diff --git a/src/librustc/ty/adjustment.rs b/src/librustc/ty/adjustment.rs
index 34977822bc69..d8ca30477205 100644
--- a/src/librustc/ty/adjustment.rs
+++ b/src/librustc/ty/adjustment.rs
@@ -139,6 +139,21 @@ pub enum AutoBorrow<'tcx> {
     RawPtr(hir::Mutability),
 }
 
+/// Information for `CoerceUnsized` impls, storing information we
+/// have computed about the coercion.
+///
+/// This struct can be obtained via the `coerce_impl_info` query.
+/// Demanding this struct also has the side-effect of reporting errors
+/// for inappropriate impls.
+#[derive(Clone, Copy, RustcEncodable, RustcDecodable, Debug)]
+pub struct CoerceUnsizedInfo {
+    /// If this is a "custom coerce" impl, then what kind of custom
+    /// coercion is it? This applies to impls of `CoerceUnsized` for
+    /// structs, primarily, where we store a bit of info about which
+    /// fields need to be coerced.
+    pub custom_kind: Option<CustomCoerceUnsized>
+}
+
 #[derive(Clone, Copy, RustcEncodable, RustcDecodable, Debug)]
 pub enum CustomCoerceUnsized {
     /// Records the index of the field being coerced.
diff --git a/src/librustc/ty/maps.rs b/src/librustc/ty/maps.rs
index ac8c38c7d585..d4214dc429ce 100644
--- a/src/librustc/ty/maps.rs
+++ b/src/librustc/ty/maps.rs
@@ -393,8 +393,8 @@ define_maps! { <'tcx>
     pub closure_type: ItemSignature(DefId) -> ty::PolyFnSig<'tcx>,
 
     /// Caches CoerceUnsized kinds for impls on custom types.
-    pub custom_coerce_unsized_kind: ItemSignature(DefId)
-        -> ty::adjustment::CustomCoerceUnsized,
+    pub coerce_unsized_info: ItemSignature(DefId)
+        -> ty::adjustment::CoerceUnsizedInfo,
 
     pub typeck_tables: TypeckTables(DefId) -> &'tcx ty::TypeckTables<'tcx>,
 
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 36d1ae74e911..d14042699581 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -2054,8 +2054,8 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         })
     }
 
-    pub fn custom_coerce_unsized_kind(self, did: DefId) -> adjustment::CustomCoerceUnsized {
-        queries::custom_coerce_unsized_kind::get(self, DUMMY_SP, did)
+    pub fn coerce_unsized_info(self, did: DefId) -> adjustment::CoerceUnsizedInfo {
+        queries::coerce_unsized_info::get(self, DUMMY_SP, did)
     }
 
     pub fn associated_item(self, def_id: DefId) -> AssociatedItem {
diff --git a/src/librustc_metadata/cstore_impl.rs b/src/librustc_metadata/cstore_impl.rs
index 2a67b79eaa52..7cac201f14f7 100644
--- a/src/librustc_metadata/cstore_impl.rs
+++ b/src/librustc_metadata/cstore_impl.rs
@@ -88,9 +88,9 @@ provide! { <'tcx> tcx, def_id, cdata
     }
     associated_item => { cdata.get_associated_item(def_id.index) }
     impl_trait_ref => { cdata.get_impl_trait(def_id.index, tcx) }
-    custom_coerce_unsized_kind => {
-        cdata.get_custom_coerce_unsized_kind(def_id.index).unwrap_or_else(|| {
-            bug!("custom_coerce_unsized_kind: `{:?}` is missing its kind", def_id);
+    coerce_unsized_info => {
+        cdata.get_coerce_unsized_info(def_id.index).unwrap_or_else(|| {
+            bug!("coerce_unsized_info: `{:?}` is missing its info", def_id);
         })
     }
     mir => {
diff --git a/src/librustc_metadata/decoder.rs b/src/librustc_metadata/decoder.rs
index 6ccdf8092f21..3de1e3442c69 100644
--- a/src/librustc_metadata/decoder.rs
+++ b/src/librustc_metadata/decoder.rs
@@ -643,10 +643,10 @@ impl<'a, 'tcx> CrateMetadata {
         self.get_impl_data(id).polarity
     }
 
-    pub fn get_custom_coerce_unsized_kind(&self,
-                                          id: DefIndex)
-                                          -> Option<ty::adjustment::CustomCoerceUnsized> {
-        self.get_impl_data(id).coerce_unsized_kind
+    pub fn get_coerce_unsized_info(&self,
+                                   id: DefIndex)
+                                   -> Option<ty::adjustment::CoerceUnsizedInfo> {
+        self.get_impl_data(id).coerce_unsized_info
     }
 
     pub fn get_impl_trait(&self,
diff --git a/src/librustc_metadata/encoder.rs b/src/librustc_metadata/encoder.rs
index 044ed529ef74..5bc84759f879 100644
--- a/src/librustc_metadata/encoder.rs
+++ b/src/librustc_metadata/encoder.rs
@@ -695,7 +695,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 let data = ImplData {
                     polarity: hir::ImplPolarity::Positive,
                     parent_impl: None,
-                    coerce_unsized_kind: None,
+                    coerce_unsized_info: None,
                     trait_ref: tcx.impl_trait_ref(def_id).map(|trait_ref| self.lazy(&trait_ref)),
                 };
 
@@ -715,13 +715,21 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                     None
                 };
 
+                // if this is an impl of `CoerceUnsized`, create its
+                // "unsized info", else just store None
+                let coerce_unsized_info =
+                    trait_ref.and_then(|t| {
+                        if Some(t.def_id) == tcx.lang_items.coerce_unsized_trait() {
+                            Some(ty::queries::coerce_unsized_info::get(tcx, item.span, def_id))
+                        } else {
+                            None
+                        }
+                    });
+
                 let data = ImplData {
                     polarity: polarity,
                     parent_impl: parent,
-                    coerce_unsized_kind: tcx.maps.custom_coerce_unsized_kind
-                        .borrow()
-                        .get(&def_id)
-                        .cloned(),
+                    coerce_unsized_info: coerce_unsized_info,
                     trait_ref: trait_ref.map(|trait_ref| self.lazy(&trait_ref)),
                 };
 
diff --git a/src/librustc_metadata/schema.rs b/src/librustc_metadata/schema.rs
index 4a20913d0b3f..abb482a50ebc 100644
--- a/src/librustc_metadata/schema.rs
+++ b/src/librustc_metadata/schema.rs
@@ -285,7 +285,9 @@ pub struct TraitData<'tcx> {
 pub struct ImplData<'tcx> {
     pub polarity: hir::ImplPolarity,
     pub parent_impl: Option<DefId>,
-    pub coerce_unsized_kind: Option<ty::adjustment::CustomCoerceUnsized>,
+
+    /// This is `Some` only for impls of `CoerceUnsized`.
+    pub coerce_unsized_info: Option<ty::adjustment::CoerceUnsizedInfo>,
     pub trait_ref: Option<Lazy<ty::TraitRef<'tcx>>>,
 }
 
diff --git a/src/librustc_trans/monomorphize.rs b/src/librustc_trans/monomorphize.rs
index fcf6937d4b6d..382ca8ef0100 100644
--- a/src/librustc_trans/monomorphize.rs
+++ b/src/librustc_trans/monomorphize.rs
@@ -287,7 +287,7 @@ pub fn custom_coerce_unsize_info<'scx, 'tcx>(scx: &SharedCrateContext<'scx, 'tcx
 
     match fulfill_obligation(scx, DUMMY_SP, trait_ref) {
         traits::VtableImpl(traits::VtableImplData { impl_def_id, .. }) => {
-            scx.tcx().custom_coerce_unsized_kind(impl_def_id)
+            scx.tcx().coerce_unsized_info(impl_def_id).custom_kind.unwrap()
         }
         vtable => {
             bug!("invalid CoerceUnsized vtable: {:?}", vtable);
diff --git a/src/librustc_typeck/coherence/builtin.rs b/src/librustc_typeck/coherence/builtin.rs
index 3cdf9fc93ae6..47b41a75cf53 100644
--- a/src/librustc_typeck/coherence/builtin.rs
+++ b/src/librustc_typeck/coherence/builtin.rs
@@ -18,6 +18,7 @@ use rustc::traits::{self, ObligationCause, Reveal};
 use rustc::ty::{self, Ty, TyCtxt};
 use rustc::ty::ParameterEnvironment;
 use rustc::ty::TypeFoldable;
+use rustc::ty::adjustment::CoerceUnsizedInfo;
 use rustc::ty::subst::Subst;
 use rustc::ty::util::CopyImplementationError;
 use rustc::infer;
@@ -159,11 +160,26 @@ fn visit_implementation_of_copy<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 }
 
 fn visit_implementation_of_coerce_unsized<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
-                                                    coerce_unsized_trait: DefId,
+                                                    _: DefId,
                                                     impl_did: DefId) {
     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}",
            impl_did);
 
+    // Just compute this for the side-effects, in particular reporting
+    // errors; other parts of the code may demand it for the info of
+    // course.
+    if impl_did.is_local() {
+        let span = tcx.def_span(impl_did);
+        ty::queries::coerce_unsized_info::get(tcx, span, impl_did);
+    }
+}
+
+pub fn coerce_unsized_info<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                                     impl_did: DefId)
+                                     -> CoerceUnsizedInfo {
+    debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
+    let coerce_unsized_trait = tcx.lang_items.coerce_unsized_trait().unwrap();
+
     let unsize_trait = match tcx.lang_items.require(UnsizeTraitLangItem) {
         Ok(id) => id,
         Err(err) => {
@@ -171,16 +187,14 @@ fn visit_implementation_of_coerce_unsized<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         }
     };
 
-    let impl_node_id = if let Some(n) = tcx.hir.as_local_node_id(impl_did) {
-        n
-    } else {
-        debug!("visit_implementation_of_coerce_unsized(): impl not \
-                in this crate");
-        return;
-    };
+    // this provider should only get invoked for local def-ids
+    let impl_node_id = tcx.hir.as_local_node_id(impl_did).unwrap_or_else(|| {
+        bug!("coerce_unsized_info: invoked for non-local def-id {:?}", impl_did)
+    });
 
     let source = tcx.item_type(impl_did);
     let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
+    assert_eq!(trait_ref.def_id, coerce_unsized_trait);
     let target = trait_ref.substs.type_at(1);
     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)",
            source,
@@ -192,6 +206,8 @@ fn visit_implementation_of_coerce_unsized<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     let target = target.subst(tcx, &param_env.free_substs);
     assert!(!source.has_escaping_regions());
 
+    let err_info = CoerceUnsizedInfo { custom_kind: None };
+
     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)",
            source,
            target);
@@ -234,7 +250,7 @@ fn visit_implementation_of_coerce_unsized<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                                definition; expected {}, found {}",
                               source_path,
                               target_path);
-                    return;
+                    return err_info;
                 }
 
                 let fields = &def_a.struct_variant().fields;
@@ -268,7 +284,7 @@ fn visit_implementation_of_coerce_unsized<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                               "the trait `CoerceUnsized` may only be implemented \
                                for a coercion between structures with one field \
                                being coerced, none found");
-                    return;
+                    return err_info;
                 } else if diff_fields.len() > 1 {
                     let item = tcx.hir.expect_item(impl_node_id);
                     let span = if let ItemImpl(.., Some(ref t), _, _) = item.node {
@@ -295,7 +311,7 @@ fn visit_implementation_of_coerce_unsized<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                                           .join(", ")));
                     err.span_label(span, &format!("requires multiple coercions"));
                     err.emit();
-                    return;
+                    return err_info;
                 }
 
                 let (i, a, b) = diff_fields[0];
@@ -309,7 +325,7 @@ fn visit_implementation_of_coerce_unsized<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                           E0376,
                           "the trait `CoerceUnsized` may only be implemented \
                            for a coercion between structures");
-                return;
+                return err_info;
             }
         };
 
@@ -331,8 +347,8 @@ fn visit_implementation_of_coerce_unsized<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             .caller_bounds);
         infcx.resolve_regions_and_report_errors(&free_regions, impl_node_id);
 
-        if let Some(kind) = kind {
-            tcx.maps.custom_coerce_unsized_kind.borrow_mut().insert(impl_did, kind);
+        CoerceUnsizedInfo {
+            custom_kind: kind
         }
-    });
+    })
 }
diff --git a/src/librustc_typeck/coherence/mod.rs b/src/librustc_typeck/coherence/mod.rs
index 9ecf42daeaae..6abd061e81f2 100644
--- a/src/librustc_typeck/coherence/mod.rs
+++ b/src/librustc_typeck/coherence/mod.rs
@@ -102,9 +102,12 @@ fn enforce_trait_manually_implementable(tcx: TyCtxt, impl_def_id: DefId, trait_d
 }
 
 pub fn provide(providers: &mut Providers) {
+    use self::builtin::coerce_unsized_info;
+
     *providers = Providers {
         coherent_trait,
         coherent_inherent_impls,
+        coerce_unsized_info,
         ..*providers
     };
 }