                            },
-                    };
-
-                    span_err!(tcx.sess, impl_item.span, E0201, "duplicate {}", desc);
+                    coherence::report_duplicate_item(tcx, impl_item.span, impl_item.name).emit();
                 }
 
                 if let hir::ImplItemKind::Const(ref ty, _) = impl_item.node {
diff --git a/src/librustc_typeck/diagnostics.rs b/src/librustc_typeck/diagnostics.rs
index cfe76206b02..14d04a5d109 100644
--- a/src/librustc_typeck/diagnostics.rs
+++ b/src/librustc_typeck/diagnostics.rs
@@ -2285,6 +2285,21 @@ impl Baz for Foo {
     type Quux = u32;
 }
 ```
+
+Note, however, that items with the same name are allowed for inherent `impl`
+blocks that don't overlap:
+
+```
+struct Foo<T>(T);
+
+impl Foo<u8> {
+    fn bar(&self) -> bool { self.0 > 5 }
+}
+
+impl Foo<bool> {
+    fn bar(&self) -> bool { self.0 }
+}
+```
 "##,
 
 E0202: r##"