Rollup merge of rust-lang#125970 - RalfJung:before_exec, r=m-ou-se

CommandExt::before_exec: deprecate safety in edition 2024

Similar to `set_var`, we had to find out after 1.0 was released that `before_exec` should have been unsafe. We partially rectified this by deprecating that function a long time ago, but since rust-lang#124636 we have the ability to also deprecate the safety of the old function and make it a *hard error* to call the old function outside `unsafe` in the next edition. So just in case anyone still uses the old function, let's ensure this can't be ignored when moving code to the new edition.

Cc `@rust-lang/libs-api`

Tracking:

- rust-lang#124866

Author: matthiaskrgr

Diff for: std/src/os/unix/process.rs

@@ -109,13 +109,21 @@ pub trait CommandExt: Sealed {
     /// Schedules a closure to be run just before the `exec` function is
     /// invoked.
     ///
-    /// This method is stable and usable, but it should be unsafe. To fix
-    /// that, it got deprecated in favor of the unsafe [`pre_exec`].
+    /// `before_exec` used to be a safe method, but it needs to be unsafe since the closure may only
+    /// perform operations that are *async-signal-safe*. Hence it got deprecated in favor of the
+    /// unsafe [`pre_exec`]. Meanwhile, Rust gained the ability to make an existing safe method
+    /// fully unsafe in a new edition, which is how `before_exec` became `unsafe`. It still also
+    /// remains deprecated; `pre_exec` should be used instead.
     ///
     /// [`pre_exec`]: CommandExt::pre_exec
     #[stable(feature = "process_exec", since = "1.15.0")]
     #[deprecated(since = "1.37.0", note = "should be unsafe, use `pre_exec` instead")]
-    fn before_exec<F>(&mut self, f: F) -> &mut process::Command
+    #[cfg_attr(bootstrap, rustc_deprecated_safe_2024)]
+    #[cfg_attr(
+        not(bootstrap),
+        rustc_deprecated_safe_2024(audit_that = "the closure is async-signal-safe")
+    )]
+    unsafe fn before_exec<F>(&mut self, f: F) -> &mut process::Command
     where
         F: FnMut() -> io::Result<()> + Send + Sync + 'static,
     {