Do the hash check only in debug mode

Author: mzohreva

ct.sh

@@ -13,6 +13,7 @@ if [ $TRAVIS_RUST_VERSION = "stable" ] || [ $TRAVIS_RUST_VERSION = "beta" ] || [
     rustup default $TRAVIS_RUST_VERSION
     # make sure that explicitly providing the default target works
     cargo test --target x86_64-unknown-linux-gnu
+    cargo test --release
     cargo test --features spin_threading
     cargo test --features rust_threading
     cargo test --features custom_time,custom_gmtime_r

mbedtls/src/ssl/async_session.rs

@@ -147,6 +147,7 @@ struct WriteTracker {
 }
 
 struct DigestAndLen {
+    #[cfg(debug_assertions)]
     digest: [u8; 20], // SHA-1
     len: usize,
 }
@@ -158,6 +159,7 @@ impl WriteTracker {
         }
     }
 
+    #[cfg(debug_assertions)]
     fn digest(buf: &[u8]) -> [u8; 20] {
         use crate::hash::{Md, Type};
         let mut out = [0u8; 20];
@@ -172,9 +174,15 @@ impl WriteTracker {
             Some(pending) => {
                 if pending.len <= buf.len() {
                     let buf = &buf[..pending.len];
+
+                    // We only do this check in debug mode since it's an expensive check.
+                    #[cfg(debug_assertions)]
                     if Self::digest(buf) == pending.digest {
                         return Ok(buf);
                     }
+
+                    #[cfg(not(debug_assertions))]
+                    return Ok(buf);
                 }
                 Err(io::Error::new(
                     io::ErrorKind::Other,
@@ -189,6 +197,7 @@ impl WriteTracker {
             &Poll::Pending => {
                 if self.pending.is_none() {
                     self.pending = Some(Box::new(DigestAndLen {
+                        #[cfg(debug_assertions)]
                         digest: Self::digest(buf),
                         len: buf.len(),
                     }));