diff --git a/src/cargo/ops/vendor.rs b/src/cargo/ops/vendor.rs
index 5e1c0568ee0..05bbb22523e 100644
--- a/src/cargo/ops/vendor.rs
+++ b/src/cargo/ops/vendor.rs
@@ -9,6 +9,8 @@ use serde::Serialize;
 use std::collections::HashSet;
 use std::collections::{BTreeMap, BTreeSet, HashMap};
 use std::fs;
+use std::fs::File;
+use std::io::{Read, Write};
 use std::path::{Path, PathBuf};
 
 pub struct VendorOptions<'a> {
@@ -182,6 +184,7 @@ fn sync(
     }
 
     let mut sources = BTreeSet::new();
+    let mut tmp_buf = [0; 64 * 1024];
     for (id, pkg) in ids.iter() {
         // Next up, copy it to the vendor directory
         let src = pkg
@@ -216,7 +219,7 @@ fn sync(
         let pathsource = PathSource::new(src, id.source_id(), config);
         let paths = pathsource.list_files(pkg)?;
         let mut map = BTreeMap::new();
-        cp_sources(src, &paths, &dst, &mut map)
+        cp_sources(src, &paths, &dst, &mut map, &mut tmp_buf)
             .chain_err(|| format!("failed to copy over vendored sources for: {}", id))?;
 
         // Finally, emit the metadata about this package
@@ -299,6 +302,7 @@ fn cp_sources(
     paths: &[PathBuf],
     dst: &Path,
     cksums: &mut BTreeMap<String, String>,
+    tmp_buf: &mut [u8],
 ) -> CargoResult<()> {
     for p in paths {
         let relative = p.strip_prefix(&src).unwrap();
@@ -334,9 +338,27 @@ fn cp_sources(
 
         paths::create_dir_all(dst.parent().unwrap())?;
 
-        paths::copy(&p, &dst)?;
-        let cksum = Sha256::new().update_path(dst)?.finish_hex();
+        let cksum = copy_and_checksum(&p, &dst, tmp_buf)?;
         cksums.insert(relative.to_str().unwrap().replace("\\", "/"), cksum);
     }
     Ok(())
 }
+
+fn copy_and_checksum(src_path: &Path, dst_path: &Path, buf: &mut [u8]) -> CargoResult<String> {
+    let mut src = File::open(src_path).chain_err(|| format!("failed to open {:?}", src_path))?;
+    let mut dst =
+        File::create(dst_path).chain_err(|| format!("failed to create {:?}", dst_path))?;
+    let mut cksum = Sha256::new();
+    loop {
+        let n = src
+            .read(buf)
+            .chain_err(|| format!("failed to read from {:?}", src_path))?;
+        if n == 0 {
+            break Ok(cksum.finish_hex());
+        }
+        let data = &buf[..n];
+        cksum.update(data);
+        dst.write_all(data)
+            .chain_err(|| format!("failed to write to {:?}", dst_path))?;
+    }
+}