runtime: merge multi & single threaded runtimes (#1716)

Simplify Tokio's runtime construct by combining both Runtime variants
into a single type. The execution style can be controlled by a
configuration setting on `Builder`.

The implication of this change is that there is no longer any way to
spawn `!Send` futures. This, however, is a temporary limitation. A
different strategy will be employed for supporting `!Send` futures.

Included in this patch is a rework of `task::JoinHandle` to support
using this type from both the thread-pool and current-thread executors.

1 files changed, 15 insertions, 5 deletions

diff --git a/tokio/src/executor/loom/std/mod.rs b/tokio/src/executor/loom/std/mod.rs
index 568912bf..eb865d98 100644
--- a/tokio/src/executor/loom/std/mod.rs
+++ b/tokio/src/executor/loom/std/mod.rs
@@ -1,10 +1,12 @@
+// rt-full implies rt-current-thread
+
 #[cfg(feature = "rt-full")]
 mod atomic_u32;
 mod atomic_usize;
-#[cfg(feature = "rt-full")]
+#[cfg(feature = "rt-current-thread")]
 mod causal_cell;
 
-#[cfg(feature = "rt-full")]
+#[cfg(feature = "rt-current-thread")]
 pub(crate) mod alloc {
     #[derive(Debug)]
     pub(crate) struct Track<T> {
@@ -26,7 +28,7 @@ pub(crate) mod alloc {
     }
 }
 
-#[cfg(feature = "rt-full")]
+#[cfg(feature = "rt-current-thread")]
 pub(crate) mod cell {
     pub(crate) use super::causal_cell::{CausalCell, CausalCheck};
 }
@@ -62,15 +64,23 @@ pub(crate) mod sync {
         pub(crate) use crate::executor::loom::std::atomic_usize::AtomicUsize;
 
         #[cfg(feature = "rt-full")]
-        pub(crate) use std::sync::atomic::{fence, spin_loop_hint, AtomicPtr};
+        pub(crate) use std::sync::atomic::spin_loop_hint;
+        #[cfg(feature = "rt-current-thread")]
+        pub(crate) use std::sync::atomic::{fence, AtomicPtr};
     }
 }
 
-#[cfg(feature = "rt-full")]
+#[cfg(feature = "rt-current-thread")]
 pub(crate) mod sys {
+    #[cfg(feature = "rt-full")]
     pub(crate) fn num_cpus() -> usize {
         usize::max(1, num_cpus::get_physical())
     }
+
+    #[cfg(not(feature = "rt-full"))]
+    pub(crate) fn num_cpus() -> usize {
+        1
+    }
 }
 
 #[cfg(any(feature = "blocking", feature = "rt-full"))]