diff --git a/src/libsyntax_ext/deriving/bounds.rs b/src/libsyntax_ext/deriving/bounds.rs
index cfc98bf3687..efb2fe5eb3b 100644
--- a/src/libsyntax_ext/deriving/bounds.rs
+++ b/src/libsyntax_ext/deriving/bounds.rs
@@ -40,6 +40,7 @@ pub fn expand_deriving_copy(cx: &mut ExtCtxt,
         additional_bounds: Vec::new(),
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
+        supports_unions: true,
         methods: Vec::new(),
         associated_types: Vec::new(),
     };
diff --git a/src/libsyntax_ext/deriving/clone.rs b/src/libsyntax_ext/deriving/clone.rs
index ce8ce2209d8..f1a3a1f41b1 100644
--- a/src/libsyntax_ext/deriving/clone.rs
+++ b/src/libsyntax_ext/deriving/clone.rs
@@ -80,6 +80,7 @@ pub fn expand_deriving_clone(cx: &mut ExtCtxt,
         additional_bounds: bounds,
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
+        supports_unions: false,
         methods: vec![MethodDef {
                           name: "clone",
                           generics: LifetimeBounds::empty(),
diff --git a/src/libsyntax_ext/deriving/cmp/eq.rs b/src/libsyntax_ext/deriving/cmp/eq.rs
index 2515435abeb..425a47a991b 100644
--- a/src/libsyntax_ext/deriving/cmp/eq.rs
+++ b/src/libsyntax_ext/deriving/cmp/eq.rs
@@ -50,6 +50,7 @@ pub fn expand_deriving_eq(cx: &mut ExtCtxt,
         additional_bounds: Vec::new(),
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
+        supports_unions: false,
         methods: vec![MethodDef {
                           name: "assert_receiver_is_total_eq",
                           generics: LifetimeBounds::empty(),
diff --git a/src/libsyntax_ext/deriving/cmp/ord.rs b/src/libsyntax_ext/deriving/cmp/ord.rs
index 31194b04fa6..6b2e36e63b6 100644
--- a/src/libsyntax_ext/deriving/cmp/ord.rs
+++ b/src/libsyntax_ext/deriving/cmp/ord.rs
@@ -32,6 +32,7 @@ pub fn expand_deriving_ord(cx: &mut ExtCtxt,
         additional_bounds: Vec::new(),
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
+        supports_unions: false,
         methods: vec![MethodDef {
                           name: "cmp",
                           generics: LifetimeBounds::empty(),
diff --git a/src/libsyntax_ext/deriving/cmp/partial_eq.rs b/src/libsyntax_ext/deriving/cmp/partial_eq.rs
index f70e0cf4ac4..64b8829dad7 100644
--- a/src/libsyntax_ext/deriving/cmp/partial_eq.rs
+++ b/src/libsyntax_ext/deriving/cmp/partial_eq.rs
@@ -97,6 +97,7 @@ pub fn expand_deriving_partial_eq(cx: &mut ExtCtxt,
         additional_bounds: Vec::new(),
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
+        supports_unions: false,
         methods: methods,
         associated_types: Vec::new(),
     };
diff --git a/src/libsyntax_ext/deriving/cmp/partial_ord.rs b/src/libsyntax_ext/deriving/cmp/partial_ord.rs
index 9e9b2f02062..99d60c43c54 100644
--- a/src/libsyntax_ext/deriving/cmp/partial_ord.rs
+++ b/src/libsyntax_ext/deriving/cmp/partial_ord.rs
@@ -88,6 +88,7 @@ pub fn expand_deriving_partial_ord(cx: &mut ExtCtxt,
         additional_bounds: vec![],
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
+        supports_unions: false,
         methods: methods,
         associated_types: Vec::new(),
     };
diff --git a/src/libsyntax_ext/deriving/debug.rs b/src/libsyntax_ext/deriving/debug.rs
index a31c695e360..b974699003b 100644
--- a/src/libsyntax_ext/deriving/debug.rs
+++ b/src/libsyntax_ext/deriving/debug.rs
@@ -35,6 +35,7 @@ pub fn expand_deriving_debug(cx: &mut ExtCtxt,
         additional_bounds: Vec::new(),
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
+        supports_unions: false,
         methods: vec![MethodDef {
                           name: "fmt",
                           generics: LifetimeBounds::empty(),
diff --git a/src/libsyntax_ext/deriving/decodable.rs b/src/libsyntax_ext/deriving/decodable.rs
index f395f7bd0c4..22b9eb8e754 100644
--- a/src/libsyntax_ext/deriving/decodable.rs
+++ b/src/libsyntax_ext/deriving/decodable.rs
@@ -62,6 +62,7 @@ fn expand_deriving_decodable_imp(cx: &mut ExtCtxt,
         additional_bounds: Vec::new(),
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
+        supports_unions: false,
         methods: vec![MethodDef {
                           name: "decode",
                           generics: LifetimeBounds {
diff --git a/src/libsyntax_ext/deriving/default.rs b/src/libsyntax_ext/deriving/default.rs
index 449c1ff066b..b15fd2b49a6 100644
--- a/src/libsyntax_ext/deriving/default.rs
+++ b/src/libsyntax_ext/deriving/default.rs
@@ -32,6 +32,7 @@ pub fn expand_deriving_default(cx: &mut ExtCtxt,
         additional_bounds: Vec::new(),
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
+        supports_unions: false,
         methods: vec![MethodDef {
                           name: "default",
                           generics: LifetimeBounds::empty(),
diff --git a/src/libsyntax_ext/deriving/encodable.rs b/src/libsyntax_ext/deriving/encodable.rs
index 940fdf03771..a4074184b6e 100644
--- a/src/libsyntax_ext/deriving/encodable.rs
+++ b/src/libsyntax_ext/deriving/encodable.rs
@@ -138,6 +138,7 @@ fn expand_deriving_encodable_imp(cx: &mut ExtCtxt,
         additional_bounds: Vec::new(),
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
+        supports_unions: false,
         methods: vec!(
             MethodDef {
                 name: "encode",
diff --git a/src/libsyntax_ext/deriving/generic/mod.rs b/src/libsyntax_ext/deriving/generic/mod.rs
index a8b682d8159..5c636d43a71 100644
--- a/src/libsyntax_ext/deriving/generic/mod.rs
+++ b/src/libsyntax_ext/deriving/generic/mod.rs
@@ -228,6 +228,9 @@ pub struct TraitDef<'a> {
     /// Is it an `unsafe` trait?
     pub is_unsafe: bool,
 
+    /// Can this trait be derived for unions?
+    pub supports_unions: bool,
+
     pub methods: Vec<MethodDef<'a>>,
 
     pub associated_types: Vec<(ast::Ident, Ty<'a>)>,
diff --git a/src/libsyntax_ext/deriving/hash.rs b/src/libsyntax_ext/deriving/hash.rs
index 81c8e7112db..0941ebca868 100644
--- a/src/libsyntax_ext/deriving/hash.rs
+++ b/src/libsyntax_ext/deriving/hash.rs
@@ -36,6 +36,7 @@ pub fn expand_deriving_hash(cx: &mut ExtCtxt,
         additional_bounds: Vec::new(),
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
+        supports_unions: false,
         methods: vec![MethodDef {
                           name: "hash",
                           generics: LifetimeBounds {
diff --git a/src/test/run-pass-fulldeps/auxiliary/custom_derive_plugin.rs b/src/test/run-pass-fulldeps/auxiliary/custom_derive_plugin.rs
index 274e430bbea..6b688b006bd 100644
--- a/src/test/run-pass-fulldeps/auxiliary/custom_derive_plugin.rs
+++ b/src/test/run-pass-fulldeps/auxiliary/custom_derive_plugin.rs
@@ -49,6 +49,7 @@ fn expand(cx: &mut ExtCtxt,
         generics: LifetimeBounds::empty(),
         associated_types: vec![],
         is_unsafe: false,
+        supports_unions: false,
         methods: vec![
             MethodDef {
                 name: "total_sum",
diff --git a/src/test/run-pass-fulldeps/auxiliary/custom_derive_plugin_attr.rs b/src/test/run-pass-fulldeps/auxiliary/custom_derive_plugin_attr.rs
index 91b4b74797a..c6174745bfc 100644
--- a/src/test/run-pass-fulldeps/auxiliary/custom_derive_plugin_attr.rs
+++ b/src/test/run-pass-fulldeps/auxiliary/custom_derive_plugin_attr.rs
@@ -51,6 +51,7 @@ fn expand(cx: &mut ExtCtxt,
         generics: LifetimeBounds::empty(),
         associated_types: vec![],
         is_unsafe: false,
+        supports_unions: false,
         methods: vec![
             MethodDef {
                 name: "total_sum",