diff --git a/src/bootstrap/channel.rs b/src/bootstrap/channel.rs
index 966bb4dbfb870..088513a76bde1 100644
--- a/src/bootstrap/channel.rs
+++ b/src/bootstrap/channel.rs
@@ -28,7 +28,7 @@ pub const CFG_RELEASE_NUM: &'static str = "1.18.0";
 // An optional number to put after the label, e.g. '.2' -> '-beta.2'
 // Be sure to make this starts with a dot to conform to semver pre-release
 // versions (section 9)
-pub const CFG_PRERELEASE_VERSION: &'static str = ".4";
+pub const CFG_PRERELEASE_VERSION: &'static str = ".5";
 
 pub struct GitInfo {
     inner: Option<Info>,
diff --git a/src/libcollections/lib.rs b/src/libcollections/lib.rs
index 8d056afdb571a..064ba1802dea6 100644
--- a/src/libcollections/lib.rs
+++ b/src/libcollections/lib.rs
@@ -64,7 +64,6 @@
 #![feature(trusted_len)]
 #![feature(unicode)]
 #![feature(unique)]
-#![feature(untagged_unions)]
 #![cfg_attr(not(test), feature(str_checked_slicing))]
 #![cfg_attr(test, feature(rand, test))]
 #![feature(offset_to)]
diff --git a/src/librustc/middle/stability.rs b/src/librustc/middle/stability.rs
index 1e856f6716ef7..aa3aec4e66271 100644
--- a/src/librustc/middle/stability.rs
+++ b/src/librustc/middle/stability.rs
@@ -590,6 +590,27 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
                 }
             }
 
+            // There's no good place to insert stability check for non-Copy unions,
+            // so semi-randomly perform it here in stability.rs
+            hir::ItemUnion(..) if !self.tcx.sess.features.borrow().untagged_unions => {
+                let def_id = self.tcx.hir.local_def_id(item.id);
+                let adt_def = self.tcx.adt_def(def_id);
+                let ty = self.tcx.type_of(def_id);
+
+                if adt_def.has_dtor(self.tcx) {
+                    emit_feature_err(&self.tcx.sess.parse_sess,
+                                     "untagged_unions", item.span, GateIssue::Language,
+                                     "unions with `Drop` implementations are unstable");
+                } else {
+                    let param_env = self.tcx.param_env(def_id);
+                    if !param_env.can_type_implement_copy(self.tcx, ty, item.span).is_ok() {
+                        emit_feature_err(&self.tcx.sess.parse_sess,
+                                        "untagged_unions", item.span, GateIssue::Language,
+                                        "unions with non-`Copy` fields are unstable");
+                    }
+                }
+            }
+
             _ => (/* pass */)
         }
         intravisit::walk_item(self, item);
diff --git a/src/librustc_data_structures/lib.rs b/src/librustc_data_structures/lib.rs
index 00c46d992bfd5..e438fb370b1c6 100644
--- a/src/librustc_data_structures/lib.rs
+++ b/src/librustc_data_structures/lib.rs
@@ -32,7 +32,6 @@
 #![feature(staged_api)]
 #![feature(unboxed_closures)]
 #![feature(fn_traits)]
-#![feature(untagged_unions)]
 #![feature(associated_consts)]
 #![feature(unsize)]
 #![feature(i128_type)]
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 9b55a860b3595..7c69234f44388 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -1202,12 +1202,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 }
             }
 
-            ast::ItemKind::Union(..) => {
-                gate_feature_post!(&self, untagged_unions,
-                                   i.span,
-                                   "unions are unstable and possibly buggy");
-            }
-
             ast::ItemKind::DefaultImpl(..) => {
                 gate_feature_post!(&self, optin_builtin_traits,
                                    i.span,
diff --git a/src/test/compile-fail/borrowck/borrowck-union-borrow-nested.rs b/src/test/compile-fail/borrowck/borrowck-union-borrow-nested.rs
index 8b6b8d9ecb08e..6298c87453ca6 100644
--- a/src/test/compile-fail/borrowck/borrowck-union-borrow-nested.rs
+++ b/src/test/compile-fail/borrowck/borrowck-union-borrow-nested.rs
@@ -8,10 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// ignore-tidy-linelength
-
-#![feature(untagged_unions)]
-
 #[derive(Clone, Copy)]
 struct S {
     a: u8,
diff --git a/src/test/compile-fail/borrowck/borrowck-union-borrow.rs b/src/test/compile-fail/borrowck/borrowck-union-borrow.rs
index ecc698acc317f..20b882e1f807c 100644
--- a/src/test/compile-fail/borrowck/borrowck-union-borrow.rs
+++ b/src/test/compile-fail/borrowck/borrowck-union-borrow.rs
@@ -10,8 +10,6 @@
 
 // ignore-tidy-linelength
 
-#![feature(untagged_unions)]
-
 #[derive(Clone, Copy)]
 union U {
     a: u8,
diff --git a/src/test/compile-fail/borrowck/borrowck-union-uninitialized.rs b/src/test/compile-fail/borrowck/borrowck-union-uninitialized.rs
index 36e062f8464e9..81376a1a169d4 100644
--- a/src/test/compile-fail/borrowck/borrowck-union-uninitialized.rs
+++ b/src/test/compile-fail/borrowck/borrowck-union-uninitialized.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 struct S {
     a: u8,
 }
diff --git a/src/test/compile-fail/privacy/union-field-privacy-1.rs b/src/test/compile-fail/privacy/union-field-privacy-1.rs
index bddcd391b205d..c716cba10c7bf 100644
--- a/src/test/compile-fail/privacy/union-field-privacy-1.rs
+++ b/src/test/compile-fail/privacy/union-field-privacy-1.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 mod m {
     pub union U {
         pub a: u8,
diff --git a/src/test/compile-fail/privacy/union-field-privacy-2.rs b/src/test/compile-fail/privacy/union-field-privacy-2.rs
index e26b5e99ec1a5..d6bb765202da5 100644
--- a/src/test/compile-fail/privacy/union-field-privacy-2.rs
+++ b/src/test/compile-fail/privacy/union-field-privacy-2.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 mod m {
     pub union U {
         pub a: u8,
diff --git a/src/test/compile-fail/union/union-const-eval.rs b/src/test/compile-fail/union/union-const-eval.rs
index b2bf173c59c86..ee4d9fe99eeb8 100644
--- a/src/test/compile-fail/union/union-const-eval.rs
+++ b/src/test/compile-fail/union/union-const-eval.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 union U {
     a: usize,
     b: usize,
diff --git a/src/test/compile-fail/union/union-const-pat.rs b/src/test/compile-fail/union/union-const-pat.rs
index 3d168980ed246..5f75e3cc4a2d6 100644
--- a/src/test/compile-fail/union/union-const-pat.rs
+++ b/src/test/compile-fail/union/union-const-pat.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 union U {
     a: usize,
     b: usize,
diff --git a/src/test/compile-fail/union/union-derive.rs b/src/test/compile-fail/union/union-derive.rs
index 26dbdfd0b4118..ffd290fb073db 100644
--- a/src/test/compile-fail/union/union-derive.rs
+++ b/src/test/compile-fail/union/union-derive.rs
@@ -10,8 +10,6 @@
 
 // Most traits cannot be derived for unions.
 
-#![feature(untagged_unions)]
-
 #[derive(
     PartialEq, //~ ERROR this trait cannot be derived for unions
     PartialOrd, //~ ERROR this trait cannot be derived for unions
diff --git a/src/test/compile-fail/union/union-empty.rs b/src/test/compile-fail/union/union-empty.rs
index ce5bbf60fee25..1f499c49a6870 100644
--- a/src/test/compile-fail/union/union-empty.rs
+++ b/src/test/compile-fail/union/union-empty.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 union U {} //~ ERROR unions cannot have zero fields
 
 fn main() {}
diff --git a/src/test/compile-fail/union/union-feature-gate.rs b/src/test/compile-fail/union/union-feature-gate.rs
index 0c2165871f6db..8a0490cdc6b6d 100644
--- a/src/test/compile-fail/union/union-feature-gate.rs
+++ b/src/test/compile-fail/union/union-feature-gate.rs
@@ -10,8 +10,28 @@
 
 // gate-test-untagged_unions
 
-union U { //~ ERROR unions are unstable and possibly buggy
+union U1 { // OK
     a: u8,
 }
 
+union U2<T: Copy> { // OK
+    a: T,
+}
+
+union U3 { //~ ERROR unions with non-`Copy` fields are unstable
+    a: String,
+}
+
+union U4<T> { //~ ERROR unions with non-`Copy` fields are unstable
+    a: T,
+}
+
+union U5 { //~ ERROR unions with `Drop` implementations are unstable
+    a: u8,
+}
+
+impl Drop for U5 {
+    fn drop(&mut self) {}
+}
+
 fn main() {}
diff --git a/src/test/compile-fail/union/union-fields.rs b/src/test/compile-fail/union/union-fields.rs
index 3ee95c2ef4258..b5d582a5746f7 100644
--- a/src/test/compile-fail/union/union-fields.rs
+++ b/src/test/compile-fail/union/union-fields.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 union U {
     a: u8,
     b: u16,
diff --git a/src/test/compile-fail/union/union-generic.rs b/src/test/compile-fail/union/union-generic.rs
index e6586b0fb7f6a..8bd422b42d879 100644
--- a/src/test/compile-fail/union/union-generic.rs
+++ b/src/test/compile-fail/union/union-generic.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 use std::rc::Rc;
 
 union U<T: Copy> {
diff --git a/src/test/compile-fail/union/union-lint-dead-code.rs b/src/test/compile-fail/union/union-lint-dead-code.rs
index 7a552c8f8b76a..a64187c5f2344 100644
--- a/src/test/compile-fail/union/union-lint-dead-code.rs
+++ b/src/test/compile-fail/union/union-lint-dead-code.rs
@@ -8,7 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
 #![deny(dead_code)]
 
 union Foo {
diff --git a/src/test/compile-fail/union/union-repr-c.rs b/src/test/compile-fail/union/union-repr-c.rs
index d7dfb126c9324..15a4197fe94c9 100644
--- a/src/test/compile-fail/union/union-repr-c.rs
+++ b/src/test/compile-fail/union/union-repr-c.rs
@@ -8,7 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
 #![allow(unused)]
 #![deny(improper_ctypes)]
 
diff --git a/src/test/compile-fail/union/union-suggest-field.rs b/src/test/compile-fail/union/union-suggest-field.rs
index 3c355989b82f0..65c7c980b8ac2 100644
--- a/src/test/compile-fail/union/union-suggest-field.rs
+++ b/src/test/compile-fail/union/union-suggest-field.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 union U {
     principal: u8,
 }
diff --git a/src/test/debuginfo/union-smoke.rs b/src/test/debuginfo/union-smoke.rs
index 0b2544151fd32..433196b526c5c 100644
--- a/src/test/debuginfo/union-smoke.rs
+++ b/src/test/debuginfo/union-smoke.rs
@@ -34,7 +34,6 @@
 #![allow(unused)]
 #![feature(omit_gdb_pretty_printer_section)]
 #![omit_gdb_pretty_printer_section]
-#![feature(untagged_unions)]
 
 union U {
     a: (u8, u8),
diff --git a/src/test/run-pass/union/auxiliary/union.rs b/src/test/run-pass/union/auxiliary/union.rs
index 0231e38a729b7..2c4945622abdc 100644
--- a/src/test/run-pass/union/auxiliary/union.rs
+++ b/src/test/run-pass/union/auxiliary/union.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 pub union U {
     pub a: u8,
     pub b: u16,
diff --git a/src/test/run-pass/union/union-backcomp.rs b/src/test/run-pass/union/union-backcomp.rs
index 0f8c996bebda8..b706a81850ce8 100644
--- a/src/test/run-pass/union/union-backcomp.rs
+++ b/src/test/run-pass/union/union-backcomp.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 macro_rules! union {
     () => (struct S;)
 }
diff --git a/src/test/run-pass/union/union-basic.rs b/src/test/run-pass/union/union-basic.rs
index dc14c12b6a22e..5e5b2d4d7ce7b 100644
--- a/src/test/run-pass/union/union-basic.rs
+++ b/src/test/run-pass/union/union-basic.rs
@@ -13,8 +13,6 @@
 // FIXME: This test case makes little-endian assumptions.
 // ignore-s390x
 
-#![feature(untagged_unions)]
-
 extern crate union;
 use std::mem::{size_of, align_of, zeroed};
 
diff --git a/src/test/run-pass/union/union-c-interop.rs b/src/test/run-pass/union/union-c-interop.rs
index 13dfd414615c5..b3df7d658b15f 100644
--- a/src/test/run-pass/union/union-c-interop.rs
+++ b/src/test/run-pass/union/union-c-interop.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 #[derive(Clone, Copy)]
 #[repr(C)]
 struct LARGE_INTEGER_U {
diff --git a/src/test/run-pass/union/union-const-trans.rs b/src/test/run-pass/union/union-const-trans.rs
index bdae1a0eaf88f..77270364bb5e9 100644
--- a/src/test/run-pass/union/union-const-trans.rs
+++ b/src/test/run-pass/union/union-const-trans.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 union U {
     a: u64,
     b: u64,
diff --git a/src/test/run-pass/union/union-inherent-method.rs b/src/test/run-pass/union/union-inherent-method.rs
index adea27bd25462..a88fdc57a3ef4 100644
--- a/src/test/run-pass/union/union-inherent-method.rs
+++ b/src/test/run-pass/union/union-inherent-method.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 union U {
     a: u8,
 }
diff --git a/src/test/run-pass/union/union-macro.rs b/src/test/run-pass/union/union-macro.rs
index a23fbc3be9e2c..b6141ae82c371 100644
--- a/src/test/run-pass/union/union-macro.rs
+++ b/src/test/run-pass/union/union-macro.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 macro_rules! duplicate {
    ($i: item) => {
         mod m1 {
diff --git a/src/test/run-pass/union/union-pat-refutability.rs b/src/test/run-pass/union/union-pat-refutability.rs
index e6144f35f1d54..81607236c9ecd 100644
--- a/src/test/run-pass/union/union-pat-refutability.rs
+++ b/src/test/run-pass/union/union-pat-refutability.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 #[repr(u32)]
 enum Tag { I, F }
 
diff --git a/src/test/run-pass/union/union-trait-impl.rs b/src/test/run-pass/union/union-trait-impl.rs
index a5a2be0133aba..1cdaff2cab7c8 100644
--- a/src/test/run-pass/union/union-trait-impl.rs
+++ b/src/test/run-pass/union/union-trait-impl.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 use std::fmt;
 
 union U {
diff --git a/src/test/run-pass/union/union-transmute.rs b/src/test/run-pass/union/union-transmute.rs
index 7a0b4c6aaca49..7233687aaab53 100644
--- a/src/test/run-pass/union/union-transmute.rs
+++ b/src/test/run-pass/union/union-transmute.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 extern crate core;
 use core::f32;
 
diff --git a/src/test/rustdoc/union.rs b/src/test/rustdoc/union.rs
index 0dcc9098ad75c..2eb25d9b7ddac 100644
--- a/src/test/rustdoc/union.rs
+++ b/src/test/rustdoc/union.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 // @has union/union.U.html
 pub union U {
     // @has - //pre "pub a: u8"
diff --git a/src/test/ui/print_type_sizes/packed.rs b/src/test/ui/print_type_sizes/packed.rs
index cd7ef86d70ee3..7f278e71b21c6 100644
--- a/src/test/ui/print_type_sizes/packed.rs
+++ b/src/test/ui/print_type_sizes/packed.rs
@@ -18,8 +18,6 @@
 // aligned (while on most it is 8-byte aligned) and so the resulting
 // padding and overall computed sizes can be quite different.
 
-#![feature(untagged_unions)]
-
 #![allow(dead_code)]
 
 #[derive(Default)]