breaking: Remove the pre-exec extension function

The purpose for removing this function is twofold:

- It is the only unsafe code in this crate that can't be realistically
  replaced with safe code.
- It is a footgun anyways, and can be done anyways with Into::into() if
   users really want it.

This is a breaking change.

Signed-off-by: John Nunley <dev@notgull.net>

src/unix.rs

@@ -20,39 +20,6 @@ pub trait CommandExt: crate::sealed::Sealed {
     /// the same semantics as the `uid` field.
     fn gid(&mut self, id: u32) -> &mut Command;
 
-    /// Schedules a closure to be run just before the `exec` function is
-    /// invoked.
-    ///
-    /// The closure is allowed to return an I/O error whose OS error code will
-    /// be communicated back to the parent and returned as an error from when
-    /// the spawn was requested.
-    ///
-    /// Multiple closures can be registered and they will be called in order of
-    /// their registration. If a closure returns `Err` then no further closures
-    /// will be called and the spawn operation will immediately return with a
-    /// failure.
-    ///
-    /// # Safety
-    ///
-    /// This closure will be run in the context of the child process after a
-    /// `fork`. This primarily means that any modifications made to memory on
-    /// behalf of this closure will **not** be visible to the parent process.
-    /// This is often a very constrained environment where normal operations
-    /// like `malloc` or acquiring a mutex are not guaranteed to work (due to
-    /// other threads perhaps still running when the `fork` was run).
-    ///
-    /// This also means that all resources such as file descriptors and
-    /// memory-mapped regions got duplicated. It is your responsibility to make
-    /// sure that the closure does not violate library invariants by making
-    /// invalid use of these duplicates.
-    ///
-    /// When this closure is run, aspects such as the stdio file descriptors and
-    /// working directory have successfully been changed, so output to these
-    /// locations may not appear where intended.
-    unsafe fn pre_exec<F>(&mut self, f: F) -> &mut Command
-    where
-        F: FnMut() -> io::Result<()> + Send + Sync + 'static;
-
     /// Performs all the required setup by this `Command`, followed by calling
     /// the `execvp` syscall.
     ///
@@ -103,14 +70,6 @@ impl CommandExt for Command {
         self
     }
 
-    unsafe fn pre_exec<F>(&mut self, f: F) -> &mut Command
-    where
-        F: FnMut() -> io::Result<()> + Send + Sync + 'static,
-    {
-        self.inner.pre_exec(f);
-        self
-    }
-
     fn exec(&mut self) -> io::Error {
         self.inner.exec()
     }