openpgp: Optimize drop(Vec<u8>::drain(..n)) in debug mode.

  - Similar to Vec<u8>::truncate(_), this operation is very slow in
    debug mode due to the dropping of drained elements.  Provide an
    optimized version in debug mode.

6 files changed, 29 insertions, 10 deletions

diff --git a/openpgp/src/armor.rs b/openpgp/src/armor.rs
index 5214d4a7..d690bfc5 100644
--- a/openpgp/src/armor.rs
+++ b/openpgp/src/armor.rs
@@ -1036,7 +1036,7 @@ impl<'a> Read for Reader<'a> {
 
             let amount = cmp::min(buf.len(), self.buffer.len());
             buf[..amount].copy_from_slice(&self.buffer[..amount]);
-            self.buffer.drain(..amount);
+            crate::vec_drain_prefix(&mut self.buffer, amount);
 
             (0, amount)
         } else {
@@ -1111,7 +1111,7 @@ impl<'a> Read for Reader<'a> {
 
                 let copied = cmp::min(buf.len(), self.buffer.len());
                 buf[..copied].copy_from_slice(&self.buffer[..copied]);
-                self.buffer.drain(..copied);
+                crate::vec_drain_prefix(&mut self.buffer, copied);
 
                 copied
             } else {
diff --git a/openpgp/src/crypto/aead.rs b/openpgp/src/crypto/aead.rs
index 35fbd074..a297b999 100644
--- a/openpgp/src/crypto/aead.rs
+++ b/openpgp/src/crypto/aead.rs
@@ -223,7 +223,7 @@ impl<'a> Decryptor<'a> {
         if self.buffer.len() > 0 {
             let to_copy = cmp::min(self.buffer.len(), plaintext.len());
             &plaintext[..to_copy].copy_from_slice(&self.buffer[..to_copy]);
-            self.buffer.drain(..to_copy);
+            crate::vec_drain_prefix(&mut self.buffer, to_copy);
 
             pos = to_copy;
             if pos == plaintext.len() {
@@ -341,7 +341,7 @@ impl<'a> Decryptor<'a> {
 
                     &plaintext[pos..pos + to_copy]
                         .copy_from_slice(&self.buffer[..to_copy]);
-                    self.buffer.drain(..to_copy);
+                    crate::vec_drain_prefix(&mut self.buffer, to_copy);
                     pos += to_copy;
                 } else {
                     pos += to_decrypt;
diff --git a/openpgp/src/crypto/symmetric.rs b/openpgp/src/crypto/symmetric.rs
index b6aa799b..fb889dd4 100644
--- a/openpgp/src/crypto/symmetric.rs
+++ b/openpgp/src/crypto/symmetric.rs
@@ -205,7 +205,7 @@ impl<R: io::Read> io::Read for Decryptor<R> {
         if self.buffer.len() > 0 {
             let to_copy = cmp::min(self.buffer.len(), plaintext.len());
             &plaintext[..to_copy].copy_from_slice(&self.buffer[..to_copy]);
-            self.buffer.drain(..to_copy);
+            crate::vec_drain_prefix(&mut self.buffer, to_copy);
             pos = to_copy;
         }
 
@@ -274,7 +274,7 @@ impl<R: io::Read> io::Read for Decryptor<R> {
                                         format!("{}", e)))?;
 
         &plaintext[pos..pos + to_copy].copy_from_slice(&self.buffer[..to_copy]);
-        self.buffer.drain(..to_copy);
+        crate::vec_drain_prefix(&mut self.buffer, to_copy);
 
         pos += to_copy;
 
diff --git a/openpgp/src/lib.rs b/openpgp/src/lib.rs
index f1d1506e..0bc4b3d2 100644
--- a/openpgp/src/lib.rs
+++ b/openpgp/src/lib.rs
@@ -90,6 +90,25 @@ fn vec_truncate(v: &mut Vec<u8>, len: usize) {
     }
 }
 
+/// Like `drop(Vec<u8>::drain(..prefix_len))`, but fast in debug
+/// builds.
+fn vec_drain_prefix(v: &mut Vec<u8>, prefix_len: usize) {
+    if cfg!(debug_assertions) {
+        // Panic like v.drain(..prefix_len).
+        assert!(prefix_len <= v.len(), "prefix len {} > vector len {}",
+                prefix_len, v.len());
+        let new_len = v.len() - prefix_len;
+        unsafe {
+            std::ptr::copy(v[prefix_len..].as_ptr(),
+                           v[..].as_mut_ptr(),
+                           new_len);
+        }
+        vec_truncate(v, new_len);
+    } else {
+        v.drain(..prefix_len);
+    }
+}
+
 // Like assert!, but checks a pattern.
 //
 //   assert_match!(Some(_) = x);
diff --git a/openpgp/src/parse/stream.rs b/openpgp/src/parse/stream.rs
index 61c12915..1b939c5b 100644
--- a/openpgp/src/parse/stream.rs
+++ b/openpgp/src/parse/stream.rs
@@ -895,7 +895,7 @@ impl<'a, H: VerificationHelper> Verifier<'a, H> {
 
             let n = cmp::min(buf.len(), reserve.len());
             &mut buf[..n].copy_from_slice(&reserve[..n]);
-            reserve.drain(..n);
+            crate::vec_drain_prefix(reserve, n);
             return Ok(n);
         }
 
@@ -1091,7 +1091,7 @@ impl<'a> Transformer<'a> {
 
         let n = cmp::min(buf.len(), self.buffer.len());
         &mut buf[..n].copy_from_slice(&self.buffer[..n]);
-        self.buffer.drain(..n);
+        crate::vec_drain_prefix(&mut self.buffer, n);
         Ok(n)
     }
 }
@@ -1809,7 +1809,7 @@ impl<'a, H: VerificationHelper + DecryptionHelper> Decryptor<'a, H> {
 
             let n = cmp::min(buf.len(), reserve.len());
             &mut buf[..n].copy_from_slice(&reserve[..n]);
-            reserve.drain(..n);
+            crate::vec_drain_prefix(reserve, n);
             return Ok(n);
         }
 
diff --git a/openpgp/src/serialize/partial_body.rs b/openpgp/src/serialize/partial_body.rs
index 26bb0e14..b85b8b9c 100644
--- a/openpgp/src/serialize/partial_body.rs
+++ b/openpgp/src/serialize/partial_body.rs
@@ -138,7 +138,7 @@ impl<'a, C: 'a> PartialBodyFilter<'a, C> {
                 // ... from our buffer first...
                 let l = cmp::min(self.buffer.len(), chunk_size);
                 inner.write_all(&self.buffer[..l])?;
-                self.buffer.drain(..l);
+                crate::vec_drain_prefix(&mut self.buffer, l);
 
                 // ... then from other.
                 if chunk_size > l {