Rollup merge of rust-lang#49461 - andreastt:child-kill-exited, r=Mark-Simulacrum

std: Child::kill() returns error if process has already exited

This patch makes it clear in std::process::Child::kill()'s API
documentation that an error is returned if the child process has
already cleanly exited.  This is implied by the example, but not
called out explicitly.

Author: kennytm

src/libstd/process.rs

@@ -1121,8 +1121,13 @@ impl ExitCode {
 }
 
 impl Child {
-    /// Forces the child to exit. This is equivalent to sending a
-    /// SIGKILL on unix platforms.
+    /// Forces the child process to exit. If the child has already exited, an [`InvalidInput`]
+    /// error is returned.
+    ///
+    /// The mapping to [`ErrorKind`]s is not part of the compatibility contract of the function,
+    /// especially the [`Other`] kind might change to more specific kinds in the future.
+    ///
+    /// This is equivalent to sending a SIGKILL on Unix platforms.
     ///
     /// # Examples
     ///
@@ -1138,6 +1143,10 @@ impl Child {
     ///     println!("yes command didn't start");
     /// }
     /// ```
+    ///
+    /// [`ErrorKind`]: ../io/enum.ErrorKind.html
+    /// [`InvalidInput`]: ../io/enum.ErrorKind.html#variant.InvalidInput
+    /// [`Other`]: ../io/enum.ErrorKind.html#variant.Other
     #[stable(feature = "process", since = "1.0.0")]
     pub fn kill(&mut self) -> io::Result<()> {
         self.handle.kill()