reorganize modules (#1766)

This patch started as an effort to make `time::Timer` private. However, in an
effort to get the build compiling again, more and more changes were made. This
probably should have been broken up, but here we are. I will attempt to
summarize the changes here.

* Feature flags are reorganized to make clearer. `net-driver` becomes
  `io-driver`. `rt-current-thread` becomes `rt-core`.

* The `Runtime` can be created without any executor. This replaces `enter`. It
  also allows creating I/O / time drivers that are standalone.

* `tokio::timer` is renamed to `tokio::time`. This brings it in line with `std`.

* `tokio::timer::Timer` is renamed to `Driver` and made private.

* The `clock` module is removed. Instead, an `Instant` type is provided. This
  type defaults to calling `std::time::Instant`. A `test-util` feature flag can
  be used to enable hooking into time.

* The `blocking` module is moved to the top level and is cleaned up.

* The `task` module is moved to the top level.

* The thread-pool's in-place blocking implementation is cleaned up.

* `runtime::Spawner` is renamed to `runtime::Handle` and can be used to "enter"
  a runtime context.

2 files changed, 62 insertions, 2 deletions

diff --git a/tokio/src/loom/std/atomic_u64.rs b/tokio/src/loom/std/atomic_u64.rs
new file mode 100644
index 00000000..206954fc
--- /dev/null
+++ b/tokio/src/loom/std/atomic_u64.rs
@@ -0,0 +1,60 @@
+//! Implementation of an atomic u64 cell. On 64 bit platforms, this is a
+//! re-export of `AtomicU64`. On 32 bit platforms, this is implemented using a
+//! `Mutex`.
+
+pub(crate) use self::imp::AtomicU64;
+
+// `AtomicU64` can only be used on targets with `target_has_atomic` is 64 or greater.
+// Once `cfg_target_has_atomic` feature is stable, we can replace it with
+// `#[cfg(target_has_atomic = "64")]`.
+// Refs: https://github.com/rust-lang/rust/tree/master/src/librustc_target
+#[cfg(not(any(target_arch = "arm", target_arch = "mips", target_arch = "powerpc")))]
+mod imp {
+    pub(crate) use std::sync::atomic::AtomicU64;
+}
+
+#[cfg(any(target_arch = "arm", target_arch = "mips", target_arch = "powerpc"))]
+mod imp {
+    use std::sync::atomic::Ordering;
+    use std::sync::Mutex;
+
+    #[derive(Debug)]
+    pub(crate) struct AtomicU64 {
+        inner: Mutex<u64>,
+    }
+
+    impl AtomicU64 {
+        pub(crate) fn new(val: u64) -> AtomicU64 {
+            AtomicU64 {
+                inner: Mutex::new(val),
+            }
+        }
+
+        pub(crate) fn load(&self, _: Ordering) -> u64 {
+            *self.inner.lock().unwrap()
+        }
+
+        pub(crate) fn store(&self, val: u64, _: Ordering) {
+            *self.inner.lock().unwrap() = val;
+        }
+
+        pub(crate) fn fetch_or(&self, val: u64, _: Ordering) -> u64 {
+            let mut lock = self.inner.lock().unwrap();
+            let prev = *lock;
+            *lock = prev | val;
+            prev
+        }
+
+        pub(crate) fn compare_and_swap(&self, old: u64, new: u64, _: Ordering) -> u64 {
+            let mut lock = self.inner.lock().unwrap();
+            let prev = *lock;
+
+            if prev != old {
+                return prev;
+            }
+
+            *lock = new;
+            prev
+        }
+    }
+}
diff --git a/tokio/src/loom/std/mod.rs b/tokio/src/loom/std/mod.rs
index e0aafa83..c5bd6039 100644
--- a/tokio/src/loom/std/mod.rs
+++ b/tokio/src/loom/std/mod.rs
@@ -1,8 +1,7 @@
-// rt-full implies rt-current-thread
-
 #![cfg_attr(not(feature = "rt-full"), allow(unused_imports, dead_code))]
 
 mod atomic_u32;
+mod atomic_u64;
 mod atomic_usize;
 mod causal_cell;
 
@@ -43,6 +42,7 @@ pub(crate) mod sync {
 
     pub(crate) mod atomic {
         pub(crate) use crate::loom::std::atomic_u32::AtomicU32;
+        pub(crate) use crate::loom::std::atomic_u64::AtomicU64;
         pub(crate) use crate::loom::std::atomic_usize::AtomicUsize;
 
         pub(crate) use std::sync::atomic::spin_loop_hint;