Don't initialize read buffer

This shouldn't be necessary, but it seems that tokio-codec doesn't
properly track the initialization of its buffer and so the old logic
resulted in a ton of overhead.

src/lib.rs

@@ -11,6 +11,7 @@ use openssl::ssl::{self, ErrorCode, ShutdownResult, Ssl, SslRef};
 use std::fmt;
 use std::io::{self, Read, Write};
 use std::pin::Pin;
+use std::slice;
 use std::task::{Context, Poll};
 use tokio::io::{AsyncRead, AsyncWrite, ReadBuf};
 
@@ -189,12 +190,23 @@ where
         ctx: &mut Context<'_>,
         buf: &mut ReadBuf<'_>,
     ) -> Poll<io::Result<()>> {
-        self.with_context(ctx, |s| match cvt(s.read(buf.initialize_unfilled()))? {
-            Poll::Ready(nread) => {
-                buf.advance(nread);
-                Poll::Ready(Ok(()))
+        self.with_context(ctx, |s| {
+            // This isn't really "proper", but rust-openssl doesn't currently expose a suitable interface even though
+            // OpenSSL itself doesn't require the buffer to be initialized. So this is good enough for now.
+            let slice = unsafe {
+                let buf = buf.unfilled_mut();
+                slice::from_raw_parts_mut(buf.as_mut_ptr().cast::<u8>(), buf.len())
+            };
+            match cvt(s.read(slice))? {
+                Poll::Ready(nread) => {
+                    unsafe {
+                        buf.assume_init(nread);
+                    }
+                    buf.advance(nread);
+                    Poll::Ready(Ok(()))
+                }
+                Poll::Pending => Poll::Pending,
             }
-            Poll::Pending => Poll::Pending,
         })
     }
 }