CommandExt::before_exec: deprecate safety in edition 2024

Author: RalfJung

library/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,
     {

tests/ui/rust-2024/unsafe-before_exec.e2024.stderr

@@ -0,0 +1,11 @@
+error[E0133]: call to unsafe function `before_exec` is unsafe and requires unsafe block
+  --> $DIR/unsafe-before_exec.rs:14:5
+   |
+LL |     cmd.before_exec(|| Ok(()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0133`.

tests/ui/rust-2024/unsafe-before_exec.rs

@@ -0,0 +1,17 @@
+//@ revisions: e2021 e2024
+//@ only-unix
+//@[e2021] edition: 2021
+//@[e2021] check-pass
+//@[e2024] edition: 2024
+//@[e2024] compile-flags: -Zunstable-options
+
+use std::process::Command;
+use std::os::unix::process::CommandExt;
+
+#[allow(deprecated)]
+fn main() {
+    let mut cmd = Command::new("sleep");
+    cmd.before_exec(|| Ok(()));
+    //[e2024]~^ ERROR call to unsafe function `before_exec` is unsafe
+    drop(cmd); // we don't actually run the command.
+}