try out the unstable copy_within interface

We won't be able to land this in master for probably several major Rust
versions, but it's nice to see it working. Tracking issue:
rust-lang/rust#54236

Cargo.toml

@@ -18,7 +18,6 @@ arrayvec = { version = "0.4.7", default-features = false, features = ["use_union
 byteorder = { version = "1.2.4", default-features = false }
 blake2b_simd = { version = "0.2.3", default-features = false }
 constant_time_eq = "0.1.3"
-copy_in_place = "0.2.0"
 rayon = { version = "1.0.2", optional = true }
 
 [dev-dependencies]

src/decode.rs

@@ -1,6 +1,5 @@
 use arrayvec::ArrayVec;
 use constant_time_eq::constant_time_eq;
-use copy_in_place::copy_in_place;
 #[cfg(feature = "std")]
 use rayon;
 
@@ -147,7 +146,7 @@ fn extract_in_place(
     if content_len <= CHUNK_SIZE {
         // This function might eventually make its way into libcore:
         // https://github.com/rust-lang/rust/pull/53652
-        copy_in_place(buf, read_offset..read_offset + content_len, write_offset);
+        buf.copy_within(read_offset..read_offset + content_len, write_offset);
     } else {
         read_offset += PARENT_SIZE;
         let left_len = hash::left_len(content_len as u64) as usize;

src/encode.rs

@@ -1,6 +1,5 @@
 use arrayvec::ArrayVec;
 use blake2b_simd;
-use copy_in_place::copy_in_place;
 use core::cmp;
 use core::fmt;
 use hash::Finalization::{self, NotRoot, Root};
@@ -198,7 +197,7 @@ fn layout_chunks_in_place(
     content_len: usize,
 ) {
     if content_len <= CHUNK_SIZE {
-        copy_in_place(buf, read_offset..read_offset + content_len, write_offset);
+        buf.copy_within(read_offset..read_offset + content_len, write_offset);
     } else {
         let left_len = hash::left_len(content_len as u64) as usize;
         let left_write_offset = write_offset + PARENT_SIZE;

src/lib.rs

@@ -1,4 +1,5 @@
 #![cfg_attr(not(feature = "std"), no_std)]
+#![feature(copy_within)]
 
 #[cfg(feature = "std")]
 extern crate core;
@@ -9,7 +10,6 @@ extern crate arrayvec;
 extern crate blake2b_simd;
 extern crate byteorder;
 extern crate constant_time_eq;
-extern crate copy_in_place;
 #[cfg(feature = "std")]
 extern crate rayon;