From 9277ddafdf7c1a3cafe17e66b43fbd222f7f89cc Mon Sep 17 00:00:00 2001
From: Sergio Benitez <sb@sergio.bz>
Date: Sun, 28 Jun 2020 14:01:06 -0700
Subject: [PATCH] Swap 'Rocket' manually without using 'replace_with'.

---
 core/lib/src/ext.rs    | 52 +-----------------------------------------
 core/lib/src/rocket.rs | 20 ++++++++++++++--
 2 files changed, 19 insertions(+), 53 deletions(-)

diff --git a/core/lib/src/ext.rs b/core/lib/src/ext.rs
index 0595d625..81236216 100644
--- a/core/lib/src/ext.rs
+++ b/core/lib/src/ext.rs
@@ -2,7 +2,7 @@ use std::io::{self, Cursor};
 use std::pin::Pin;
 use std::task::{Poll, Context};
 
-use futures::{ready, Future, future::BoxFuture, stream::Stream};
+use futures::{ready, future::BoxFuture, stream::Stream};
 use tokio::io::{AsyncRead, AsyncReadExt as _};
 
 use crate::http::hyper;
@@ -111,53 +111,3 @@ impl AsyncRead for AsyncReadBody {
         }
     }
 }
-
-// The code below was adapted from the `replace_with` crate and reproduced here
-// under the rights granted by the MIT license. The code is copyright the
-// `replace_with` developers. See LICENSE-MIT for the full text.
-
-struct OnUnwind<F: FnOnce()>(std::mem::ManuallyDrop<F>);
-
-impl<F: FnOnce()> Drop for OnUnwind<F> {
-    #[inline(always)]
-    fn drop(&mut self) {
-        (unsafe { std::ptr::read(&*self.0) })();
-    }
-}
-
-#[inline(always)]
-pub async fn async_on_unwind<F, Fut, T, P: FnOnce()>(f: F, p: P) -> T
-    where F: FnOnce() -> Fut, Fut: Future<Output = T>,
-{
-    let x = OnUnwind(std::mem::ManuallyDrop::new(p));
-    let t = f().await;
-    let _ = unsafe { std::ptr::read(&*x.0) };
-    std::mem::forget(x);
-    t
-}
-
-#[inline]
-pub async fn async_replace_with_or_else<T, Fut, F, D>(dest: &mut T, d: D, f: F)
-    where Fut: Future<Output = T>,
-          F: FnOnce(T) -> Fut,
-          D: FnOnce() -> T,
-{
-    unsafe {
-        let old = std::ptr::read(dest);
-        let new = async_on_unwind(
-            || async move { f(old).await },
-            || std::ptr::write(dest, d()),
-        ).await;
-
-        std::ptr::write(dest, new);
-    }
-}
-
-
-#[inline]
-pub async fn async_replace_with<T, Fut, F>(dest: &mut T, f: F)
-    where Fut: Future<Output = T>,
-          F: FnOnce(T) -> Fut,
-{
-    async_replace_with_or_else(dest, || std::process::abort(), f).await
-}
diff --git a/core/lib/src/rocket.rs b/core/lib/src/rocket.rs
index f92fe8fd..f830633c 100644
--- a/core/lib/src/rocket.rs
+++ b/core/lib/src/rocket.rs
@@ -24,7 +24,7 @@ use crate::outcome::Outcome;
 use crate::error::{LaunchError, LaunchErrorKind};
 use crate::fairing::{Fairing, Fairings};
 use crate::logger::PaintExt;
-use crate::ext::{AsyncReadExt, async_replace_with};
+use crate::ext::AsyncReadExt;
 use crate::shutdown::{ShutdownHandle, ShutdownHandleManaged};
 
 use crate::http::{Method, Status, Header};
@@ -109,6 +109,21 @@ impl Rocket {
         self
     }
 
+    // Create a "dummy" instance of `Rocket` to use while mem-swapping `self`.
+    fn dummy() -> Rocket {
+        Rocket {
+            manifest: vec![],
+            config: Config::development(),
+            router: Router::new(),
+            default_catchers: HashMap::new(),
+            catchers: HashMap::new(),
+            managed_state: Container::new(),
+            fairings: Fairings::new(),
+            shutdown_handle: ShutdownHandle(mpsc::channel(1).0),
+            shutdown_receiver: None,
+        }
+    }
+
     // Instead of requiring the user to individually `await` each call to
     // `attach()`, some operations are queued in `self.pending`. Functions that
     // want to provide read access to any data from the Cargo, such as
@@ -133,7 +148,8 @@ impl Rocket {
                     PreLaunchOp::Mount(base, routes) => self._mount(base, routes),
                     PreLaunchOp::Register(catchers) => self._register(catchers),
                     PreLaunchOp::Attach(fairing) => {
-                        async_replace_with(self, |rocket| rocket._attach(fairing)).await;
+                        let rocket = mem::replace(self, Rocket::dummy());
+                        *self = rocket._attach(fairing).await;
                         self.manifest.append(&mut manifest);
                         manifest = mem::replace(&mut self.manifest, vec![]);
                     }