diff --git a/src/librustc/middle/lint.rs b/src/librustc/middle/lint.rs
index c3e57bd9c60d..693f6fb35f4f 100644
--- a/src/librustc/middle/lint.rs
+++ b/src/librustc/middle/lint.rs
@@ -98,6 +98,7 @@ pub enum Lint {
     UnusedMut,
     UnnecessaryAllocation,
     DeadCode,
+    VisiblePrivateTypes,
     UnnecessaryTypecast,
 
     MissingDoc,
@@ -312,6 +313,12 @@ static lint_table: &'static [(&'static str, LintSpec)] = &[
         desc: "detect piece of code that will never be used",
         default: warn
     }),
+    ("visible_private_types",
+     LintSpec {
+        lint: VisiblePrivateTypes,
+        desc: "detect use of private types in exported type signatures",
+        default: warn
+    }),
 
     ("missing_doc",
      LintSpec {
diff --git a/src/librustc/middle/privacy.rs b/src/librustc/middle/privacy.rs
index 453160dc0011..afe4d0010361 100644
--- a/src/librustc/middle/privacy.rs
+++ b/src/librustc/middle/privacy.rs
@@ -16,6 +16,7 @@ use std::mem::replace;
 use collections::{HashSet, HashMap};
 
 use metadata::csearch;
+use middle::lint;
 use middle::resolve;
 use middle::ty;
 use middle::typeck::{MethodMap, MethodOrigin, MethodParam};
@@ -1169,6 +1170,251 @@ impl SanePrivacyVisitor {
     }
 }
 
+struct VisiblePrivateTypesVisitor<'a> {
+    tcx: ty::ctxt,
+    exported_items: &'a ExportedItems,
+    public_items: &'a PublicItems,
+}
+
+struct CheckTypeForPrivatenessVisitor<'a, 'b> {
+    inner: &'b VisiblePrivateTypesVisitor<'a>,
+    /// whether the type refers to private types.
+    contains_private: bool,
+    /// whether we've recurred at all (i.e. if we're pointing at the
+    /// first type on which visit_ty was called).
+    at_outer_type: bool,
+    // whether that first type is a public path.
+    outer_type_is_public_path: bool,
+}
+
+impl<'a> VisiblePrivateTypesVisitor<'a> {
+    fn path_is_private_type(&self, path_id: ast::NodeId) -> bool {
+        let did = match self.tcx.def_map.borrow().get().find_copy(&path_id) {
+            // `int` etc. (None doesn't seem to occur.)
+            None | Some(ast::DefPrimTy(..)) => return false,
+            Some(def) => def_id_of_def(def)
+        };
+        // A path can only be private if:
+        // it's in this crate...
+        is_local(did) &&
+            // ... it's not exported (obviously) ...
+            !self.exported_items.contains(&did.node) &&
+            // .. and it corresponds to a type in the AST (this returns None for
+            // type parameters)
+            self.tcx.map.find(did.node).is_some()
+    }
+
+    fn trait_is_public(&self, trait_id: ast::NodeId) -> bool {
+        // FIXME: this would preferably be using `exported_items`, but all
+        // traits are exported currently (see `EmbargoVisitor.exported_trait`)
+        self.public_items.contains(&trait_id)
+    }
+}
+
+impl<'a, 'b> Visitor<()> for CheckTypeForPrivatenessVisitor<'a, 'b> {
+    fn visit_ty(&mut self, ty: &ast::Ty, _: ()) {
+        match ty.node {
+            ast::TyPath(_, _, path_id) => {
+                if self.inner.path_is_private_type(path_id) {
+                    self.contains_private = true;
+                    // found what we're looking for so let's stop
+                    // working.
+                    return
+                } else if self.at_outer_type {
+                    self.outer_type_is_public_path = true;
+                }
+            }
+            _ => {}
+        }
+        self.at_outer_type = false;
+        visit::walk_ty(self, ty, ())
+    }
+
+    // don't want to recurse into [, .. expr]
+    fn visit_expr(&mut self, _: &ast::Expr, _: ()) {}
+}
+
+impl<'a> Visitor<()> for VisiblePrivateTypesVisitor<'a> {
+    fn visit_item(&mut self, item: &ast::Item, _: ()) {
+        match item.node {
+            // contents of a private mod can be reexported, so we need
+            // to check internals.
+            ast::ItemMod(_) => {}
+
+            // An `extern {}` doesn't introduce a new privacy
+            // namespace (the contents have their own privacies).
+            ast::ItemForeignMod(_) => {}
+
+            ast::ItemTrait(..) if !self.trait_is_public(item.id) => return,
+
+            // impls need some special handling to try to offer useful
+            // error messages without (too many) false positives
+            // (i.e. we could just return here to not check them at
+            // all, or some worse estimation of whether an impl is
+            // publically visible.
+            ast::ItemImpl(ref g, ref trait_ref, self_, ref methods) => {
+                // `impl [... for] Private` is never visible.
+                let self_contains_private;
+                // impl [... for] Public<...>, but not `impl [... for]
+                // ~[Public]` or `(Public,)` etc.
+                let self_is_public_path;
+
+                // check the properties of the Self type:
+                {
+                    let mut visitor = CheckTypeForPrivatenessVisitor {
+                        inner: self,
+                        contains_private: false,
+                        at_outer_type: true,
+                        outer_type_is_public_path: false,
+                    };
+                    visitor.visit_ty(self_, ());
+                    self_contains_private = visitor.contains_private;
+                    self_is_public_path = visitor.outer_type_is_public_path;
+                }
+
+                // miscellanous info about the impl
+
+                // `true` iff this is `impl Private for ...`.
+                let not_private_trait =
+                    trait_ref.as_ref().map_or(true, // no trait counts as public trait
+                                              |tr| {
+                        let did = ty::trait_ref_to_def_id(self.tcx, tr);
+
+                        !is_local(did) || self.trait_is_public(did.node)
+                    });
+
+                // `true` iff this is a trait impl or at least one method is public.
+                //
+                // `impl Public { $( fn ...() {} )* }` is not visible.
+                //
+                // This is required over just using the methods' privacy
+                // directly because we might have `impl<T: Foo<Private>> ...`,
+                // and we shouldn't warn about the generics if all the methods
+                // are private (because `T` won't be visible externally).
+                let trait_or_some_public_method =
+                    trait_ref.is_some() ||
+                    methods.iter().any(|m| self.exported_items.contains(&m.id));
+
+                if !self_contains_private &&
+                        not_private_trait &&
+                        trait_or_some_public_method {
+
+                    visit::walk_generics(self, g, ());
+
+                    match *trait_ref {
+                        None => {
+                            for method in methods.iter() {
+                                visit::walk_method_helper(self, *method, ())
+                            }
+                        }
+                        Some(ref tr) => {
+                            // Any private types in a trait impl fall into two
+                            // categories.
+                            // 1. mentioned in the trait definition
+                            // 2. mentioned in the type params/generics
+                            //
+                            // Those in 1. can only occur if the trait is in
+                            // this crate and will've been warned about on the
+                            // trait definition (there's no need to warn twice
+                            // so we don't check the methods).
+                            //
+                            // Those in 2. are warned via walk_generics and this
+                            // call here.
+                            visit::walk_trait_ref_helper(self, tr, ())
+                        }
+                    }
+                } else if trait_ref.is_none() && self_is_public_path {
+                    // impl Public<Private> { ... }. Any public static
+                    // methods will be visible as `Public::foo`.
+                    let mut found_pub_static = false;
+                    for method in methods.iter() {
+                        if method.explicit_self.node == ast::SelfStatic &&
+                            self.exported_items.contains(&method.id) {
+                            found_pub_static = true;
+                            visit::walk_method_helper(self, *method, ());
+                        }
+                    }
+                    if found_pub_static {
+                        visit::walk_generics(self, g, ())
+                    }
+                }
+                return
+            }
+
+            // `type ... = ...;` can contain private types, because
+            // we're introducing a new name.
+            ast::ItemTy(..) => return,
+
+            // not at all public, so we don't care
+            _ if !self.exported_items.contains(&item.id) => return,
+
+            _ => {}
+        }
+
+        // we've carefully constructed it so that if we're here, then
+        // any `visit_ty`'s will be called on things that are in
+        // public signatures, i.e. things that we're interested in for
+        // this visitor.
+        visit::walk_item(self, item, ());
+    }
+
+    fn visit_foreign_item(&mut self, item: &ast::ForeignItem, _: ()) {
+        if self.exported_items.contains(&item.id) {
+            visit::walk_foreign_item(self, item, ())
+        }
+    }
+
+    fn visit_fn(&mut self,
+                fk: &visit::FnKind, fd: &ast::FnDecl, b: &ast::Block, s: Span, id: ast::NodeId,
+                _: ()) {
+        // needs special handling for methods.
+        if self.exported_items.contains(&id) {
+            visit::walk_fn(self, fk, fd, b, s, id, ());
+        }
+    }
+
+    fn visit_ty(&mut self, t: &ast::Ty, _: ()) {
+        match t.node {
+            ast::TyPath(ref p, _, path_id) => {
+                if self.path_is_private_type(path_id) {
+                    self.tcx.sess.add_lint(lint::VisiblePrivateTypes,
+                                           path_id, p.span,
+                                           ~"private type in exported type signature");
+                }
+            }
+            _ => {}
+        }
+        visit::walk_ty(self, t, ())
+    }
+
+    fn visit_variant(&mut self, v: &ast::Variant, g: &ast::Generics, _: ()) {
+        if self.exported_items.contains(&v.node.id) {
+            visit::walk_variant(self, v, g, ());
+        }
+    }
+
+    fn visit_struct_field(&mut self, s: &ast::StructField, _: ()) {
+        match s.node.kind {
+            // the only way to get here is by being inside a public
+            // struct/enum variant, so the only way to have a private
+            // field is with an explicit `priv`.
+            ast::NamedField(_, ast::Private) => {}
+
+            _ => visit::walk_struct_field(self, s, ())
+        }
+    }
+
+
+    // we don't need to introspect into these at all: an
+    // expression/block context can't possibly contain exported
+    // things, and neither do view_items. (Making them no-ops stops us
+    // from traversing the whole AST without having to be super
+    // careful about our `walk_...` calls above.)
+    fn visit_view_item(&mut self, _: &ast::ViewItem, _: ()) {}
+    fn visit_block(&mut self, _: &ast::Block, _: ()) {}
+    fn visit_expr(&mut self, _: &ast::Expr, _: ()) {}
+}
+
 pub fn check_crate(tcx: ty::ctxt,
                    method_map: &MethodMap,
                    exp_map2: &resolve::ExportMap2,
@@ -1225,5 +1471,14 @@ pub fn check_crate(tcx: ty::ctxt,
     }
 
     let EmbargoVisitor { exported_items, public_items, .. } = visitor;
+
+    {
+        let mut visitor = VisiblePrivateTypesVisitor {
+            tcx: tcx,
+            exported_items: &exported_items,
+            public_items: &public_items
+        };
+        visit::walk_crate(&mut visitor, krate, ());
+    }
     return (exported_items, public_items);
 }
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 248ba593c1f0..39989977d69f 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -193,9 +193,11 @@ fn walk_explicit_self<E: Clone, V: Visitor<E>>(visitor: &mut V,
     }
 }
 
-fn walk_trait_ref<E: Clone, V: Visitor<E>>(visitor: &mut V,
-                                           trait_ref: &TraitRef,
-                                           env: E) {
+/// Like with walk_method_helper this doesn't correspond to a method
+/// in Visitor, and so it gets a _helper suffix.
+pub fn walk_trait_ref_helper<E: Clone, V: Visitor<E>>(visitor: &mut V,
+                                                      trait_ref: &TraitRef,
+                                                      env: E) {
     visitor.visit_path(&trait_ref.path, trait_ref.ref_id, env)
 }
 
@@ -239,7 +241,8 @@ pub fn walk_item<E: Clone, V: Visitor<E>>(visitor: &mut V, item: &Item, env: E)
                  ref methods) => {
             visitor.visit_generics(type_parameters, env.clone());
             match *trait_reference {
-                Some(ref trait_reference) => walk_trait_ref(visitor, trait_reference, env.clone()),
+                Some(ref trait_reference) => walk_trait_ref_helper(visitor,
+                                                                   trait_reference, env.clone()),
                 None => ()
             }
             visitor.visit_ty(typ, env.clone());
@@ -459,7 +462,7 @@ pub fn walk_ty_param_bounds<E: Clone, V: Visitor<E>>(visitor: &mut V,
     for bound in bounds.iter() {
         match *bound {
             TraitTyParamBound(ref typ) => {
-                walk_trait_ref(visitor, typ, env.clone())
+                walk_trait_ref_helper(visitor, typ, env.clone())
             }
             RegionTyParamBound => {}
         }
diff --git a/src/test/compile-fail/lint-dead-code-1.rs b/src/test/compile-fail/lint-dead-code-1.rs
index b6965a003e0a..629a203fcbb6 100644
--- a/src/test/compile-fail/lint-dead-code-1.rs
+++ b/src/test/compile-fail/lint-dead-code-1.rs
@@ -11,6 +11,7 @@
 #[no_std];
 #[allow(unused_variable)];
 #[allow(non_camel_case_types)];
+#[allow(visible_private_types)];
 #[deny(dead_code)];
 
 #[crate_type="lib"];
diff --git a/src/test/compile-fail/lint-visible-private-types.rs b/src/test/compile-fail/lint-visible-private-types.rs
new file mode 100644
index 000000000000..6d77f8b324c5
--- /dev/null
+++ b/src/test/compile-fail/lint-visible-private-types.rs
@@ -0,0 +1,129 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[feature(struct_variant)];
+#[deny(visible_private_types)];
+#[allow(dead_code)];
+#[crate_type="lib"];
+
+struct Private<T>;
+pub struct Public<T>;
+
+impl Private<Public<int>> {
+    pub fn a(&self) -> Private<int> { fail!() }
+    fn b(&self) -> Private<int> { fail!() }
+
+    pub fn c() -> Private<int> { fail!() }
+    fn d() -> Private<int> { fail!() }
+}
+impl Private<int> {
+    pub fn e(&self) -> Private<int> { fail!() }
+    fn f(&self) -> Private<int> { fail!() }
+}
+
+impl Public<Private<int>> {
+    pub fn a(&self) -> Private<int> { fail!() }
+    fn b(&self) -> Private<int> { fail!() }
+
+    pub fn c() -> Private<int> { fail!() } //~ ERROR private type in exported type signature
+    fn d() -> Private<int> { fail!() }
+}
+impl Public<int> {
+    pub fn e(&self) -> Private<int> { fail!() } //~ ERROR private type in exported type signature
+    fn f(&self) -> Private<int> { fail!() }
+}
+
+pub fn x(_: Private<int>) {} //~ ERROR private type in exported type signature
+
+fn y(_: Private<int>) {}
+
+
+pub struct Foo {
+    x: Private<int>, //~ ERROR private type in exported type signature
+    priv y: Private<int>
+}
+
+struct Bar {
+    x: Private<int>,
+}
+
+pub enum Baz {
+    Baz1(Private<int>), //~ ERROR private type in exported type signature
+    Baz2 {
+        x: Private<int>, //~ ERROR private type in exported type signature
+        priv y: Private<int>
+    },
+
+    priv Baz3(Private<int>),
+    priv Baz4 {
+        x: Private<int>,
+    }
+}
+
+enum Qux {
+    Qux1(Private<int>),
+    Qux2 {
+        x: Private<int>,
+    }
+}
+
+pub trait PubTrait {
+    fn foo(&self) -> Private<int> { fail!( )} //~ ERROR private type in exported type signature
+    fn bar(&self) -> Private<int>; //~ ERROR private type in exported type signature
+    fn baz() -> Private<int>; //~ ERROR private type in exported type signature
+}
+
+impl PubTrait for Public<int> {
+    fn bar(&self) -> Private<int> { fail!() }
+    fn baz() -> Private<int> { fail!() }
+}
+impl PubTrait for Public<Private<int>> {
+    fn bar(&self) -> Private<int> { fail!() }
+    fn baz() -> Private<int> { fail!() }
+}
+
+impl PubTrait for Private<int> {
+    fn bar(&self) -> Private<int> { fail!() }
+    fn baz() -> Private<int> { fail!() }
+}
+impl PubTrait for (Private<int>,) {
+    fn bar(&self) -> Private<int> { fail!() }
+    fn baz() -> Private<int> { fail!() }
+}
+
+
+trait PrivTrait {
+    fn foo(&self) -> Private<int> { fail!( )}
+    fn bar(&self) -> Private<int>;
+}
+impl PrivTrait for Private<int> {
+    fn bar(&self) -> Private<int> { fail!() }
+}
+impl PrivTrait for (Private<int>,) {
+    fn bar(&self) -> Private<int> { fail!() }
+}
+
+pub trait ParamTrait<T> {
+    fn foo() -> T;
+}
+
+impl ParamTrait<Private<int>> //~ ERROR private type in exported type signature
+   for Public<int> {
+    fn foo() -> Private<int> { fail!() }
+}
+
+impl ParamTrait<Private<int>> for Private<int> {
+    fn foo() -> Private<int> { fail!( )}
+}
+
+impl<T: ParamTrait<Private<int>>>  //~ ERROR private type in exported type signature
+     ParamTrait<T> for Public<i8> {
+    fn foo() -> T { fail!() }
+}