Add fast-and-unsound feature

In some cases, speed could be preferable over soundness if we control
the implementation of `Read` that is used for the underlying stream.
By default the feature is _NOT_ activated, keeping the current behavior as-is.
However, when the feature is activated, it gives better performance when the user
ensures correctness by verifying the behavior of the underlying `Read` implementation.

Author: woutersl

Cargo.toml

@@ -26,6 +26,7 @@ native-tls-vendored = ["native-tls", "native-tls-crate/vendored"]
 rustls-tls-native-roots = ["__rustls-tls", "rustls-native-certs"]
 rustls-tls-webpki-roots = ["__rustls-tls", "webpki-roots"]
 __rustls-tls = ["rustls", "rustls-pki-types"]
+fast-and-unsound = []
 
 [dependencies]
 data-encoding = { version = "2", optional = true }

src/protocol/frame/mod.rs

@@ -225,7 +225,13 @@ impl FrameCodec {
     fn read_in(&mut self, stream: &mut impl Read) -> io::Result<usize> {
         let len = self.in_buffer.len();
         debug_assert!(self.in_buffer.capacity() > len);
+        #[cfg(not(feature = "fast-and-unsound"))]
         self.in_buffer.resize(self.in_buffer.capacity(), 0);
+        #[cfg(feature = "fast-and-unsound")]
+        // SAFETY: this is known to be unsound,
+        // a strange implementation of `Read` could read uninitialized memory.
+        // However, if speed is preferable over soundness `fast-and-unsound` can be activated.
+        unsafe { self.in_buffer.set_len(self.in_buffer.capacity()); }
         let size = stream.read(&mut self.in_buffer[len..]);
         self.in_buffer.truncate(len + size.as_ref().copied().unwrap_or(0));
         size