Auto merge of #114589 - ijackson:exit-code-default, r=dtolnay

impl Default for ExitCode

As suggested here
  #106425 (comment)

Needs FCP since this is an insta-stable impl.

Ideally we would have `impl From<ExitCode> for ExitStatus` and implement the default `ExitStatus` using that.   That is sadly not so easy because of the various strange confusions about `ExitCode` (unix: exit status) vs `ExitStatus` (unix: wait status) in the not-really-unix platforms in `library//src/sys/unix/process`.  I'll try to follow that up.

library/std/src/process.rs

@@ -1960,6 +1960,14 @@ impl ExitCode {
     }
 }
 
+/// The default value is [`ExitCode::SUCCESS`]
+#[stable(feature = "process-exitcode-default", since = "CURRENT_RUSTC_VERSION")]
+impl Default for ExitCode {
+    fn default() -> Self {
+        ExitCode::SUCCESS
+    }
+}
+
 #[stable(feature = "process_exitcode", since = "1.61.0")]
 impl From<u8> for ExitCode {
     /// Construct an `ExitCode` from an arbitrary u8 value.