diff --git a/.travis.yml b/.travis.yml
index ba96dd4e..41ac09c7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,6 +2,19 @@ language: rust
 
 matrix:
   include:
+    - env: TARGET=x86_64-unknown-linux-gnu
+      rust: beta
+      if: branch != master
+
+    # wait for 1.29-beta
+    # - env: TARGET=thumbv6m-none-eabi
+    #   rust: beta
+    #   if: branch != master
+
+    - env: TARGET=thumbv7m-none-eabi
+      rust: beta
+      if: branch != master
+
     - env: TARGET=x86_64-unknown-linux-gnu
       rust: nightly
 
diff --git a/ci/script.sh b/ci/script.sh
index eafda3fd..335da344 100644
--- a/ci/script.sh
+++ b/ci/script.sh
@@ -1,22 +1,28 @@
 set -euxo pipefail
 
 main() {
-    cargo check --target $TARGET
+    cargo check --target $TARGET --no-default-features
+    if [ $TRAVIS_RUST_VERSION = nightly ]; then
+        cargo check --target $TARGET
+    fi
 
     if [ $TARGET = x86_64-unknown-linux-gnu ]; then
-        cargo test --target $TARGET
         cargo test --target $TARGET --no-default-features
-        cargo test --target $TARGET --release
         cargo test --target $TARGET --release --no-default-features
 
-        export RUSTFLAGS="-Z sanitizer=thread"
-        export RUST_TEST_THREADS=1
-        export TSAN_OPTIONS="suppressions=$(pwd)/blacklist.txt"
+        if [ $TRAVIS_RUST_VERSION = nightly ]; then
+            cargo test --target $TARGET
+            cargo test --target $TARGET --release
 
-        cargo test --test tsan --target $TARGET
-        cargo test --test tsan --target $TARGET --no-default-features
-        cargo test --test tsan --target $TARGET --release
-        cargo test --test tsan --target $TARGET --release --no-default-features
+            export RUSTFLAGS="-Z sanitizer=thread"
+            export RUST_TEST_THREADS=1
+            export TSAN_OPTIONS="suppressions=$(pwd)/blacklist.txt"
+
+            cargo test --test tsan --target $TARGET
+            cargo test --test tsan --target $TARGET --no-default-features
+            cargo test --test tsan --target $TARGET --release
+            cargo test --test tsan --target $TARGET --release --no-default-features
+        fi
     fi
 }
 
diff --git a/src/__core.rs b/src/__core.rs
index 763d0500..001a8024 100644
--- a/src/__core.rs
+++ b/src/__core.rs
@@ -1,51 +1,20 @@
 /// Temporary fork of some stuff in `core` that's doesn't have a `const fn` API
 
 pub mod mem {
-    pub use core::mem::{replace, zeroed};
+    #[cfg(not(feature = "const-fn"))]
+    pub use core::mem::uninitialized;
+    pub use core::mem::{replace, zeroed, ManuallyDrop};
 
-    use core::ops::{Deref, DerefMut};
-
-    #[allow(unions_with_drop_fields)]
-    pub union ManuallyDrop<T> {
-        value: T,
-    }
-
-    impl<T> ManuallyDrop<T> {
-        #[inline]
-        const_fn!(
-            pub const fn new(value: T) -> ManuallyDrop<T> {
-                ManuallyDrop { value: value }
-            }
-        );
-    }
-
-    impl<T> Deref for ManuallyDrop<T> {
-        type Target = T;
-
-        #[inline]
-        fn deref(&self) -> &Self::Target {
-            unsafe { &self.value }
+    #[cfg(feature = "const-fn")]
+    pub const unsafe fn uninitialized<T>() -> T {
+        #[allow(unions_with_drop_fields)]
+        union U<T> {
+            none: (),
+            some: T,
         }
+
+        U { none: () }.some
     }
-
-    impl<T> DerefMut for ManuallyDrop<T> {
-        #[inline]
-        fn deref_mut(&mut self) -> &mut Self::Target {
-            unsafe { &mut self.value }
-        }
-    }
-
-    const_fn!(
-        pub const unsafe fn uninitialized<T>() -> T {
-            #[allow(unions_with_drop_fields)]
-            union U<T> {
-                none: (),
-                some: T,
-            }
-
-            U { none: () }.some
-        }
-    );
 }
 
 #[cfg(feature = "const-fn")] // Remove this if there are more tests
@@ -61,14 +30,16 @@ mod test {
         static mut I: i32 = unsafe { __core::mem::uninitialized() };
         // Initialize before drop
         unsafe { core::ptr::write(&mut I as *mut i32, 42) };
-        unsafe{ assert_eq!(I, 42) };
+        unsafe { assert_eq!(I, 42) };
     }
 
     #[cfg(feature = "const-fn")]
     #[test]
     fn static_new_manually_drop() {
         static mut M: ManuallyDrop<i32> = ManuallyDrop::new(42);
-        unsafe { assert_eq!(*M, 42); }
+        unsafe {
+            assert_eq!(*M, 42);
+        }
         // Drop before deinitialization
         unsafe { core::ptr::drop_in_place(&mut M as &mut i32 as *mut i32) };
     }
diff --git a/src/lib.rs b/src/lib.rs
index 8f934071..dfa5463b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -83,8 +83,9 @@
 #![deny(missing_docs)]
 #![deny(warnings)]
 #![cfg_attr(feature = "const-fn", feature(const_fn))]
+#![cfg_attr(feature = "const-fn", feature(const_manually_drop_new))]
+#![cfg_attr(feature = "const-fn", feature(untagged_unions))]
 #![cfg_attr(feature = "smaller-atomics", feature(core_intrinsics))]
-#![feature(untagged_unions)]
 #![no_std]
 
 extern crate generic_array;