rust: Update the memmap2 crate to version 0.4.0

This version:

* Optionally implements the `StableDeref` trait, removing the need
  for a wrapper type doing that
* Adds support for mapping empty files, which can routinely happen
  for dirstate-v2 data files. This was a cause of some failing tests
  when running with `run-tests.py --rhg --extra-config-opt rhg.status=1`

Differential Revision: https://phab.mercurial-scm.org/D11446

rust/Cargo.lock

@@ -1,5 +1,6 b''
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+
 [[package]]
 name = "adler"
 version = "0.2.3"
@@ -511,11 +512,12 b' checksum = "0ee1c47aaa256ecabcaea351eae4'
 
 [[package]]
 name = "memmap2"
-version = "0.3.1"
+version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "00b6c2ebff6180198788f5db08d7ce3bc1d0b617176678831a7510825973e357"
+checksum = "de5d3112c080d58ce560081baeaab7e1e864ca21795ddbf533d5b1842bb1ecf8"
 dependencies = [
  "libc",
+ "stable_deref_trait",
 ]
 
 [[package]]

rust/hg-core/Cargo.toml

@@ -29,7 +29,7 b' tempfile = "3.1.0"'
 crossbeam-channel = "0.4"
 micro-timer = "0.3.0"
 log = "0.4.8"
-memmap2 = "0.3.1"
+memmap2 = {version = "0.4", features = ["stable_deref_trait"]}
 zstd = "0.5.3"
 format-bytes = "0.2.2"
 

rust/hg-core/src/repo.rs

@@ -290,7 +290,7 b' impl Repo {'
                 .mmap_open(docket.data_filename())
                 .io_not_found_as_none()?
             {
-                OwningDirstateMap::new_empty(MmapWrapper(data_mmap))
+                OwningDirstateMap::new_empty(data_mmap)
             } else {
                 OwningDirstateMap::new_empty(Vec::new())
             };
@@ -407,16 +407,3 b' impl<T, E> LazyCell<T, E> {'
         Ok(RefMut::map(borrowed, |option| option.as_mut().unwrap()))
     }
 }
-
-// TODO: remove this when https://github.com/RazrFalcon/memmap2-rs/pull/22 is on crates.io
-struct MmapWrapper(memmap2::Mmap);
-
-impl std::ops::Deref for MmapWrapper {
-    type Target = [u8];
-
-    fn deref(&self) -> &[u8] {
-        self.0.deref()
-    }
-}
-
-unsafe impl stable_deref_trait::StableDeref for MmapWrapper {}