Remove usage of 'never_type' nightly feature

Summary: It doesn't look like the never type is going to be stabilized anytime soon, so lets just use the `never-say-never` crate instead.

Reviewed By: dtolnay

Differential Revision: D41459064

fbshipit-source-id: 6bc82377242b31171251b2393a534f76c7f3d97a

reverie-ptrace/src/gdbstub/response.rs

@@ -66,12 +66,6 @@ impl WriteResponse for ResponseOk {
     }
 }
 
-impl WriteResponse for ! {
-    fn write_response(&self, f: &mut ResponseWriter) {
-        ResponseNone.write_response(f)
-    }
-}
-
 impl<T> WriteResponse for ResponseAsPlain<T>
 where
     T: AsRef<[u8]>,

reverie-ptrace/src/lib.rs

@@ -29,7 +29,6 @@
 #![deny(missing_docs)]
 #![deny(rustdoc::broken_intra_doc_links)]
 #![feature(internal_output_capture)]
-#![feature(never_type)]
 
 mod children;
 mod cp;

reverie-ptrace/src/task.rs

@@ -43,6 +43,7 @@ use reverie::Frame;
 use reverie::GlobalRPC;
 use reverie::GlobalTool;
 use reverie::Guest;
+use reverie::Never;
 use reverie::Pid;
 #[cfg(target_arch = "x86_64")]
 use reverie::Rdtsc;
@@ -1275,7 +1276,7 @@ impl<L: Tool + 'static> TracedTask<L> {
 
         // Wait on a future that will never complete. This pending future will
         // be dropped when the channel receives the event just sent.
-        future::pending::<!>().await
+        future::pending().await
     }
 
     /// Marks the current task as exited via a channel. The receiver end of the
@@ -1698,7 +1699,7 @@ impl<L: Tool + 'static> TracedTask<L> {
             Ok(_) => {
                 // Drop the handle_syscall_event future.
                 self.notifier.notify_one();
-                future::pending::<!>().await
+                future::pending().await
             }
             Err(err) => self.abort(Err(err)).await,
         }
@@ -2161,7 +2162,7 @@ impl<L: Tool + 'static> Guest<L> for TracedTask<L> {
     }
 
     #[allow(unreachable_code)]
-    async fn tail_inject<S: SyscallInfo>(&mut self, syscall: S) -> ! {
+    async fn tail_inject<S: SyscallInfo>(&mut self, syscall: S) -> Never {
         // Call a non-templatized function to reduce code bloat.
         let (nr, args) = syscall.into_parts();
         self.do_tail_inject(nr, args).await

reverie/Cargo.toml

@@ -18,6 +18,7 @@ lazy_static = "1.4"
 libc = "0.2.137"
 linked-hash-map = { version = "0.5", features = ["serde_impl"] }
 memmap = "0.7"
+never-say-never = "6"
 nix = "0.25"
 object = { version = "0.29", features = ["write"] }
 procfs = "0.9"

reverie/src/guest.rs

@@ -21,6 +21,7 @@ use crate::timer::TimerSchedule;
 use crate::tool::GlobalRPC;
 use crate::tool::GlobalTool;
 use crate::tool::Tool;
+use crate::Never;
 use crate::Pid;
 
 /// A representation of a guest task (thread).
@@ -152,7 +153,7 @@ pub trait Guest<T: Tool>: Send + GlobalRPC<T::GlobalState> {
     ///     }
     /// }
     /// ```
-    async fn tail_inject<S: SyscallInfo>(&mut self, syscall: S) -> !;
+    async fn tail_inject<S: SyscallInfo>(&mut self, syscall: S) -> Never;
 
     /// Like [`Guest::inject`], but will retry the syscall if `EINTR` or
     /// `ERESTARTSYS` are returned.
@@ -350,7 +351,7 @@ where
         self.inner.inject(syscall).await
     }
 
-    async fn tail_inject<S: SyscallInfo>(&mut self, syscall: S) -> ! {
+    async fn tail_inject<S: SyscallInfo>(&mut self, syscall: S) -> Never {
         #![allow(unreachable_code)]
         self.inner.tail_inject(syscall).await
     }

reverie/src/lib.rs

@@ -9,7 +9,6 @@
 #![doc = include_str!("../../README.md")]
 #![deny(missing_docs)]
 #![deny(rustdoc::broken_intra_doc_links)]
-#![feature(never_type)]
 
 mod auxv;
 mod backtrace;
@@ -65,3 +64,6 @@ pub use nix::sys::signal::Signal;
 pub use raw_cpuid::CpuIdResult;
 /// typed syscalls.
 pub use reverie_syscalls as syscalls;
+
+/// `Never` type is a stopgap for the unstable `!` type (i.e., the never type).
+pub type Never = never_say_never::Never;