Document that `BorrowedFd` may be used to do a `dup`.

library/std/src/os/unix/io/mod.rs

@@ -26,20 +26,30 @@
 //! that they don't outlive the resource they point to. These are safe to
 //! use. `BorrowedFd` values may be used in APIs which provide safe access to
 //! any system call except for:
+//!
 //!  - `close`, because that would end the dynamic lifetime of the resource
 //!    without ending the lifetime of the file descriptor.
+//!
 //!  - `dup2`/`dup3`, in the second argument, because this argument is
 //!    closed and assigned a new resource, which may break the assumptions
 //!    other code using that file descriptor.
-//! This list doesn't include `mmap`, since `mmap` does do a proper borrow of
-//! its file descriptor argument. That said, `mmap` is unsafe for other
-//! reasons: it operates on raw pointers, and it can have undefined behavior if
-//! the underlying storage is mutated. Mutations may come from other processes,
-//! or from the same process if the API provides `BorrowedFd` access, since as
-//! mentioned earlier, `BorrowedFd` values may be used in APIs which provide
-//! safe access to any system call. Consequently, code using `mmap` and
-//! presenting a safe API must take full responsibility for ensuring that safe
-//! Rust code cannot evoke undefined behavior through it.
+//!
+//! `BorrowedFd` values may be used in APIs which provide safe access to `dup`
+//! system calls, so types implementing `AsFd` or `From<OwnedFd>` should not
+//! assume they always have exclusive access to the underlying file
+//! description.
+//!
+//! `BorrowedFd` values may also be used with `mmap`, since `mmap` uses the
+//! provided file descriptor in a manner similar to `dup` and does not require
+//! the `BorrowedFd` passed to it to live for the lifetime of the resulting
+//! mapping. That said, `mmap` is unsafe for other reasons: it operates on raw
+//! pointers, and it can have undefined behavior if the underlying storage is
+//! mutated. Mutations may come from other processes, or from the same process
+//! if the API provides `BorrowedFd` access, since as mentioned earlier,
+//! `BorrowedFd` values may be used in APIs which provide safe access to any
+//! system call. Consequently, code using `mmap` and presenting a safe API must
+//! take full responsibility for ensuring that safe Rust code cannot evoke
+//! undefined behavior through it.
 //!
 //! Like boxes, `OwnedFd` values conceptually own the resource they point to,
 //! and free (close) it when they are dropped.

library/std/src/os/windows/io/handle.rs

@@ -189,7 +189,7 @@ pub(crate) fn duplicate(
         access: c::DWORD,
         inherit: bool,
         options: c::DWORD,
-    ) -> io::Result<Self> {
+    ) -> io::Result<OwnedHandle> {
         let handle = self.as_raw_handle();
 
         // `Stdin`, `Stdout`, and `Stderr` can all hold null handles, such as
@@ -197,7 +197,7 @@ pub(crate) fn duplicate(
         // if we passed it a null handle, but we can treat null as a valid
         // handle which doesn't do any I/O, and allow it to be duplicated.
         if handle.is_null() {
-            return unsafe { Ok(Self::from_raw_handle(handle)) };
+            return unsafe { Ok(OwnedHandle::from_raw_handle(handle)) };
         }
 
         let mut ret = ptr::null_mut();
@@ -213,7 +213,7 @@ pub(crate) fn duplicate(
                 options,
             )
         })?;
-        unsafe { Ok(Self::from_raw_handle(ret)) }
+        unsafe { Ok(OwnedHandle::from_raw_handle(ret)) }
     }
 }
 

library/std/src/os/windows/io/mod.rs

@@ -36,6 +36,12 @@
 //! dynamic lifetime of the resource without ending the lifetime of the
 //! handle or socket.
 //!
+//! `BorrowedHandle` and `BorrowedSocket` values may be used in APIs which
+//! provide safe access to `DuplicateHandle` and `WSADuplicateSocketW` and
+//! related functions, so types implementing `AsHandle`, `AsSocket`,
+//! `From<OwnedHandle>`, or `From<OwnedSocket>` should not assume they always
+//! have exclusive access to the underlying object.
+//!
 //! Like boxes, `OwnedHandle` and `OwnedSocket` values conceptually own the
 //! resource they point to, and free (close) it when they are dropped.
 //!

library/std/src/sys/windows/handle.rs

@@ -218,7 +218,7 @@ pub fn duplicate(
         inherit: bool,
         options: c::DWORD,
     ) -> io::Result<Self> {
-        Ok(Self(self.0.duplicate(access, inherit, options)?))
+        Ok(Self(self.0.as_handle().duplicate(access, inherit, options)?))
     }
 
     /// Performs a synchronous read.