Adding upstream version 110.0.1.upstream/110.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

5 files changed, 2887 insertions, 0 deletions

diff --git a/third_party/rust/memmap2/src/advice.rs b/third_party/rust/memmap2/src/advice.rs
new file mode 100644
index 0000000000..185743e077
--- /dev/null
+++ b/third_party/rust/memmap2/src/advice.rs
@@ -0,0 +1,273 @@
+// The use statement is needed for the `cargo docs`
+#[allow(unused_imports)]
+use crate::{Mmap, MmapMut};
+
+/// Values supported by [Mmap::advise] and [MmapMut::advise] functions.
+/// See [madvise()](https://man7.org/linux/man-pages/man2/madvise.2.html) map page.
+#[repr(i32)]
+#[derive(Clone, Copy, Debug, Eq, PartialEq, Hash)]
+pub enum Advice {
+    /// **MADV_NORMAL**
+    ///
+    /// No special treatment.  This is the default.
+    Normal = libc::MADV_NORMAL,
+
+    /// **MADV_RANDOM**
+    ///
+    /// Expect page references in random order.  (Hence, read
+    /// ahead may be less useful than normally.)
+    Random = libc::MADV_RANDOM,
+
+    /// **MADV_SEQUENTIAL**
+    ///
+    /// Expect page references in sequential order.  (Hence, pages
+    /// in the given range can be aggressively read ahead, and may
+    /// be freed soon after they are accessed.)
+    Sequential = libc::MADV_SEQUENTIAL,
+
+    /// **MADV_WILLNEED**
+    ///
+    /// Expect access in the near future.  (Hence, it might be a
+    /// good idea to read some pages ahead.)
+    WillNeed = libc::MADV_WILLNEED,
+
+    /// **MADV_DONTNEED**
+    ///
+    /// Do not expect access in the near future.  (For the time
+    /// being, the application is finished with the given range,
+    /// so the kernel can free resources associated with it.)
+    ///
+    /// After a successful MADV_DONTNEED operation, the semantics
+    /// of memory access in the specified region are changed:
+    /// subsequent accesses of pages in the range will succeed,
+    /// but will result in either repopulating the memory contents
+    /// from the up-to-date contents of the underlying mapped file
+    /// (for shared file mappings, shared anonymous mappings, and
+    /// shmem-based techniques such as System V shared memory
+    /// segments) or zero-fill-on-demand pages for anonymous
+    /// private mappings.
+    ///
+    /// Note that, when applied to shared mappings, MADV_DONTNEED
+    /// might not lead to immediate freeing of the pages in the
+    /// range.  The kernel is free to delay freeing the pages
+    /// until an appropriate moment.  The resident set size (RSS)
+    /// of the calling process will be immediately reduced
+    /// however.
+    ///
+    /// **MADV_DONTNEED** cannot be applied to locked pages, Huge TLB
+    /// pages, or VM_PFNMAP pages.  (Pages marked with the kernel-
+    /// internal VM_PFNMAP flag are special memory areas that are
+    /// not managed by the virtual memory subsystem.  Such pages
+    /// are typically created by device drivers that map the pages
+    /// into user space.)
+    DontNeed = libc::MADV_DONTNEED,
+
+    //
+    // The rest are Linux-specific
+    //
+    /// **MADV_FREE** - Linux (since Linux 4.5) and Darwin
+    ///
+    /// The application no longer requires the pages in the range
+    /// specified by addr and len.  The kernel can thus free these
+    /// pages, but the freeing could be delayed until memory
+    /// pressure occurs.  For each of the pages that has been
+    /// marked to be freed but has not yet been freed, the free
+    /// operation will be canceled if the caller writes into the
+    /// page.  After a successful MADV_FREE operation, any stale
+    /// data (i.e., dirty, unwritten pages) will be lost when the
+    /// kernel frees the pages.  However, subsequent writes to
+    /// pages in the range will succeed and then kernel cannot
+    /// free those dirtied pages, so that the caller can always
+    /// see just written data.  If there is no subsequent write,
+    /// the kernel can free the pages at any time.  Once pages in
+    /// the range have been freed, the caller will see zero-fill-
+    /// on-demand pages upon subsequent page references.
+    ///
+    /// The MADV_FREE operation can be applied only to private
+    /// anonymous pages (see mmap(2)).  In Linux before version
+    /// 4.12, when freeing pages on a swapless system, the pages
+    /// in the given range are freed instantly, regardless of
+    /// memory pressure.
+    #[cfg(any(target_os = "linux", target_os = "macos", target_os = "ios"))]
+    Free = libc::MADV_FREE,
+
+    /// **MADV_REMOVE** - Linux only (since Linux 2.6.16)
+    ///
+    /// Free up a given range of pages and its associated backing
+    /// store.  This is equivalent to punching a hole in the
+    /// corresponding byte range of the backing store (see
+    /// fallocate(2)).  Subsequent accesses in the specified
+    /// address range will see bytes containing zero.
+    ///
+    /// The specified address range must be mapped shared and
+    /// writable.  This flag cannot be applied to locked pages,
+    /// Huge TLB pages, or VM_PFNMAP pages.
+    ///
+    /// In the initial implementation, only tmpfs(5) was supported
+    /// **MADV_REMOVE**; but since Linux 3.5, any filesystem which
+    /// supports the fallocate(2) FALLOC_FL_PUNCH_HOLE mode also
+    /// supports MADV_REMOVE.  Hugetlbfs fails with the error
+    /// EINVAL and other filesystems fail with the error
+    /// EOPNOTSUPP.
+    #[cfg(target_os = "linux")]
+    Remove = libc::MADV_REMOVE,
+
+    /// **MADV_DONTFORK** - Linux only (since Linux 2.6.16)
+    ///
+    /// Do not make the pages in this range available to the child
+    /// after a fork(2).  This is useful to prevent copy-on-write
+    /// semantics from changing the physical location of a page if
+    /// the parent writes to it after a fork(2).  (Such page
+    /// relocations cause problems for hardware that DMAs into the
+    /// page.)
+    #[cfg(target_os = "linux")]
+    DontFork = libc::MADV_DONTFORK,
+
+    /// **MADV_DOFORK** - Linux only (since Linux 2.6.16)
+    ///
+    /// Undo the effect of MADV_DONTFORK, restoring the default
+    /// behavior, whereby a mapping is inherited across fork(2).
+    #[cfg(target_os = "linux")]
+    DoFork = libc::MADV_DOFORK,
+
+    /// **MADV_MERGEABLE** - Linux only (since Linux 2.6.32)
+    ///
+    /// Enable Kernel Samepage Merging (KSM) for the pages in the
+    /// range specified by addr and length.  The kernel regularly
+    /// scans those areas of user memory that have been marked as
+    /// mergeable, looking for pages with identical content.
+    /// These are replaced by a single write-protected page (which
+    /// is automatically copied if a process later wants to update
+    /// the content of the page).  KSM merges only private
+    /// anonymous pages (see mmap(2)).
+    ///
+    /// The KSM feature is intended for applications that generate
+    /// many instances of the same data (e.g., virtualization
+    /// systems such as KVM).  It can consume a lot of processing
+    /// power; use with care.  See the Linux kernel source file
+    /// Documentation/admin-guide/mm/ksm.rst for more details.
+    ///
+    /// The MADV_MERGEABLE and MADV_UNMERGEABLE operations are
+    /// available only if the kernel was configured with
+    /// CONFIG_KSM.
+    #[cfg(target_os = "linux")]
+    Mergeable = libc::MADV_MERGEABLE,
+
+    /// **MADV_UNMERGEABLE** - Linux only (since Linux 2.6.32)
+    ///
+    /// Undo the effect of an earlier MADV_MERGEABLE operation on
+    /// the specified address range; KSM unmerges whatever pages
+    /// it had merged in the address range specified by addr and
+    /// length.
+    #[cfg(target_os = "linux")]
+    Unmergeable = libc::MADV_UNMERGEABLE,
+
+    /// **MADV_HUGEPAGE** - Linux only (since Linux 2.6.38)
+    ///
+    /// Enable Transparent Huge Pages (THP) for pages in the range
+    /// specified by addr and length.  Currently, Transparent Huge
+    /// Pages work only with private anonymous pages (see
+    /// mmap(2)).  The kernel will regularly scan the areas marked
+    /// as huge page candidates to replace them with huge pages.
+    /// The kernel will also allocate huge pages directly when the
+    /// region is naturally aligned to the huge page size (see
+    /// posix_memalign(2)).
+    ///
+    /// This feature is primarily aimed at applications that use
+    /// large mappings of data and access large regions of that
+    /// memory at a time (e.g., virtualization systems such as
+    /// QEMU).  It can very easily waste memory (e.g., a 2 MB
+    /// mapping that only ever accesses 1 byte will result in 2 MB
+    /// of wired memory instead of one 4 KB page).  See the Linux
+    /// kernel source file
+    /// Documentation/admin-guide/mm/transhuge.rst for more
+    /// details.
+    ///
+    /// Most common kernels configurations provide MADV_HUGEPAGE-
+    /// style behavior by default, and thus MADV_HUGEPAGE is
+    /// normally not necessary.  It is mostly intended for
+    /// embedded systems, where MADV_HUGEPAGE-style behavior may
+    /// not be enabled by default in the kernel.  On such systems,
+    /// this flag can be used in order to selectively enable THP.
+    /// Whenever MADV_HUGEPAGE is used, it should always be in
+    /// regions of memory with an access pattern that the
+    /// developer knows in advance won't risk to increase the
+    /// memory footprint of the application when transparent
+    /// hugepages are enabled.
+    ///
+    /// The MADV_HUGEPAGE and MADV_NOHUGEPAGE operations are
+    /// available only if the kernel was configured with
+    /// CONFIG_TRANSPARENT_HUGEPAGE.
+    #[cfg(target_os = "linux")]
+    HugePage = libc::MADV_HUGEPAGE,
+
+    /// **MADV_NOHUGEPAGE** - Linux only (since Linux 2.6.38)
+    ///
+    /// Ensures that memory in the address range specified by addr
+    /// and length will not be backed by transparent hugepages.
+    #[cfg(target_os = "linux")]
+    NoHugePage = libc::MADV_NOHUGEPAGE,
+
+    /// **MADV_DONTDUMP** - Linux only (since Linux 3.4)
+    ///
+    /// Exclude from a core dump those pages in the range
+    /// specified by addr and length.  This is useful in
+    /// applications that have large areas of memory that are
+    /// known not to be useful in a core dump.  The effect of
+    /// **MADV_DONTDUMP** takes precedence over the bit mask that is
+    /// set via the `/proc/[pid]/coredump_filter` file (see
+    /// core(5)).
+    #[cfg(target_os = "linux")]
+    DontDump = libc::MADV_DONTDUMP,
+
+    /// **MADV_DODUMP** - Linux only (since Linux 3.4)
+    ///
+    /// Undo the effect of an earlier MADV_DONTDUMP.
+    #[cfg(target_os = "linux")]
+    DoDump = libc::MADV_DODUMP,
+
+    /// **MADV_HWPOISON** - Linux only (since Linux 2.6.32)
+    ///
+    /// Poison the pages in the range specified by addr and length
+    /// and handle subsequent references to those pages like a
+    /// hardware memory corruption.  This operation is available
+    /// only for privileged (CAP_SYS_ADMIN) processes.  This
+    /// operation may result in the calling process receiving a
+    /// SIGBUS and the page being unmapped.
+    ///
+    /// This feature is intended for testing of memory error-
+    /// handling code; it is available only if the kernel was
+    /// configured with CONFIG_MEMORY_FAILURE.
+    #[cfg(target_os = "linux")]
+    HwPoison = libc::MADV_HWPOISON,
+
+    /// **MADV_ZERO_WIRED_PAGES** - Darwin only
+    ///
+    /// Indicates that the application would like the wired pages in this address range to be
+    /// zeroed out if the address range is deallocated without first unwiring the pages (i.e.
+    /// a munmap(2) without a preceding munlock(2) or the application quits).  This is used
+    /// with madvise() system call.
+    #[cfg(any(target_os = "macos", target_os = "ios"))]
+    ZeroWiredPages = libc::MADV_ZERO_WIRED_PAGES,
+
+    /// **MADV_FREE_REUSABLE** - Darwin only
+    ///
+    /// Behaves like **MADV_FREE**, but the freed pages are accounted for in the RSS of the process.
+    #[cfg(any(target_os = "macos", target_os = "ios"))]
+    FreeReusable = libc::MADV_FREE_REUSABLE,
+
+    /// **MADV_FREE_REUSE** - Darwin only
+    ///
+    /// Marks a memory region previously freed by **MADV_FREE_REUSABLE** as non-reusable, accounts
+    /// for the pages in the RSS of the process. Pages that have been freed will be replaced by
+    /// zero-filled pages on demand, other pages will be left as is.
+    #[cfg(any(target_os = "macos", target_os = "ios"))]
+    FreeReuse = libc::MADV_FREE_REUSE,
+}
+
+// Future expansion:
+// MADV_SOFT_OFFLINE  (since Linux 2.6.33)
+// MADV_WIPEONFORK  (since Linux 4.14)
+// MADV_KEEPONFORK  (since Linux 4.14)
+// MADV_COLD  (since Linux 5.4)
+// MADV_PAGEOUT  (since Linux 5.4)
diff --git a/third_party/rust/memmap2/src/lib.rs b/third_party/rust/memmap2/src/lib.rs
new file mode 100644
index 0000000000..58df7589bb
--- /dev/null
+++ b/third_party/rust/memmap2/src/lib.rs
@@ -0,0 +1,1703 @@
+//! A cross-platform Rust API for memory mapped buffers.
+//!
+//! The core functionality is provided by either [`Mmap`] or [`MmapMut`],
+//! which correspond to mapping a [`File`] to a [`&[u8]`](https://doc.rust-lang.org/std/primitive.slice.html)
+//! or [`&mut [u8]`](https://doc.rust-lang.org/std/primitive.slice.html)
+//! respectively. Both function by dereferencing to a slice, allowing the
+//! [`Mmap`]/[`MmapMut`] to be used in the same way you would the equivelant slice
+//! types.
+//!
+//! [`File`]: std::fs::File
+//!
+//! # Examples
+//!
+//! For simple cases [`Mmap`] can be used directly:
+//!
+//! ```
+//! use std::fs::File;
+//! use std::io::Read;
+//!
+//! use memmap2::Mmap;
+//!
+//! # fn main() -> std::io::Result<()> {
+//! let mut file = File::open("LICENSE-APACHE")?;
+//!
+//! let mut contents = Vec::new();
+//! file.read_to_end(&mut contents)?;
+//!
+//! let mmap = unsafe { Mmap::map(&file)?  };
+//!
+//! assert_eq!(&contents[..], &mmap[..]);
+//! # Ok(())
+//! # }
+//! ```
+//!
+//! However for cases which require configuration of the mapping, then
+//! you can use [`MmapOptions`] in order to further configure a mapping
+//! before you create it.
+
+#[cfg_attr(unix, path = "unix.rs")]
+#[cfg_attr(windows, path = "windows.rs")]
+#[cfg_attr(not(any(unix, windows)), path = "stub.rs")]
+mod os;
+use crate::os::{file_len, MmapInner};
+
+#[cfg(unix)]
+mod advice;
+#[cfg(unix)]
+pub use crate::advice::Advice;
+
+use std::fmt;
+#[cfg(not(any(unix, windows)))]
+use std::fs::File;
+use std::io::{Error, ErrorKind, Result};
+use std::isize;
+use std::mem;
+use std::ops::{Deref, DerefMut};
+#[cfg(unix)]
+use std::os::unix::io::{AsRawFd, RawFd};
+#[cfg(windows)]
+use std::os::windows::io::{AsRawHandle, RawHandle};
+use std::slice;
+
+#[cfg(not(any(unix, windows)))]
+pub struct MmapRawDescriptor<'a>(&'a File);
+
+#[cfg(unix)]
+pub struct MmapRawDescriptor(RawFd);
+
+#[cfg(windows)]
+pub struct MmapRawDescriptor(RawHandle);
+
+pub trait MmapAsRawDesc {
+    fn as_raw_desc(&self) -> MmapRawDescriptor;
+}
+
+#[cfg(not(any(unix, windows)))]
+impl MmapAsRawDesc for &File {
+    fn as_raw_desc(&self) -> MmapRawDescriptor {
+        MmapRawDescriptor(self)
+    }
+}
+
+#[cfg(unix)]
+impl MmapAsRawDesc for RawFd {
+    fn as_raw_desc(&self) -> MmapRawDescriptor {
+        MmapRawDescriptor(*self)
+    }
+}
+
+#[cfg(unix)]
+impl<'a, T> MmapAsRawDesc for &'a T
+where
+    T: AsRawFd,
+{
+    fn as_raw_desc(&self) -> MmapRawDescriptor {
+        MmapRawDescriptor(self.as_raw_fd())
+    }
+}
+
+#[cfg(windows)]
+impl MmapAsRawDesc for RawHandle {
+    fn as_raw_desc(&self) -> MmapRawDescriptor {
+        MmapRawDescriptor(*self)
+    }
+}
+
+#[cfg(windows)]
+impl<'a, T> MmapAsRawDesc for &'a T
+where
+    T: AsRawHandle,
+{
+    fn as_raw_desc(&self) -> MmapRawDescriptor {
+        MmapRawDescriptor(self.as_raw_handle())
+    }
+}
+
+/// A memory map builder, providing advanced options and flags for specifying memory map behavior.
+///
+/// `MmapOptions` can be used to create an anonymous memory map using [`map_anon()`], or a
+/// file-backed memory map using one of [`map()`], [`map_mut()`], [`map_exec()`],
+/// [`map_copy()`], or [`map_copy_read_only()`].
+///
+/// ## Safety
+///
+/// All file-backed memory map constructors are marked `unsafe` because of the potential for
+/// *Undefined Behavior* (UB) using the map if the underlying file is subsequently modified, in or
+/// out of process. Applications must consider the risk and take appropriate precautions when
+/// using file-backed maps. Solutions such as file permissions, locks or process-private (e.g.
+/// unlinked) files exist but are platform specific and limited.
+///
+/// [`map_anon()`]: MmapOptions::map_anon()
+/// [`map()`]: MmapOptions::map()
+/// [`map_mut()`]: MmapOptions::map_mut()
+/// [`map_exec()`]: MmapOptions::map_exec()
+/// [`map_copy()`]: MmapOptions::map_copy()
+/// [`map_copy_read_only()`]: MmapOptions::map_copy_read_only()
+#[derive(Clone, Debug, Default)]
+pub struct MmapOptions {
+    offset: u64,
+    len: Option<usize>,
+    stack: bool,
+    populate: bool,
+}
+
+impl MmapOptions {
+    /// Creates a new set of options for configuring and creating a memory map.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use memmap2::{MmapMut, MmapOptions};
+    /// # use std::io::Result;
+    ///
+    /// # fn main() -> Result<()> {
+    /// // Create a new memory map builder.
+    /// let mut mmap_options = MmapOptions::new();
+    ///
+    /// // Configure the memory map builder using option setters, then create
+    /// // a memory map using one of `mmap_options.map_anon`, `mmap_options.map`,
+    /// // `mmap_options.map_mut`, `mmap_options.map_exec`, or `mmap_options.map_copy`:
+    /// let mut mmap: MmapMut = mmap_options.len(36).map_anon()?;
+    ///
+    /// // Use the memory map:
+    /// mmap.copy_from_slice(b"...data to copy to the memory map...");
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn new() -> MmapOptions {
+        MmapOptions::default()
+    }
+
+    /// Configures the memory map to start at byte `offset` from the beginning of the file.
+    ///
+    /// This option has no effect on anonymous memory maps.
+    ///
+    /// By default, the offset is 0.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use memmap2::MmapOptions;
+    /// use std::fs::File;
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// let mmap = unsafe {
+    ///     MmapOptions::new()
+    ///                 .offset(30)
+    ///                 .map(&File::open("LICENSE-APACHE")?)?
+    /// };
+    /// assert_eq!(&b"Apache License"[..],
+    ///            &mmap[..14]);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn offset(&mut self, offset: u64) -> &mut Self {
+        self.offset = offset;
+        self
+    }
+
+    /// Configures the created memory mapped buffer to be `len` bytes long.
+    ///
+    /// This option is mandatory for anonymous memory maps.
+    ///
+    /// For file-backed memory maps, the length will default to the file length.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use memmap2::MmapOptions;
+    /// use std::fs::File;
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// let mmap = unsafe {
+    ///     MmapOptions::new()
+    ///                 .len(9)
+    ///                 .map(&File::open("README.md")?)?
+    /// };
+    /// assert_eq!(&b"# memmap2"[..], &mmap[..]);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn len(&mut self, len: usize) -> &mut Self {
+        self.len = Some(len);
+        self
+    }
+
+    /// Returns the configured length, or the length of the provided file.
+    fn get_len<T: MmapAsRawDesc>(&self, file: &T) -> Result<usize> {
+        self.len.map(Ok).unwrap_or_else(|| {
+            let desc = file.as_raw_desc();
+            let file_len = file_len(desc.0)?;
+
+            if file_len < self.offset {
+                return Err(Error::new(
+                    ErrorKind::InvalidData,
+                    "memory map offset is larger than length",
+                ));
+            }
+            let len = file_len - self.offset;
+
+            // Rust's slice cannot be larger than isize::MAX.
+            // See https://doc.rust-lang.org/std/slice/fn.from_raw_parts.html
+            //
+            // This is not a problem on 64-bit targets, but on 32-bit one
+            // having a file or an anonymous mapping larger than 2GB is quite normal
+            // and we have to prevent it.
+            //
+            // The code below is essentially the same as in Rust's std:
+            // https://github.com/rust-lang/rust/blob/db78ab70a88a0a5e89031d7ee4eccec835dcdbde/library/alloc/src/raw_vec.rs#L495
+            if mem::size_of::<usize>() < 8 && len > isize::MAX as u64 {
+                return Err(Error::new(
+                    ErrorKind::InvalidData,
+                    "memory map length overflows isize",
+                ));
+            }
+
+            Ok(len as usize)
+        })
+    }
+
+    /// Configures the anonymous memory map to be suitable for a process or thread stack.
+    ///
+    /// This option corresponds to the `MAP_STACK` flag on Linux. It has no effect on Windows.
+    ///
+    /// This option has no effect on file-backed memory maps.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use memmap2::MmapOptions;
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// let stack = MmapOptions::new().stack().len(4096).map_anon();
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn stack(&mut self) -> &mut Self {
+        self.stack = true;
+        self
+    }
+
+    /// Populate (prefault) page tables for a mapping.
+    ///
+    /// For a file mapping, this causes read-ahead on the file. This will help to reduce blocking on page faults later.
+    ///
+    /// This option corresponds to the `MAP_POPULATE` flag on Linux. It has no effect on Windows.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use memmap2::MmapOptions;
+    /// use std::fs::File;
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// let file = File::open("LICENSE-MIT")?;
+    ///
+    /// let mmap = unsafe {
+    ///     MmapOptions::new().populate().map(&file)?
+    /// };
+    ///
+    /// assert_eq!(&b"Copyright"[..], &mmap[..9]);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn populate(&mut self) -> &mut Self {
+        self.populate = true;
+        self
+    }
+
+    /// Creates a read-only memory map backed by a file.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails, which can happen for a
+    /// variety of reasons, such as when the file is not open with read permissions.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use memmap2::MmapOptions;
+    /// use std::fs::File;
+    /// use std::io::Read;
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// let mut file = File::open("LICENSE-APACHE")?;
+    ///
+    /// let mut contents = Vec::new();
+    /// file.read_to_end(&mut contents)?;
+    ///
+    /// let mmap = unsafe {
+    ///     MmapOptions::new().map(&file)?
+    /// };
+    ///
+    /// assert_eq!(&contents[..], &mmap[..]);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub unsafe fn map<T: MmapAsRawDesc>(&self, file: T) -> Result<Mmap> {
+        let desc = file.as_raw_desc();
+
+        MmapInner::map(self.get_len(&file)?, desc.0, self.offset, self.populate)
+            .map(|inner| Mmap { inner })
+    }
+
+    /// Creates a readable and executable memory map backed by a file.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails, which can happen for a
+    /// variety of reasons, such as when the file is not open with read permissions.
+    pub unsafe fn map_exec<T: MmapAsRawDesc>(&self, file: T) -> Result<Mmap> {
+        let desc = file.as_raw_desc();
+
+        MmapInner::map_exec(self.get_len(&file)?, desc.0, self.offset, self.populate)
+            .map(|inner| Mmap { inner })
+    }
+
+    /// Creates a writeable memory map backed by a file.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails, which can happen for a
+    /// variety of reasons, such as when the file is not open with read and write permissions.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # extern crate memmap2;
+    /// # extern crate tempfile;
+    /// #
+    /// use std::fs::OpenOptions;
+    /// use std::path::PathBuf;
+    ///
+    /// use memmap2::MmapOptions;
+    /// #
+    /// # fn main() -> std::io::Result<()> {
+    /// # let tempdir = tempfile::tempdir()?;
+    /// let path: PathBuf = /* path to file */
+    /// #   tempdir.path().join("map_mut");
+    /// let file = OpenOptions::new().read(true).write(true).create(true).open(&path)?;
+    /// file.set_len(13)?;
+    ///
+    /// let mut mmap = unsafe {
+    ///     MmapOptions::new().map_mut(&file)?
+    /// };
+    ///
+    /// mmap.copy_from_slice(b"Hello, world!");
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub unsafe fn map_mut<T: MmapAsRawDesc>(&self, file: T) -> Result<MmapMut> {
+        let desc = file.as_raw_desc();
+
+        MmapInner::map_mut(self.get_len(&file)?, desc.0, self.offset, self.populate)
+            .map(|inner| MmapMut { inner })
+    }
+
+    /// Creates a copy-on-write memory map backed by a file.
+    ///
+    /// Data written to the memory map will not be visible by other processes,
+    /// and will not be carried through to the underlying file.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails, which can happen for a
+    /// variety of reasons, such as when the file is not open with writable permissions.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use memmap2::MmapOptions;
+    /// use std::fs::File;
+    /// use std::io::Write;
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// let file = File::open("LICENSE-APACHE")?;
+    /// let mut mmap = unsafe { MmapOptions::new().map_copy(&file)? };
+    /// (&mut mmap[..]).write_all(b"Hello, world!")?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub unsafe fn map_copy<T: MmapAsRawDesc>(&self, file: T) -> Result<MmapMut> {
+        let desc = file.as_raw_desc();
+
+        MmapInner::map_copy(self.get_len(&file)?, desc.0, self.offset, self.populate)
+            .map(|inner| MmapMut { inner })
+    }
+
+    /// Creates a copy-on-write read-only memory map backed by a file.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails, which can happen for a
+    /// variety of reasons, such as when the file is not open with read permissions.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use memmap2::MmapOptions;
+    /// use std::fs::File;
+    /// use std::io::Read;
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// let mut file = File::open("README.md")?;
+    ///
+    /// let mut contents = Vec::new();
+    /// file.read_to_end(&mut contents)?;
+    ///
+    /// let mmap = unsafe {
+    ///     MmapOptions::new().map_copy_read_only(&file)?
+    /// };
+    ///
+    /// assert_eq!(&contents[..], &mmap[..]);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub unsafe fn map_copy_read_only<T: MmapAsRawDesc>(&self, file: T) -> Result<Mmap> {
+        let desc = file.as_raw_desc();
+
+        MmapInner::map_copy_read_only(self.get_len(&file)?, desc.0, self.offset, self.populate)
+            .map(|inner| Mmap { inner })
+    }
+
+    /// Creates an anonymous memory map.
+    ///
+    /// The memory map length should be configured using [`MmapOptions::len()`]
+    /// before creating an anonymous memory map, otherwise a zero-length mapping
+    /// will be crated.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails or
+    /// when `len > isize::MAX`.
+    pub fn map_anon(&self) -> Result<MmapMut> {
+        let len = self.len.unwrap_or(0);
+
+        // See get_len() for details.
+        if mem::size_of::<usize>() < 8 && len > isize::MAX as usize {
+            return Err(Error::new(
+                ErrorKind::InvalidData,
+                "memory map length overflows isize",
+            ));
+        }
+
+        MmapInner::map_anon(len, self.stack).map(|inner| MmapMut { inner })
+    }
+
+    /// Creates a raw memory map.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails, which can happen for a
+    /// variety of reasons, such as when the file is not open with read and write permissions.
+    pub fn map_raw<T: MmapAsRawDesc>(&self, file: T) -> Result<MmapRaw> {
+        let desc = file.as_raw_desc();
+
+        MmapInner::map_mut(self.get_len(&file)?, desc.0, self.offset, self.populate)
+            .map(|inner| MmapRaw { inner })
+    }
+}
+
+/// A handle to an immutable memory mapped buffer.
+///
+/// A `Mmap` may be backed by a file, or it can be anonymous map, backed by volatile memory. Use
+/// [`MmapOptions`] or [`map()`] to create a file-backed memory map. To create an immutable
+/// anonymous memory map, first create a mutable anonymous memory map, and then make it immutable
+/// with [`MmapMut::make_read_only()`].
+///
+/// A file backed `Mmap` is created by `&File` reference, and will remain valid even after the
+/// `File` is dropped. In other words, the `Mmap` handle is completely independent of the `File`
+/// used to create it. For consistency, on some platforms this is achieved by duplicating the
+/// underlying file handle. The memory will be unmapped when the `Mmap` handle is dropped.
+///
+/// Dereferencing and accessing the bytes of the buffer may result in page faults (e.g. swapping
+/// the mapped pages into physical memory) though the details of this are platform specific.
+///
+/// `Mmap` is [`Sync`](std::marker::Sync) and [`Send`](std::marker::Send).
+///
+/// ## Safety
+///
+/// All file-backed memory map constructors are marked `unsafe` because of the potential for
+/// *Undefined Behavior* (UB) using the map if the underlying file is subsequently modified, in or
+/// out of process. Applications must consider the risk and take appropriate precautions when using
+/// file-backed maps. Solutions such as file permissions, locks or process-private (e.g. unlinked)
+/// files exist but are platform specific and limited.
+///
+/// ## Example
+///
+/// ```
+/// use memmap2::MmapOptions;
+/// use std::io::Write;
+/// use std::fs::File;
+///
+/// # fn main() -> std::io::Result<()> {
+/// let file = File::open("README.md")?;
+/// let mmap = unsafe { MmapOptions::new().map(&file)? };
+/// assert_eq!(b"# memmap2", &mmap[0..9]);
+/// # Ok(())
+/// # }
+/// ```
+///
+/// See [`MmapMut`] for the mutable version.
+///
+/// [`map()`]: Mmap::map()
+pub struct Mmap {
+    inner: MmapInner,
+}
+
+impl Mmap {
+    /// Creates a read-only memory map backed by a file.
+    ///
+    /// This is equivalent to calling `MmapOptions::new().map(file)`.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails, which can happen for a
+    /// variety of reasons, such as when the file is not open with read permissions.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use std::fs::File;
+    /// use std::io::Read;
+    ///
+    /// use memmap2::Mmap;
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// let mut file = File::open("LICENSE-APACHE")?;
+    ///
+    /// let mut contents = Vec::new();
+    /// file.read_to_end(&mut contents)?;
+    ///
+    /// let mmap = unsafe { Mmap::map(&file)?  };
+    ///
+    /// assert_eq!(&contents[..], &mmap[..]);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub unsafe fn map<T: MmapAsRawDesc>(file: T) -> Result<Mmap> {
+        MmapOptions::new().map(file)
+    }
+
+    /// Transition the memory map to be writable.
+    ///
+    /// If the memory map is file-backed, the file must have been opened with write permissions.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails, which can happen for a
+    /// variety of reasons, such as when the file is not open with writable permissions.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # extern crate memmap2;
+    /// # extern crate tempfile;
+    /// #
+    /// use memmap2::Mmap;
+    /// use std::ops::DerefMut;
+    /// use std::io::Write;
+    /// # use std::fs::OpenOptions;
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// # let tempdir = tempfile::tempdir()?;
+    /// let file = /* file opened with write permissions */
+    /// #          OpenOptions::new()
+    /// #                      .read(true)
+    /// #                      .write(true)
+    /// #                      .create(true)
+    /// #                      .open(tempdir.path()
+    /// #                      .join("make_mut"))?;
+    /// # file.set_len(128)?;
+    /// let mmap = unsafe { Mmap::map(&file)? };
+    /// // ... use the read-only memory map ...
+    /// let mut mut_mmap = mmap.make_mut()?;
+    /// mut_mmap.deref_mut().write_all(b"hello, world!")?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn make_mut(mut self) -> Result<MmapMut> {
+        self.inner.make_mut()?;
+        Ok(MmapMut { inner: self.inner })
+    }
+
+    /// Advise OS how this memory map will be accessed. Only supported on Unix.
+    ///
+    /// See [madvise()](https://man7.org/linux/man-pages/man2/madvise.2.html) map page.
+    #[cfg(unix)]
+    pub fn advise(&self, advice: Advice) -> Result<()> {
+        self.inner.advise(advice)
+    }
+
+    /// Lock the whole memory map into RAM. Only supported on Unix.
+    ///
+    /// See [mlock()](https://man7.org/linux/man-pages/man2/mlock.2.html) map page.
+    #[cfg(unix)]
+    pub fn lock(&mut self) -> Result<()> {
+        self.inner.lock()
+    }
+
+    /// Unlock the whole memory map. Only supported on Unix.
+    ///
+    /// See [munlock()](https://man7.org/linux/man-pages/man2/munlock.2.html) map page.
+    #[cfg(unix)]
+    pub fn unlock(&mut self) -> Result<()> {
+        self.inner.unlock()
+    }
+}
+
+#[cfg(feature = "stable_deref_trait")]
+unsafe impl stable_deref_trait::StableDeref for Mmap {}
+
+impl Deref for Mmap {
+    type Target = [u8];
+
+    #[inline]
+    fn deref(&self) -> &[u8] {
+        unsafe { slice::from_raw_parts(self.inner.ptr(), self.inner.len()) }
+    }
+}
+
+impl AsRef<[u8]> for Mmap {
+    #[inline]
+    fn as_ref(&self) -> &[u8] {
+        self.deref()
+    }
+}
+
+impl fmt::Debug for Mmap {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("Mmap")
+            .field("ptr", &self.as_ptr())
+            .field("len", &self.len())
+            .finish()
+    }
+}
+
+/// A handle to a raw memory mapped buffer.
+///
+/// This struct never hands out references to its interior, only raw pointers.
+/// This can be helpful when creating shared memory maps between untrusted processes.
+pub struct MmapRaw {
+    inner: MmapInner,
+}
+
+impl MmapRaw {
+    /// Creates a writeable memory map backed by a file.
+    ///
+    /// This is equivalent to calling `MmapOptions::new().map_raw(file)`.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails, which can happen for a
+    /// variety of reasons, such as when the file is not open with read and write permissions.
+    pub fn map_raw<T: MmapAsRawDesc>(file: T) -> Result<MmapRaw> {
+        MmapOptions::new().map_raw(file)
+    }
+
+    /// Returns a raw pointer to the memory mapped file.
+    ///
+    /// Before dereferencing this pointer, you have to make sure that the file has not been
+    /// truncated since the memory map was created.
+    /// Avoiding this will not introduce memory safety issues in Rust terms,
+    /// but will cause SIGBUS (or equivalent) signal.
+    #[inline]
+    pub fn as_ptr(&self) -> *const u8 {
+        self.inner.ptr()
+    }
+
+    /// Returns an unsafe mutable pointer to the memory mapped file.
+    ///
+    /// Before dereferencing this pointer, you have to make sure that the file has not been
+    /// truncated since the memory map was created.
+    /// Avoiding this will not introduce memory safety issues in Rust terms,
+    /// but will cause SIGBUS (or equivalent) signal.
+    #[inline]
+    pub fn as_mut_ptr(&self) -> *mut u8 {
+        self.inner.ptr() as _
+    }
+
+    /// Returns the length in bytes of the memory map.
+    ///
+    /// Note that truncating the file can cause the length to change (and render this value unusable).
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.inner.len()
+    }
+
+    /// Flushes outstanding memory map modifications to disk.
+    ///
+    /// When this method returns with a non-error result, all outstanding changes to a file-backed
+    /// memory map are guaranteed to be durably stored. The file's metadata (including last
+    /// modification timestamp) may not be updated.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # extern crate memmap2;
+    /// # extern crate tempfile;
+    /// #
+    /// use std::fs::OpenOptions;
+    /// use std::io::Write;
+    /// use std::path::PathBuf;
+    /// use std::slice;
+    ///
+    /// use memmap2::MmapRaw;
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// let tempdir = tempfile::tempdir()?;
+    /// let path: PathBuf = /* path to file */
+    /// #   tempdir.path().join("flush");
+    /// let file = OpenOptions::new().read(true).write(true).create(true).open(&path)?;
+    /// file.set_len(128)?;
+    ///
+    /// let mut mmap = unsafe { MmapRaw::map_raw(&file)? };
+    ///
+    /// let mut memory = unsafe { slice::from_raw_parts_mut(mmap.as_mut_ptr(), 128) };
+    /// memory.write_all(b"Hello, world!")?;
+    /// mmap.flush()?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn flush(&self) -> Result<()> {
+        let len = self.len();
+        self.inner.flush(0, len)
+    }
+
+    /// Asynchronously flushes outstanding memory map modifications to disk.
+    ///
+    /// This method initiates flushing modified pages to durable storage, but it will not wait for
+    /// the operation to complete before returning. The file's metadata (including last
+    /// modification timestamp) may not be updated.
+    pub fn flush_async(&self) -> Result<()> {
+        let len = self.len();
+        self.inner.flush_async(0, len)
+    }
+
+    /// Flushes outstanding memory map modifications in the range to disk.
+    ///
+    /// The offset and length must be in the bounds of the memory map.
+    ///
+    /// When this method returns with a non-error result, all outstanding changes to a file-backed
+    /// memory in the range are guaranteed to be durable stored. The file's metadata (including
+    /// last modification timestamp) may not be updated. It is not guaranteed the only the changes
+    /// in the specified range are flushed; other outstanding changes to the memory map may be
+    /// flushed as well.
+    pub fn flush_range(&self, offset: usize, len: usize) -> Result<()> {
+        self.inner.flush(offset, len)
+    }
+
+    /// Asynchronously flushes outstanding memory map modifications in the range to disk.
+    ///
+    /// The offset and length must be in the bounds of the memory map.
+    ///
+    /// This method initiates flushing modified pages to durable storage, but it will not wait for
+    /// the operation to complete before returning. The file's metadata (including last
+    /// modification timestamp) may not be updated. It is not guaranteed that the only changes
+    /// flushed are those in the specified range; other outstanding changes to the memory map may
+    /// be flushed as well.
+    pub fn flush_async_range(&self, offset: usize, len: usize) -> Result<()> {
+        self.inner.flush_async(offset, len)
+    }
+
+    /// Advise OS how this memory map will be accessed. Only supported on Unix.
+    ///
+    /// See [madvise()](https://man7.org/linux/man-pages/man2/madvise.2.html) map page.
+    #[cfg(unix)]
+    pub fn advise(&self, advice: Advice) -> Result<()> {
+        self.inner.advise(advice)
+    }
+
+    /// Lock the whole memory map into RAM. Only supported on Unix.
+    ///
+    /// See [mlock()](https://man7.org/linux/man-pages/man2/mlock.2.html) map page.
+    #[cfg(unix)]
+    pub fn lock(&mut self) -> Result<()> {
+        self.inner.lock()
+    }
+
+    /// Unlock the whole memory map. Only supported on Unix.
+    ///
+    /// See [munlock()](https://man7.org/linux/man-pages/man2/munlock.2.html) map page.
+    #[cfg(unix)]
+    pub fn unlock(&mut self) -> Result<()> {
+        self.inner.unlock()
+    }
+}
+
+impl fmt::Debug for MmapRaw {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("MmapRaw")
+            .field("ptr", &self.as_ptr())
+            .field("len", &self.len())
+            .finish()
+    }
+}
+
+/// A handle to a mutable memory mapped buffer.
+///
+/// A file-backed `MmapMut` buffer may be used to read from or write to a file. An anonymous
+/// `MmapMut` buffer may be used any place that an in-memory byte buffer is needed. Use
+/// [`MmapMut::map_mut()`] and [`MmapMut::map_anon()`] to create a mutable memory map of the
+/// respective types, or [`MmapOptions::map_mut()`] and [`MmapOptions::map_anon()`] if non-default
+/// options are required.
+///
+/// A file backed `MmapMut` is created by `&File` reference, and will remain valid even after the
+/// `File` is dropped. In other words, the `MmapMut` handle is completely independent of the `File`
+/// used to create it. For consistency, on some platforms this is achieved by duplicating the
+/// underlying file handle. The memory will be unmapped when the `MmapMut` handle is dropped.
+///
+/// Dereferencing and accessing the bytes of the buffer may result in page faults (e.g. swapping
+/// the mapped pages into physical memory) though the details of this are platform specific.
+///
+/// `Mmap` is [`Sync`](std::marker::Sync) and [`Send`](std::marker::Send).
+///
+/// See [`Mmap`] for the immutable version.
+///
+/// ## Safety
+///
+/// All file-backed memory map constructors are marked `unsafe` because of the potential for
+/// *Undefined Behavior* (UB) using the map if the underlying file is subsequently modified, in or
+/// out of process. Applications must consider the risk and take appropriate precautions when using
+/// file-backed maps. Solutions such as file permissions, locks or process-private (e.g. unlinked)
+/// files exist but are platform specific and limited.
+pub struct MmapMut {
+    inner: MmapInner,
+}
+
+impl MmapMut {
+    /// Creates a writeable memory map backed by a file.
+    ///
+    /// This is equivalent to calling `MmapOptions::new().map_mut(file)`.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails, which can happen for a
+    /// variety of reasons, such as when the file is not open with read and write permissions.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # extern crate memmap2;
+    /// # extern crate tempfile;
+    /// #
+    /// use std::fs::OpenOptions;
+    /// use std::path::PathBuf;
+    ///
+    /// use memmap2::MmapMut;
+    /// #
+    /// # fn main() -> std::io::Result<()> {
+    /// # let tempdir = tempfile::tempdir()?;
+    /// let path: PathBuf = /* path to file */
+    /// #   tempdir.path().join("map_mut");
+    /// let file = OpenOptions::new()
+    ///                        .read(true)
+    ///                        .write(true)
+    ///                        .create(true)
+    ///                        .open(&path)?;
+    /// file.set_len(13)?;
+    ///
+    /// let mut mmap = unsafe { MmapMut::map_mut(&file)? };
+    ///
+    /// mmap.copy_from_slice(b"Hello, world!");
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub unsafe fn map_mut<T: MmapAsRawDesc>(file: T) -> Result<MmapMut> {
+        MmapOptions::new().map_mut(file)
+    }
+
+    /// Creates an anonymous memory map.
+    ///
+    /// This is equivalent to calling `MmapOptions::new().len(length).map_anon()`.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails or
+    /// when `len > isize::MAX`.
+    pub fn map_anon(length: usize) -> Result<MmapMut> {
+        MmapOptions::new().len(length).map_anon()
+    }
+
+    /// Flushes outstanding memory map modifications to disk.
+    ///
+    /// When this method returns with a non-error result, all outstanding changes to a file-backed
+    /// memory map are guaranteed to be durably stored. The file's metadata (including last
+    /// modification timestamp) may not be updated.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # extern crate memmap2;
+    /// # extern crate tempfile;
+    /// #
+    /// use std::fs::OpenOptions;
+    /// use std::io::Write;
+    /// use std::path::PathBuf;
+    ///
+    /// use memmap2::MmapMut;
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// # let tempdir = tempfile::tempdir()?;
+    /// let path: PathBuf = /* path to file */
+    /// #   tempdir.path().join("flush");
+    /// let file = OpenOptions::new().read(true).write(true).create(true).open(&path)?;
+    /// file.set_len(128)?;
+    ///
+    /// let mut mmap = unsafe { MmapMut::map_mut(&file)? };
+    ///
+    /// (&mut mmap[..]).write_all(b"Hello, world!")?;
+    /// mmap.flush()?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn flush(&self) -> Result<()> {
+        let len = self.len();
+        self.inner.flush(0, len)
+    }
+
+    /// Asynchronously flushes outstanding memory map modifications to disk.
+    ///
+    /// This method initiates flushing modified pages to durable storage, but it will not wait for
+    /// the operation to complete before returning. The file's metadata (including last
+    /// modification timestamp) may not be updated.
+    pub fn flush_async(&self) -> Result<()> {
+        let len = self.len();
+        self.inner.flush_async(0, len)
+    }
+
+    /// Flushes outstanding memory map modifications in the range to disk.
+    ///
+    /// The offset and length must be in the bounds of the memory map.
+    ///
+    /// When this method returns with a non-error result, all outstanding changes to a file-backed
+    /// memory in the range are guaranteed to be durable stored. The file's metadata (including
+    /// last modification timestamp) may not be updated. It is not guaranteed the only the changes
+    /// in the specified range are flushed; other outstanding changes to the memory map may be
+    /// flushed as well.
+    pub fn flush_range(&self, offset: usize, len: usize) -> Result<()> {
+        self.inner.flush(offset, len)
+    }
+
+    /// Asynchronously flushes outstanding memory map modifications in the range to disk.
+    ///
+    /// The offset and length must be in the bounds of the memory map.
+    ///
+    /// This method initiates flushing modified pages to durable storage, but it will not wait for
+    /// the operation to complete before returning. The file's metadata (including last
+    /// modification timestamp) may not be updated. It is not guaranteed that the only changes
+    /// flushed are those in the specified range; other outstanding changes to the memory map may
+    /// be flushed as well.
+    pub fn flush_async_range(&self, offset: usize, len: usize) -> Result<()> {
+        self.inner.flush_async(offset, len)
+    }
+
+    /// Returns an immutable version of this memory mapped buffer.
+    ///
+    /// If the memory map is file-backed, the file must have been opened with read permissions.
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails, which can happen for a
+    /// variety of reasons, such as when the file has not been opened with read permissions.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # extern crate memmap2;
+    /// #
+    /// use std::io::Write;
+    /// use std::path::PathBuf;
+    ///
+    /// use memmap2::{Mmap, MmapMut};
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// let mut mmap = MmapMut::map_anon(128)?;
+    ///
+    /// (&mut mmap[..]).write(b"Hello, world!")?;
+    ///
+    /// let mmap: Mmap = mmap.make_read_only()?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn make_read_only(mut self) -> Result<Mmap> {
+        self.inner.make_read_only()?;
+        Ok(Mmap { inner: self.inner })
+    }
+
+    /// Transition the memory map to be readable and executable.
+    ///
+    /// If the memory map is file-backed, the file must have been opened with execute permissions.
+    ///
+    /// On systems with separate instructions and data caches (a category that includes many ARM
+    /// chips), a platform-specific call may be needed to ensure that the changes are visible to the
+    /// execution unit (e.g. when using this function to implement a JIT compiler).  For more
+    /// details, see [this ARM write-up](https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/caches-and-self-modifying-code)
+    /// or the `man` page for [`sys_icache_invalidate`](https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/sys_icache_invalidate.3.html).
+    ///
+    /// # Errors
+    ///
+    /// This method returns an error when the underlying system call fails, which can happen for a
+    /// variety of reasons, such as when the file has not been opened with execute permissions.
+    pub fn make_exec(mut self) -> Result<Mmap> {
+        self.inner.make_exec()?;
+        Ok(Mmap { inner: self.inner })
+    }
+
+    /// Advise OS how this memory map will be accessed. Only supported on Unix.
+    ///
+    /// See [madvise()](https://man7.org/linux/man-pages/man2/madvise.2.html) map page.
+    #[cfg(unix)]
+    pub fn advise(&self, advice: Advice) -> Result<()> {
+        self.inner.advise(advice)
+    }
+
+    /// Lock the whole memory map into RAM. Only supported on Unix.
+    ///
+    /// See [mlock()](https://man7.org/linux/man-pages/man2/mlock.2.html) map page.
+    #[cfg(unix)]
+    pub fn lock(&mut self) -> Result<()> {
+        self.inner.lock()
+    }
+
+    /// Unlock the whole memory map. Only supported on Unix.
+    ///
+    /// See [munlock()](https://man7.org/linux/man-pages/man2/munlock.2.html) map page.
+    #[cfg(unix)]
+    pub fn unlock(&mut self) -> Result<()> {
+        self.inner.unlock()
+    }
+}
+
+#[cfg(feature = "stable_deref_trait")]
+unsafe impl stable_deref_trait::StableDeref for MmapMut {}
+
+impl Deref for MmapMut {
+    type Target = [u8];
+
+    #[inline]
+    fn deref(&self) -> &[u8] {
+        unsafe { slice::from_raw_parts(self.inner.ptr(), self.inner.len()) }
+    }
+}
+
+impl DerefMut for MmapMut {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut [u8] {
+        unsafe { slice::from_raw_parts_mut(self.inner.mut_ptr(), self.inner.len()) }
+    }
+}
+
+impl AsRef<[u8]> for MmapMut {
+    #[inline]
+    fn as_ref(&self) -> &[u8] {
+        self.deref()
+    }
+}
+
+impl AsMut<[u8]> for MmapMut {
+    #[inline]
+    fn as_mut(&mut self) -> &mut [u8] {
+        self.deref_mut()
+    }
+}
+
+impl fmt::Debug for MmapMut {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("MmapMut")
+            .field("ptr", &self.as_ptr())
+            .field("len", &self.len())
+            .finish()
+    }
+}
+
+#[cfg(test)]
+mod test {
+    extern crate tempfile;
+
+    #[cfg(unix)]
+    use crate::advice::Advice;
+    use std::fs::OpenOptions;
+    use std::io::{Read, Write};
+    #[cfg(unix)]
+    use std::os::unix::io::AsRawFd;
+    #[cfg(windows)]
+    use std::os::windows::fs::OpenOptionsExt;
+
+    #[cfg(windows)]
+    const GENERIC_ALL: u32 = 0x10000000;
+
+    use super::{Mmap, MmapMut, MmapOptions};
+
+    #[test]
+    fn map_file() {
+        let expected_len = 128;
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmap");
+
+        let file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .unwrap();
+
+        file.set_len(expected_len as u64).unwrap();
+
+        let mut mmap = unsafe { MmapMut::map_mut(&file).unwrap() };
+        let len = mmap.len();
+        assert_eq!(expected_len, len);
+
+        let zeros = vec![0; len];
+        let incr: Vec<u8> = (0..len as u8).collect();
+
+        // check that the mmap is empty
+        assert_eq!(&zeros[..], &mmap[..]);
+
+        // write values into the mmap
+        (&mut mmap[..]).write_all(&incr[..]).unwrap();
+
+        // read values back
+        assert_eq!(&incr[..], &mmap[..]);
+    }
+
+    #[test]
+    #[cfg(unix)]
+    fn map_fd() {
+        let expected_len = 128;
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmap");
+
+        let file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .unwrap();
+
+        file.set_len(expected_len as u64).unwrap();
+
+        let mut mmap = unsafe { MmapMut::map_mut(file.as_raw_fd()).unwrap() };
+        let len = mmap.len();
+        assert_eq!(expected_len, len);
+
+        let zeros = vec![0; len];
+        let incr: Vec<u8> = (0..len as u8).collect();
+
+        // check that the mmap is empty
+        assert_eq!(&zeros[..], &mmap[..]);
+
+        // write values into the mmap
+        (&mut mmap[..]).write_all(&incr[..]).unwrap();
+
+        // read values back
+        assert_eq!(&incr[..], &mmap[..]);
+    }
+
+    /// Checks that "mapping" a 0-length file derefs to an empty slice.
+    #[test]
+    fn map_empty_file() {
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmap");
+
+        let file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .unwrap();
+        let mmap = unsafe { Mmap::map(&file).unwrap() };
+        assert!(mmap.is_empty());
+        let mmap = unsafe { MmapMut::map_mut(&file).unwrap() };
+        assert!(mmap.is_empty());
+    }
+
+    #[test]
+    fn map_anon() {
+        let expected_len = 128;
+        let mut mmap = MmapMut::map_anon(expected_len).unwrap();
+        let len = mmap.len();
+        assert_eq!(expected_len, len);
+
+        let zeros = vec![0; len];
+        let incr: Vec<u8> = (0..len as u8).collect();
+
+        // check that the mmap is empty
+        assert_eq!(&zeros[..], &mmap[..]);
+
+        // write values into the mmap
+        (&mut mmap[..]).write_all(&incr[..]).unwrap();
+
+        // read values back
+        assert_eq!(&incr[..], &mmap[..]);
+    }
+
+    #[test]
+    fn map_anon_zero_len() {
+        assert!(MmapOptions::new().map_anon().unwrap().is_empty())
+    }
+
+    #[test]
+    #[cfg(target_pointer_width = "32")]
+    fn map_anon_len_overflow() {
+        let res = MmapMut::map_anon(0x80000000);
+
+        assert_eq!(
+            res.unwrap_err().to_string(),
+            "memory map length overflows isize"
+        );
+    }
+
+    #[test]
+    fn file_write() {
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmap");
+
+        let mut file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .unwrap();
+        file.set_len(128).unwrap();
+
+        let write = b"abc123";
+        let mut read = [0u8; 6];
+
+        let mut mmap = unsafe { MmapMut::map_mut(&file).unwrap() };
+        (&mut mmap[..]).write_all(write).unwrap();
+        mmap.flush().unwrap();
+
+        file.read_exact(&mut read).unwrap();
+        assert_eq!(write, &read);
+    }
+
+    #[test]
+    fn flush_range() {
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmap");
+
+        let file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .unwrap();
+        file.set_len(128).unwrap();
+        let write = b"abc123";
+
+        let mut mmap = unsafe {
+            MmapOptions::new()
+                .offset(2)
+                .len(write.len())
+                .map_mut(&file)
+                .unwrap()
+        };
+        (&mut mmap[..]).write_all(write).unwrap();
+        mmap.flush_async_range(0, write.len()).unwrap();
+        mmap.flush_range(0, write.len()).unwrap();
+    }
+
+    #[test]
+    fn map_copy() {
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmap");
+
+        let mut file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .unwrap();
+        file.set_len(128).unwrap();
+
+        let nulls = b"\0\0\0\0\0\0";
+        let write = b"abc123";
+        let mut read = [0u8; 6];
+
+        let mut mmap = unsafe { MmapOptions::new().map_copy(&file).unwrap() };
+
+        (&mut mmap[..]).write_all(write).unwrap();
+        mmap.flush().unwrap();
+
+        // The mmap contains the write
+        (&mmap[..]).read_exact(&mut read).unwrap();
+        assert_eq!(write, &read);
+
+        // The file does not contain the write
+        file.read_exact(&mut read).unwrap();
+        assert_eq!(nulls, &read);
+
+        // another mmap does not contain the write
+        let mmap2 = unsafe { MmapOptions::new().map(&file).unwrap() };
+        (&mmap2[..]).read_exact(&mut read).unwrap();
+        assert_eq!(nulls, &read);
+    }
+
+    #[test]
+    fn map_copy_read_only() {
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmap");
+
+        let file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .unwrap();
+        file.set_len(128).unwrap();
+
+        let nulls = b"\0\0\0\0\0\0";
+        let mut read = [0u8; 6];
+
+        let mmap = unsafe { MmapOptions::new().map_copy_read_only(&file).unwrap() };
+        (&mmap[..]).read_exact(&mut read).unwrap();
+        assert_eq!(nulls, &read);
+
+        let mmap2 = unsafe { MmapOptions::new().map(&file).unwrap() };
+        (&mmap2[..]).read_exact(&mut read).unwrap();
+        assert_eq!(nulls, &read);
+    }
+
+    #[test]
+    fn map_offset() {
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmap");
+
+        let file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .unwrap();
+
+        let offset = u32::max_value() as u64 + 2;
+        let len = 5432;
+        file.set_len(offset + len as u64).unwrap();
+
+        // Check inferred length mmap.
+        let mmap = unsafe { MmapOptions::new().offset(offset).map_mut(&file).unwrap() };
+        assert_eq!(len, mmap.len());
+
+        // Check explicit length mmap.
+        let mut mmap = unsafe {
+            MmapOptions::new()
+                .offset(offset)
+                .len(len)
+                .map_mut(&file)
+                .unwrap()
+        };
+        assert_eq!(len, mmap.len());
+
+        let zeros = vec![0; len];
+        let incr: Vec<_> = (0..len).map(|i| i as u8).collect();
+
+        // check that the mmap is empty
+        assert_eq!(&zeros[..], &mmap[..]);
+
+        // write values into the mmap
+        (&mut mmap[..]).write_all(&incr[..]).unwrap();
+
+        // read values back
+        assert_eq!(&incr[..], &mmap[..]);
+    }
+
+    #[test]
+    fn index() {
+        let mut mmap = MmapMut::map_anon(128).unwrap();
+        mmap[0] = 42;
+        assert_eq!(42, mmap[0]);
+    }
+
+    #[test]
+    fn sync_send() {
+        let mmap = MmapMut::map_anon(129).unwrap();
+
+        fn is_sync_send<T>(_val: T)
+        where
+            T: Sync + Send,
+        {
+        }
+
+        is_sync_send(mmap);
+    }
+
+    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+    fn jit_x86(mut mmap: MmapMut) {
+        mmap[0] = 0xB8; // mov eax, 0xAB
+        mmap[1] = 0xAB;
+        mmap[2] = 0x00;
+        mmap[3] = 0x00;
+        mmap[4] = 0x00;
+        mmap[5] = 0xC3; // ret
+
+        let mmap = mmap.make_exec().expect("make_exec");
+
+        let jitfn: extern "C" fn() -> u8 = unsafe { std::mem::transmute(mmap.as_ptr()) };
+        assert_eq!(jitfn(), 0xab);
+    }
+
+    #[test]
+    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+    fn jit_x86_anon() {
+        jit_x86(MmapMut::map_anon(4096).unwrap());
+    }
+
+    #[test]
+    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+    fn jit_x86_file() {
+        let tempdir = tempfile::tempdir().unwrap();
+        let mut options = OpenOptions::new();
+        #[cfg(windows)]
+        options.access_mode(GENERIC_ALL);
+
+        let file = options
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&tempdir.path().join("jit_x86"))
+            .expect("open");
+
+        file.set_len(4096).expect("set_len");
+        jit_x86(unsafe { MmapMut::map_mut(&file).expect("map_mut") });
+    }
+
+    #[test]
+    fn mprotect_file() {
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmap");
+
+        let mut options = OpenOptions::new();
+        #[cfg(windows)]
+        options.access_mode(GENERIC_ALL);
+
+        let mut file = options
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .expect("open");
+        file.set_len(256_u64).expect("set_len");
+
+        let mmap = unsafe { MmapMut::map_mut(&file).expect("map_mut") };
+
+        let mmap = mmap.make_read_only().expect("make_read_only");
+        let mut mmap = mmap.make_mut().expect("make_mut");
+
+        let write = b"abc123";
+        let mut read = [0u8; 6];
+
+        (&mut mmap[..]).write_all(write).unwrap();
+        mmap.flush().unwrap();
+
+        // The mmap contains the write
+        (&mmap[..]).read_exact(&mut read).unwrap();
+        assert_eq!(write, &read);
+
+        // The file should contain the write
+        file.read_exact(&mut read).unwrap();
+        assert_eq!(write, &read);
+
+        // another mmap should contain the write
+        let mmap2 = unsafe { MmapOptions::new().map(&file).unwrap() };
+        (&mmap2[..]).read_exact(&mut read).unwrap();
+        assert_eq!(write, &read);
+
+        let mmap = mmap.make_exec().expect("make_exec");
+
+        drop(mmap);
+    }
+
+    #[test]
+    fn mprotect_copy() {
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmap");
+
+        let mut options = OpenOptions::new();
+        #[cfg(windows)]
+        options.access_mode(GENERIC_ALL);
+
+        let mut file = options
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .expect("open");
+        file.set_len(256_u64).expect("set_len");
+
+        let mmap = unsafe { MmapOptions::new().map_copy(&file).expect("map_mut") };
+
+        let mmap = mmap.make_read_only().expect("make_read_only");
+        let mut mmap = mmap.make_mut().expect("make_mut");
+
+        let nulls = b"\0\0\0\0\0\0";
+        let write = b"abc123";
+        let mut read = [0u8; 6];
+
+        (&mut mmap[..]).write_all(write).unwrap();
+        mmap.flush().unwrap();
+
+        // The mmap contains the write
+        (&mmap[..]).read_exact(&mut read).unwrap();
+        assert_eq!(write, &read);
+
+        // The file does not contain the write
+        file.read_exact(&mut read).unwrap();
+        assert_eq!(nulls, &read);
+
+        // another mmap does not contain the write
+        let mmap2 = unsafe { MmapOptions::new().map(&file).unwrap() };
+        (&mmap2[..]).read_exact(&mut read).unwrap();
+        assert_eq!(nulls, &read);
+
+        let mmap = mmap.make_exec().expect("make_exec");
+
+        drop(mmap);
+    }
+
+    #[test]
+    fn mprotect_anon() {
+        let mmap = MmapMut::map_anon(256).expect("map_mut");
+
+        let mmap = mmap.make_read_only().expect("make_read_only");
+        let mmap = mmap.make_mut().expect("make_mut");
+        let mmap = mmap.make_exec().expect("make_exec");
+        drop(mmap);
+    }
+
+    #[test]
+    fn raw() {
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmapraw");
+
+        let mut options = OpenOptions::new();
+        let mut file = options
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .expect("open");
+        file.write_all(b"abc123").unwrap();
+        let mmap = MmapOptions::new().map_raw(&file).unwrap();
+        assert_eq!(mmap.len(), 6);
+        assert!(!mmap.as_ptr().is_null());
+        assert_eq!(unsafe { std::ptr::read(mmap.as_ptr()) }, b'a');
+    }
+
+    /// Something that relies on StableDeref
+    #[test]
+    #[cfg(feature = "stable_deref_trait")]
+    fn owning_ref() {
+        extern crate owning_ref;
+
+        let mut map = MmapMut::map_anon(128).unwrap();
+        map[10] = 42;
+        let owning = owning_ref::OwningRef::new(map);
+        let sliced = owning.map(|map| &map[10..20]);
+        assert_eq!(42, sliced[0]);
+
+        let map = sliced.into_owner().make_read_only().unwrap();
+        let owning = owning_ref::OwningRef::new(map);
+        let sliced = owning.map(|map| &map[10..20]);
+        assert_eq!(42, sliced[0]);
+    }
+
+    #[test]
+    #[cfg(unix)]
+    fn advise() {
+        let expected_len = 128;
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmap_advise");
+
+        let file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .unwrap();
+
+        file.set_len(expected_len as u64).unwrap();
+
+        // Test MmapMut::advise
+        let mut mmap = unsafe { MmapMut::map_mut(&file).unwrap() };
+        mmap.advise(Advice::Random)
+            .expect("mmap advising should be supported on unix");
+
+        let len = mmap.len();
+        assert_eq!(expected_len, len);
+
+        let zeros = vec![0; len];
+        let incr: Vec<u8> = (0..len as u8).collect();
+
+        // check that the mmap is empty
+        assert_eq!(&zeros[..], &mmap[..]);
+
+        // write values into the mmap
+        (&mut mmap[..]).write_all(&incr[..]).unwrap();
+
+        // read values back
+        assert_eq!(&incr[..], &mmap[..]);
+
+        // Set advice and Read from the read-only map
+        let mmap = unsafe { Mmap::map(&file).unwrap() };
+
+        mmap.advise(Advice::Random)
+            .expect("mmap advising should be supported on unix");
+
+        // read values back
+        assert_eq!(&incr[..], &mmap[..]);
+    }
+
+    /// Returns true if a non-zero amount of memory is locked.
+    #[cfg(target_os = "linux")]
+    fn is_locked() -> bool {
+        let status = &std::fs::read_to_string("/proc/self/status")
+            .expect("/proc/self/status should be available");
+        for line in status.lines() {
+            if line.starts_with("VmLck:") {
+                let numbers = line.replace(|c: char| !c.is_ascii_digit(), "");
+                return numbers != "0";
+            }
+        }
+        panic!("cannot get VmLck information")
+    }
+
+    #[test]
+    #[cfg(unix)]
+    fn lock() {
+        let tempdir = tempfile::tempdir().unwrap();
+        let path = tempdir.path().join("mmap_lock");
+
+        let file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)
+            .unwrap();
+        file.set_len(128).unwrap();
+
+        let mut mmap = unsafe { Mmap::map(&file).unwrap() };
+        #[cfg(target_os = "linux")]
+        assert!(!is_locked());
+
+        mmap.lock().expect("mmap lock should be supported on unix");
+        #[cfg(target_os = "linux")]
+        assert!(is_locked());
+
+        mmap.lock()
+            .expect("mmap lock again should not cause problems");
+        #[cfg(target_os = "linux")]
+        assert!(is_locked());
+
+        mmap.unlock()
+            .expect("mmap unlock should be supported on unix");
+        #[cfg(target_os = "linux")]
+        assert!(!is_locked());
+
+        mmap.unlock()
+            .expect("mmap unlock again should not cause problems");
+        #[cfg(target_os = "linux")]
+        assert!(!is_locked());
+    }
+}
diff --git a/third_party/rust/memmap2/src/stub.rs b/third_party/rust/memmap2/src/stub.rs
new file mode 100644
index 0000000000..e756da4f68
--- /dev/null
+++ b/third_party/rust/memmap2/src/stub.rs
@@ -0,0 +1,81 @@
+use std::fs::File;
+use std::io;
+
+// A stable alternative to https://doc.rust-lang.org/stable/std/primitive.never.html
+enum Never {}
+
+pub struct MmapInner {
+    never: Never,
+}
+
+impl MmapInner {
+    fn new() -> io::Result<MmapInner> {
+        Err(io::Error::new(
+            io::ErrorKind::Other,
+            "platform not supported",
+        ))
+    }
+
+    pub fn map(_: usize, _: &File, _: u64, _: bool) -> io::Result<MmapInner> {
+        MmapInner::new()
+    }
+
+    pub fn map_exec(_: usize, _: &File, _: u64, _: bool) -> io::Result<MmapInner> {
+        MmapInner::new()
+    }
+
+    pub fn map_mut(_: usize, _: &File, _: u64, _: bool) -> io::Result<MmapInner> {
+        MmapInner::new()
+    }
+
+    pub fn map_copy(_: usize, _: &File, _: u64, _: bool) -> io::Result<MmapInner> {
+        MmapInner::new()
+    }
+
+    pub fn map_copy_read_only(_: usize, _: &File, _: u64, _: bool) -> io::Result<MmapInner> {
+        MmapInner::new()
+    }
+
+    pub fn map_anon(_: usize, _: bool) -> io::Result<MmapInner> {
+        MmapInner::new()
+    }
+
+    pub fn flush(&self, _: usize, _: usize) -> io::Result<()> {
+        match self.never {}
+    }
+
+    pub fn flush_async(&self, _: usize, _: usize) -> io::Result<()> {
+        match self.never {}
+    }
+
+    pub fn make_read_only(&mut self) -> io::Result<()> {
+        match self.never {}
+    }
+
+    pub fn make_exec(&mut self) -> io::Result<()> {
+        match self.never {}
+    }
+
+    pub fn make_mut(&mut self) -> io::Result<()> {
+        match self.never {}
+    }
+
+    #[inline]
+    pub fn ptr(&self) -> *const u8 {
+        match self.never {}
+    }
+
+    #[inline]
+    pub fn mut_ptr(&mut self) -> *mut u8 {
+        match self.never {}
+    }
+
+    #[inline]
+    pub fn len(&self) -> usize {
+        match self.never {}
+    }
+}
+
+pub fn file_len(file: &File) -> io::Result<u64> {
+    Ok(file.metadata()?.len())
+}
diff --git a/third_party/rust/memmap2/src/unix.rs b/third_party/rust/memmap2/src/unix.rs
new file mode 100644
index 0000000000..158d78c1f9
--- /dev/null
+++ b/third_party/rust/memmap2/src/unix.rs
@@ -0,0 +1,314 @@
+extern crate libc;
+
+use std::fs::File;
+use std::mem::ManuallyDrop;
+use std::os::unix::io::{FromRawFd, RawFd};
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::{io, ptr};
+
+use crate::advice::Advice;
+
+#[cfg(any(
+    all(target_os = "linux", not(target_arch = "mips")),
+    target_os = "freebsd",
+    target_os = "android"
+))]
+const MAP_STACK: libc::c_int = libc::MAP_STACK;
+
+#[cfg(not(any(
+    all(target_os = "linux", not(target_arch = "mips")),
+    target_os = "freebsd",
+    target_os = "android"
+)))]
+const MAP_STACK: libc::c_int = 0;
+
+#[cfg(any(target_os = "linux", target_os = "android"))]
+const MAP_POPULATE: libc::c_int = libc::MAP_POPULATE;
+
+#[cfg(not(any(target_os = "linux", target_os = "android")))]
+const MAP_POPULATE: libc::c_int = 0;
+
+pub struct MmapInner {
+    ptr: *mut libc::c_void,
+    len: usize,
+}
+
+impl MmapInner {
+    /// Creates a new `MmapInner`.
+    ///
+    /// This is a thin wrapper around the `mmap` sytem call.
+    fn new(
+        len: usize,
+        prot: libc::c_int,
+        flags: libc::c_int,
+        file: RawFd,
+        offset: u64,
+    ) -> io::Result<MmapInner> {
+        let alignment = offset % page_size() as u64;
+        let aligned_offset = offset - alignment;
+        let aligned_len = len + alignment as usize;
+
+        // `libc::mmap` does not support zero-size mappings. POSIX defines:
+        //
+        // https://pubs.opengroup.org/onlinepubs/9699919799/functions/mmap.html
+        // > If `len` is zero, `mmap()` shall fail and no mapping shall be established.
+        //
+        // So if we would create such a mapping, crate a one-byte mapping instead:
+        let aligned_len = aligned_len.max(1);
+
+        // Note that in that case `MmapInner::len` is still set to zero,
+        // and `Mmap` will still dereferences to an empty slice.
+        //
+        // If this mapping is backed by an empty file, we create a mapping larger than the file.
+        // This is unusual but well-defined. On the same man page, POSIX further defines:
+        //
+        // > The `mmap()` function can be used to map a region of memory that is larger
+        // > than the current size of the object.
+        //
+        // (The object here is the file.)
+        //
+        // > Memory access within the mapping but beyond the current end of the underlying
+        // > objects may result in SIGBUS signals being sent to the process. The reason for this
+        // > is that the size of the object can be manipulated by other processes and can change
+        // > at any moment. The implementation should tell the application that a memory reference
+        // > is outside the object where this can be detected; otherwise, written data may be lost
+        // > and read data may not reflect actual data in the object.
+        //
+        // Because `MmapInner::len` is not incremented, this increment of `aligned_len`
+        // will not allow accesses past the end of the file and will not cause SIGBUS.
+        //
+        // (SIGBUS is still possible by mapping a non-empty file and then truncating it
+        // to a shorter size, but that is unrelated to this handling of empty files.)
+
+        unsafe {
+            let ptr = libc::mmap(
+                ptr::null_mut(),
+                aligned_len as libc::size_t,
+                prot,
+                flags,
+                file,
+                aligned_offset as libc::off_t,
+            );
+
+            if ptr == libc::MAP_FAILED {
+                Err(io::Error::last_os_error())
+            } else {
+                Ok(MmapInner {
+                    ptr: ptr.offset(alignment as isize),
+                    len,
+                })
+            }
+        }
+    }
+
+    pub fn map(len: usize, file: RawFd, offset: u64, populate: bool) -> io::Result<MmapInner> {
+        let populate = if populate { MAP_POPULATE } else { 0 };
+        MmapInner::new(
+            len,
+            libc::PROT_READ,
+            libc::MAP_SHARED | populate,
+            file,
+            offset,
+        )
+    }
+
+    pub fn map_exec(len: usize, file: RawFd, offset: u64, populate: bool) -> io::Result<MmapInner> {
+        let populate = if populate { MAP_POPULATE } else { 0 };
+        MmapInner::new(
+            len,
+            libc::PROT_READ | libc::PROT_EXEC,
+            libc::MAP_SHARED | populate,
+            file,
+            offset,
+        )
+    }
+
+    pub fn map_mut(len: usize, file: RawFd, offset: u64, populate: bool) -> io::Result<MmapInner> {
+        let populate = if populate { MAP_POPULATE } else { 0 };
+        MmapInner::new(
+            len,
+            libc::PROT_READ | libc::PROT_WRITE,
+            libc::MAP_SHARED | populate,
+            file,
+            offset,
+        )
+    }
+
+    pub fn map_copy(len: usize, file: RawFd, offset: u64, populate: bool) -> io::Result<MmapInner> {
+        let populate = if populate { MAP_POPULATE } else { 0 };
+        MmapInner::new(
+            len,
+            libc::PROT_READ | libc::PROT_WRITE,
+            libc::MAP_PRIVATE | populate,
+            file,
+            offset,
+        )
+    }
+
+    pub fn map_copy_read_only(
+        len: usize,
+        file: RawFd,
+        offset: u64,
+        populate: bool,
+    ) -> io::Result<MmapInner> {
+        let populate = if populate { MAP_POPULATE } else { 0 };
+        MmapInner::new(
+            len,
+            libc::PROT_READ,
+            libc::MAP_PRIVATE | populate,
+            file,
+            offset,
+        )
+    }
+
+    /// Open an anonymous memory map.
+    pub fn map_anon(len: usize, stack: bool) -> io::Result<MmapInner> {
+        let stack = if stack { MAP_STACK } else { 0 };
+        MmapInner::new(
+            len,
+            libc::PROT_READ | libc::PROT_WRITE,
+            libc::MAP_PRIVATE | libc::MAP_ANON | stack,
+            -1,
+            0,
+        )
+    }
+
+    pub fn flush(&self, offset: usize, len: usize) -> io::Result<()> {
+        let alignment = (self.ptr as usize + offset) % page_size();
+        let offset = offset as isize - alignment as isize;
+        let len = len + alignment;
+        let result =
+            unsafe { libc::msync(self.ptr.offset(offset), len as libc::size_t, libc::MS_SYNC) };
+        if result == 0 {
+            Ok(())
+        } else {
+            Err(io::Error::last_os_error())
+        }
+    }
+
+    pub fn flush_async(&self, offset: usize, len: usize) -> io::Result<()> {
+        let alignment = (self.ptr as usize + offset) % page_size();
+        let offset = offset as isize - alignment as isize;
+        let len = len + alignment;
+        let result =
+            unsafe { libc::msync(self.ptr.offset(offset), len as libc::size_t, libc::MS_ASYNC) };
+        if result == 0 {
+            Ok(())
+        } else {
+            Err(io::Error::last_os_error())
+        }
+    }
+
+    fn mprotect(&mut self, prot: libc::c_int) -> io::Result<()> {
+        unsafe {
+            let alignment = self.ptr as usize % page_size();
+            let ptr = self.ptr.offset(-(alignment as isize));
+            let len = self.len + alignment;
+            let len = len.max(1);
+            if libc::mprotect(ptr, len, prot) == 0 {
+                Ok(())
+            } else {
+                Err(io::Error::last_os_error())
+            }
+        }
+    }
+
+    pub fn make_read_only(&mut self) -> io::Result<()> {
+        self.mprotect(libc::PROT_READ)
+    }
+
+    pub fn make_exec(&mut self) -> io::Result<()> {
+        self.mprotect(libc::PROT_READ | libc::PROT_EXEC)
+    }
+
+    pub fn make_mut(&mut self) -> io::Result<()> {
+        self.mprotect(libc::PROT_READ | libc::PROT_WRITE)
+    }
+
+    #[inline]
+    pub fn ptr(&self) -> *const u8 {
+        self.ptr as *const u8
+    }
+
+    #[inline]
+    pub fn mut_ptr(&mut self) -> *mut u8 {
+        self.ptr as *mut u8
+    }
+
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.len
+    }
+
+    pub fn advise(&self, advice: Advice) -> io::Result<()> {
+        unsafe {
+            if libc::madvise(self.ptr, self.len, advice as i32) != 0 {
+                Err(io::Error::last_os_error())
+            } else {
+                Ok(())
+            }
+        }
+    }
+
+    pub fn lock(&self) -> io::Result<()> {
+        unsafe {
+            if libc::mlock(self.ptr, self.len) != 0 {
+                Err(io::Error::last_os_error())
+            } else {
+                Ok(())
+            }
+        }
+    }
+
+    pub fn unlock(&self) -> io::Result<()> {
+        unsafe {
+            if libc::munlock(self.ptr, self.len) != 0 {
+                Err(io::Error::last_os_error())
+            } else {
+                Ok(())
+            }
+        }
+    }
+}
+
+impl Drop for MmapInner {
+    fn drop(&mut self) {
+        let alignment = self.ptr as usize % page_size();
+        let len = self.len + alignment;
+        let len = len.max(1);
+        // Any errors during unmapping/closing are ignored as the only way
+        // to report them would be through panicking which is highly discouraged
+        // in Drop impls, c.f. https://github.com/rust-lang/lang-team/issues/97
+        unsafe {
+            let ptr = self.ptr.offset(-(alignment as isize));
+            libc::munmap(ptr, len as libc::size_t);
+        }
+    }
+}
+
+unsafe impl Sync for MmapInner {}
+unsafe impl Send for MmapInner {}
+
+fn page_size() -> usize {
+    static PAGE_SIZE: AtomicUsize = AtomicUsize::new(0);
+
+    match PAGE_SIZE.load(Ordering::Relaxed) {
+        0 => {
+            let page_size = unsafe { libc::sysconf(libc::_SC_PAGESIZE) as usize };
+
+            PAGE_SIZE.store(page_size, Ordering::Relaxed);
+
+            page_size
+        }
+        page_size => page_size,
+    }
+}
+
+pub fn file_len(file: RawFd) -> io::Result<u64> {
+    // SAFETY: We must not close the passed-in fd by dropping the File we create,
+    // we ensure this by immediately wrapping it in a ManuallyDrop.
+    unsafe {
+        let file = ManuallyDrop::new(File::from_raw_fd(file));
+        Ok(file.metadata()?.len())
+    }
+}
diff --git a/third_party/rust/memmap2/src/windows.rs b/third_party/rust/memmap2/src/windows.rs
new file mode 100644
index 0000000000..d580995e65
--- /dev/null
+++ b/third_party/rust/memmap2/src/windows.rs
@@ -0,0 +1,516 @@
+#![allow(non_camel_case_types)]
+#![allow(non_snake_case)]
+
+use std::fs::File;
+use std::mem::ManuallyDrop;
+use std::os::raw::c_void;
+use std::os::windows::io::{FromRawHandle, RawHandle};
+use std::{io, mem, ptr};
+
+type BOOL = i32;
+type WORD = u16;
+type DWORD = u32;
+type WCHAR = u16;
+type HANDLE = *mut c_void;
+type LPHANDLE = *mut HANDLE;
+type LPVOID = *mut c_void;
+type LPCVOID = *const c_void;
+type ULONG_PTR = usize;
+type SIZE_T = ULONG_PTR;
+type LPCWSTR = *const WCHAR;
+type PDWORD = *mut DWORD;
+type DWORD_PTR = ULONG_PTR;
+type LPSECURITY_ATTRIBUTES = *mut SECURITY_ATTRIBUTES;
+type LPSYSTEM_INFO = *mut SYSTEM_INFO;
+
+const INVALID_HANDLE_VALUE: HANDLE = -1isize as HANDLE;
+
+const DUPLICATE_SAME_ACCESS: DWORD = 0x00000002;
+
+const STANDARD_RIGHTS_REQUIRED: DWORD = 0x000F0000;
+
+const SECTION_QUERY: DWORD = 0x0001;
+const SECTION_MAP_WRITE: DWORD = 0x0002;
+const SECTION_MAP_READ: DWORD = 0x0004;
+const SECTION_MAP_EXECUTE: DWORD = 0x0008;
+const SECTION_EXTEND_SIZE: DWORD = 0x0010;
+const SECTION_MAP_EXECUTE_EXPLICIT: DWORD = 0x0020;
+const SECTION_ALL_ACCESS: DWORD = STANDARD_RIGHTS_REQUIRED
+    | SECTION_QUERY
+    | SECTION_MAP_WRITE
+    | SECTION_MAP_READ
+    | SECTION_MAP_EXECUTE
+    | SECTION_EXTEND_SIZE;
+
+const PAGE_READONLY: DWORD = 0x02;
+const PAGE_READWRITE: DWORD = 0x04;
+const PAGE_WRITECOPY: DWORD = 0x08;
+const PAGE_EXECUTE_READ: DWORD = 0x20;
+const PAGE_EXECUTE_READWRITE: DWORD = 0x40;
+const PAGE_EXECUTE_WRITECOPY: DWORD = 0x80;
+
+const FILE_MAP_WRITE: DWORD = SECTION_MAP_WRITE;
+const FILE_MAP_READ: DWORD = SECTION_MAP_READ;
+const FILE_MAP_ALL_ACCESS: DWORD = SECTION_ALL_ACCESS;
+const FILE_MAP_EXECUTE: DWORD = SECTION_MAP_EXECUTE_EXPLICIT;
+const FILE_MAP_COPY: DWORD = 0x00000001;
+
+#[repr(C)]
+struct SECURITY_ATTRIBUTES {
+    nLength: DWORD,
+    lpSecurityDescriptor: LPVOID,
+    bInheritHandle: BOOL,
+}
+
+#[repr(C)]
+struct SYSTEM_INFO {
+    wProcessorArchitecture: WORD,
+    wReserved: WORD,
+    dwPageSize: DWORD,
+    lpMinimumApplicationAddress: LPVOID,
+    lpMaximumApplicationAddress: LPVOID,
+    dwActiveProcessorMask: DWORD_PTR,
+    dwNumberOfProcessors: DWORD,
+    dwProcessorType: DWORD,
+    dwAllocationGranularity: DWORD,
+    wProcessorLevel: WORD,
+    wProcessorRevision: WORD,
+}
+
+#[repr(C)]
+#[derive(Copy, Clone)]
+pub struct FILETIME {
+    pub dwLowDateTime: DWORD,
+    pub dwHighDateTime: DWORD,
+}
+
+extern "system" {
+    fn GetCurrentProcess() -> HANDLE;
+
+    fn CloseHandle(hObject: HANDLE) -> BOOL;
+
+    fn DuplicateHandle(
+        hSourceProcessHandle: HANDLE,
+        hSourceHandle: HANDLE,
+        hTargetProcessHandle: HANDLE,
+        lpTargetHandle: LPHANDLE,
+        dwDesiredAccess: DWORD,
+        bInheritHandle: BOOL,
+        dwOptions: DWORD,
+    ) -> BOOL;
+
+    fn CreateFileMappingW(
+        hFile: HANDLE,
+        lpFileMappingAttributes: LPSECURITY_ATTRIBUTES,
+        flProtect: DWORD,
+        dwMaximumSizeHigh: DWORD,
+        dwMaximumSizeLow: DWORD,
+        lpName: LPCWSTR,
+    ) -> HANDLE;
+
+    fn FlushFileBuffers(hFile: HANDLE) -> BOOL;
+
+    fn FlushViewOfFile(lpBaseAddress: LPCVOID, dwNumberOfBytesToFlush: SIZE_T) -> BOOL;
+
+    fn UnmapViewOfFile(lpBaseAddress: LPCVOID) -> BOOL;
+
+    fn MapViewOfFile(
+        hFileMappingObject: HANDLE,
+        dwDesiredAccess: DWORD,
+        dwFileOffsetHigh: DWORD,
+        dwFileOffsetLow: DWORD,
+        dwNumberOfBytesToMap: SIZE_T,
+    ) -> LPVOID;
+
+    fn VirtualProtect(
+        lpAddress: LPVOID,
+        dwSize: SIZE_T,
+        flNewProtect: DWORD,
+        lpflOldProtect: PDWORD,
+    ) -> BOOL;
+
+    fn GetSystemInfo(lpSystemInfo: LPSYSTEM_INFO);
+}
+
+/// Returns a fixed pointer that is valid for `slice::from_raw_parts::<u8>` with `len == 0`.
+fn empty_slice_ptr() -> *mut c_void {
+    std::ptr::NonNull::<u8>::dangling().cast().as_ptr()
+}
+
+pub struct MmapInner {
+    handle: Option<RawHandle>,
+    ptr: *mut c_void,
+    len: usize,
+    copy: bool,
+}
+
+impl MmapInner {
+    /// Creates a new `MmapInner`.
+    ///
+    /// This is a thin wrapper around the `CreateFileMappingW` and `MapViewOfFile` system calls.
+    pub fn new(
+        handle: RawHandle,
+        protect: DWORD,
+        access: DWORD,
+        offset: u64,
+        len: usize,
+        copy: bool,
+    ) -> io::Result<MmapInner> {
+        let alignment = offset % allocation_granularity() as u64;
+        let aligned_offset = offset - alignment as u64;
+        let aligned_len = len + alignment as usize;
+        if aligned_len == 0 {
+            // `CreateFileMappingW` documents:
+            //
+            // https://docs.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-createfilemappingw
+            // > An attempt to map a file with a length of 0 (zero) fails with an error code
+            // > of ERROR_FILE_INVALID. Applications should test for files with a length of 0
+            // > (zero) and reject those files.
+            //
+            // For such files, don’t create a mapping at all and use a marker pointer instead.
+            return Ok(MmapInner {
+                handle: None,
+                ptr: empty_slice_ptr(),
+                len: 0,
+                copy,
+            });
+        }
+
+        unsafe {
+            let mapping = CreateFileMappingW(handle, ptr::null_mut(), protect, 0, 0, ptr::null());
+            if mapping.is_null() {
+                return Err(io::Error::last_os_error());
+            }
+
+            let ptr = MapViewOfFile(
+                mapping,
+                access,
+                (aligned_offset >> 16 >> 16) as DWORD,
+                (aligned_offset & 0xffffffff) as DWORD,
+                aligned_len as SIZE_T,
+            );
+            CloseHandle(mapping);
+            if ptr.is_null() {
+                return Err(io::Error::last_os_error());
+            }
+
+            let mut new_handle = 0 as RawHandle;
+            let cur_proc = GetCurrentProcess();
+            let ok = DuplicateHandle(
+                cur_proc,
+                handle,
+                cur_proc,
+                &mut new_handle,
+                0,
+                0,
+                DUPLICATE_SAME_ACCESS,
+            );
+            if ok == 0 {
+                UnmapViewOfFile(ptr);
+                return Err(io::Error::last_os_error());
+            }
+
+            Ok(MmapInner {
+                handle: Some(new_handle),
+                ptr: ptr.offset(alignment as isize),
+                len: len as usize,
+                copy,
+            })
+        }
+    }
+
+    pub fn map(
+        len: usize,
+        handle: RawHandle,
+        offset: u64,
+        _populate: bool,
+    ) -> io::Result<MmapInner> {
+        let write = protection_supported(handle, PAGE_READWRITE);
+        let exec = protection_supported(handle, PAGE_EXECUTE_READ);
+        let mut access = FILE_MAP_READ;
+        let protection = match (write, exec) {
+            (true, true) => {
+                access |= FILE_MAP_WRITE | FILE_MAP_EXECUTE;
+                PAGE_EXECUTE_READWRITE
+            }
+            (true, false) => {
+                access |= FILE_MAP_WRITE;
+                PAGE_READWRITE
+            }
+            (false, true) => {
+                access |= FILE_MAP_EXECUTE;
+                PAGE_EXECUTE_READ
+            }
+            (false, false) => PAGE_READONLY,
+        };
+
+        let mut inner = MmapInner::new(handle, protection, access, offset, len, false)?;
+        if write || exec {
+            inner.make_read_only()?;
+        }
+        Ok(inner)
+    }
+
+    pub fn map_exec(
+        len: usize,
+        handle: RawHandle,
+        offset: u64,
+        _populate: bool,
+    ) -> io::Result<MmapInner> {
+        let write = protection_supported(handle, PAGE_READWRITE);
+        let mut access = FILE_MAP_READ | FILE_MAP_EXECUTE;
+        let protection = if write {
+            access |= FILE_MAP_WRITE;
+            PAGE_EXECUTE_READWRITE
+        } else {
+            PAGE_EXECUTE_READ
+        };
+
+        let mut inner = MmapInner::new(handle, protection, access, offset, len, false)?;
+        if write {
+            inner.make_exec()?;
+        }
+        Ok(inner)
+    }
+
+    pub fn map_mut(
+        len: usize,
+        handle: RawHandle,
+        offset: u64,
+        _populate: bool,
+    ) -> io::Result<MmapInner> {
+        let exec = protection_supported(handle, PAGE_EXECUTE_READ);
+        let mut access = FILE_MAP_READ | FILE_MAP_WRITE;
+        let protection = if exec {
+            access |= FILE_MAP_EXECUTE;
+            PAGE_EXECUTE_READWRITE
+        } else {
+            PAGE_READWRITE
+        };
+
+        let mut inner = MmapInner::new(handle, protection, access, offset, len, false)?;
+        if exec {
+            inner.make_mut()?;
+        }
+        Ok(inner)
+    }
+
+    pub fn map_copy(
+        len: usize,
+        handle: RawHandle,
+        offset: u64,
+        _populate: bool,
+    ) -> io::Result<MmapInner> {
+        let exec = protection_supported(handle, PAGE_EXECUTE_READWRITE);
+        let mut access = FILE_MAP_COPY;
+        let protection = if exec {
+            access |= FILE_MAP_EXECUTE;
+            PAGE_EXECUTE_WRITECOPY
+        } else {
+            PAGE_WRITECOPY
+        };
+
+        let mut inner = MmapInner::new(handle, protection, access, offset, len, true)?;
+        if exec {
+            inner.make_mut()?;
+        }
+        Ok(inner)
+    }
+
+    pub fn map_copy_read_only(
+        len: usize,
+        handle: RawHandle,
+        offset: u64,
+        _populate: bool,
+    ) -> io::Result<MmapInner> {
+        let write = protection_supported(handle, PAGE_READWRITE);
+        let exec = protection_supported(handle, PAGE_EXECUTE_READ);
+        let mut access = FILE_MAP_COPY;
+        let protection = if exec {
+            access |= FILE_MAP_EXECUTE;
+            PAGE_EXECUTE_WRITECOPY
+        } else {
+            PAGE_WRITECOPY
+        };
+
+        let mut inner = MmapInner::new(handle, protection, access, offset, len, true)?;
+        if write || exec {
+            inner.make_read_only()?;
+        }
+        Ok(inner)
+    }
+
+    pub fn map_anon(len: usize, _stack: bool) -> io::Result<MmapInner> {
+        // Ensure a non-zero length for the underlying mapping
+        let mapped_len = len.max(1);
+        unsafe {
+            // Create a mapping and view with maximum access permissions, then use `VirtualProtect`
+            // to set the actual `Protection`. This way, we can set more permissive protection later
+            // on.
+            // Also see https://msdn.microsoft.com/en-us/library/windows/desktop/aa366537.aspx
+
+            let mapping = CreateFileMappingW(
+                INVALID_HANDLE_VALUE,
+                ptr::null_mut(),
+                PAGE_EXECUTE_READWRITE,
+                (mapped_len >> 16 >> 16) as DWORD,
+                (mapped_len & 0xffffffff) as DWORD,
+                ptr::null(),
+            );
+            if mapping.is_null() {
+                return Err(io::Error::last_os_error());
+            }
+            let access = FILE_MAP_ALL_ACCESS | FILE_MAP_EXECUTE;
+            let ptr = MapViewOfFile(mapping, access, 0, 0, mapped_len as SIZE_T);
+            CloseHandle(mapping);
+
+            if ptr.is_null() {
+                return Err(io::Error::last_os_error());
+            }
+
+            let mut old = 0;
+            let result = VirtualProtect(ptr, mapped_len as SIZE_T, PAGE_READWRITE, &mut old);
+            if result != 0 {
+                Ok(MmapInner {
+                    handle: None,
+                    ptr,
+                    len: len as usize,
+                    copy: false,
+                })
+            } else {
+                Err(io::Error::last_os_error())
+            }
+        }
+    }
+
+    pub fn flush(&self, offset: usize, len: usize) -> io::Result<()> {
+        self.flush_async(offset, len)?;
+
+        if let Some(handle) = self.handle {
+            let ok = unsafe { FlushFileBuffers(handle) };
+            if ok == 0 {
+                return Err(io::Error::last_os_error());
+            }
+        }
+
+        Ok(())
+    }
+
+    pub fn flush_async(&self, offset: usize, len: usize) -> io::Result<()> {
+        if self.ptr == empty_slice_ptr() {
+            return Ok(());
+        }
+        let result = unsafe { FlushViewOfFile(self.ptr.add(offset), len as SIZE_T) };
+        if result != 0 {
+            Ok(())
+        } else {
+            Err(io::Error::last_os_error())
+        }
+    }
+
+    fn virtual_protect(&mut self, protect: DWORD) -> io::Result<()> {
+        if self.ptr == empty_slice_ptr() {
+            return Ok(());
+        }
+        unsafe {
+            let alignment = self.ptr as usize % allocation_granularity();
+            let ptr = self.ptr.offset(-(alignment as isize));
+            let aligned_len = self.len as SIZE_T + alignment as SIZE_T;
+
+            let mut old = 0;
+            let result = VirtualProtect(ptr, aligned_len, protect, &mut old);
+
+            if result != 0 {
+                Ok(())
+            } else {
+                Err(io::Error::last_os_error())
+            }
+        }
+    }
+
+    pub fn make_read_only(&mut self) -> io::Result<()> {
+        self.virtual_protect(PAGE_READONLY)
+    }
+
+    pub fn make_exec(&mut self) -> io::Result<()> {
+        if self.copy {
+            self.virtual_protect(PAGE_EXECUTE_WRITECOPY)
+        } else {
+            self.virtual_protect(PAGE_EXECUTE_READ)
+        }
+    }
+
+    pub fn make_mut(&mut self) -> io::Result<()> {
+        if self.copy {
+            self.virtual_protect(PAGE_WRITECOPY)
+        } else {
+            self.virtual_protect(PAGE_READWRITE)
+        }
+    }
+
+    #[inline]
+    pub fn ptr(&self) -> *const u8 {
+        self.ptr as *const u8
+    }
+
+    #[inline]
+    pub fn mut_ptr(&mut self) -> *mut u8 {
+        self.ptr as *mut u8
+    }
+
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.len
+    }
+}
+
+impl Drop for MmapInner {
+    fn drop(&mut self) {
+        if self.ptr == empty_slice_ptr() {
+            return;
+        }
+        let alignment = self.ptr as usize % allocation_granularity();
+        // Any errors during unmapping/closing are ignored as the only way
+        // to report them would be through panicking which is highly discouraged
+        // in Drop impls, c.f. https://github.com/rust-lang/lang-team/issues/97
+        unsafe {
+            let ptr = self.ptr.offset(-(alignment as isize));
+            UnmapViewOfFile(ptr);
+
+            if let Some(handle) = self.handle {
+                CloseHandle(handle);
+            }
+        }
+    }
+}
+
+unsafe impl Sync for MmapInner {}
+unsafe impl Send for MmapInner {}
+
+fn protection_supported(handle: RawHandle, protection: DWORD) -> bool {
+    unsafe {
+        let mapping = CreateFileMappingW(handle, ptr::null_mut(), protection, 0, 0, ptr::null());
+        if mapping.is_null() {
+            return false;
+        }
+        CloseHandle(mapping);
+        true
+    }
+}
+
+fn allocation_granularity() -> usize {
+    unsafe {
+        let mut info = mem::zeroed();
+        GetSystemInfo(&mut info);
+        info.dwAllocationGranularity as usize
+    }
+}
+
+pub fn file_len(handle: RawHandle) -> io::Result<u64> {
+    // SAFETY: We must not close the passed-in fd by dropping the File we create,
+    // we ensure this by immediately wrapping it in a ManuallyDrop.
+    unsafe {
+        let file = ManuallyDrop::new(File::from_raw_handle(handle));
+        Ok(file.metadata()?.len())
+    }
+}