openpgp,buffered-reader: Optimize Vec<u8>::truncate manually

  - On debug builds, Vec<u8>::truncate is very, very slow.  For
    instance, running the decrypt_test_stream test takes 51 seconds on
    my (Neal's) computer using Vec<u8>::truncate and <0.1 seconds using
    `unsafe { v.set_len(len); }`.

    The issue is that the compiler calls drop on every element that is
    dropped, even though a u8 doesn't have a drop implementation.  The
    compiler optimizes this away at high optimization levels, but
    those levels make debugging harder.

1 files changed, 3 insertions, 2 deletions

diff --git a/openpgp/src/parse/partial_body.rs b/openpgp/src/parse/partial_body.rs
index bc4e6309..222e2894 100644
--- a/openpgp/src/parse/partial_body.rs
+++ b/openpgp/src/parse/partial_body.rs
@@ -4,6 +4,8 @@ use std::io;
 use std::io::{Error, ErrorKind};
 
 use buffered_reader::{buffered_reader_generic_read_impl, BufferedReader};
+
+use crate::vec_truncate;
 use crate::packet::header::BodyLength;
 use crate::parse::{Cookie, Hashing};
 
@@ -203,8 +205,7 @@ impl<T: BufferedReader<Cookie>> BufferedReaderPartialBodyFilter<T> {
                 }
             }
         }
-
-        buffer.truncate(amount_buffered);
+        vec_truncate(&mut buffer, amount_buffered);
         buffer.shrink_to_fit();
 
         // We're done.