Auto merge of #82973 - ijackson:exitstatuserror, r=yaahc

Provide ExitStatusError

Closes #73125

In MR #81452 "Add #[must_use] to [...] process::ExitStatus" we concluded that the existing arrangements in are too awkward so adding that `#[must_use]` is blocked on improving the ergonomics.

I wrote a mini-RFC-style discusion of the approach in https://github.com/rust-lang/rust/issues/73125#issuecomment-771092741

1 files changed, 28 insertions, 3 deletions

diff --git a/library/std/src/sys/unix/process/process_unix.rs b/library/std/src/sys/unix/process/process_unix.rs
index 08b500b9c82..7f8065e7500 100644
--- a/library/std/src/sys/unix/process/process_unix.rs
+++ b/library/std/src/sys/unix/process/process_unix.rs
@@ -1,7 +1,9 @@
-use crate::convert::TryInto;
+use crate::convert::{TryFrom, TryInto};
 use crate::fmt;
 use crate::io::{self, Error, ErrorKind};
 use crate::mem;
+use crate::num::NonZeroI32;
+use crate::os::raw::NonZero_c_int;
 use crate::ptr;
 use crate::sys;
 use crate::sys::cvt;
@@ -491,8 +493,16 @@ impl ExitStatus {
         libc::WIFEXITED(self.0)
     }
 
-    pub fn success(&self) -> bool {
-        self.code() == Some(0)
+    pub fn exit_ok(&self) -> Result<(), ExitStatusError> {
+        // This assumes that WIFEXITED(status) && WEXITSTATUS==0 corresponds to status==0.  This is
+        // true on all actual versios of Unix, is widely assumed, and is specified in SuS
+        // https://pubs.opengroup.org/onlinepubs/9699919799/functions/wait.html .  If it is not
+        // true for a platform pretending to be Unix, the tests (our doctests, and also
+        // procsss_unix/tests.rs) will spot it.  `ExitStatusError::code` assumes this too.
+        match NonZero_c_int::try_from(self.0) {
+            /* was nonzero */ Ok(failure) => Err(ExitStatusError(failure)),
+            /* was zero, couldn't convert */ Err(_) => Ok(()),
+        }
     }
 
     pub fn code(&self) -> Option<i32> {
@@ -547,6 +557,21 @@ impl fmt::Display for ExitStatus {
     }
 }
 
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct ExitStatusError(NonZero_c_int);
+
+impl Into<ExitStatus> for ExitStatusError {
+    fn into(self) -> ExitStatus {
+        ExitStatus(self.0.into())
+    }
+}
+
+impl ExitStatusError {
+    pub fn code(self) -> Option<NonZeroI32> {
+        ExitStatus(self.0.into()).code().map(|st| st.try_into().unwrap())
+    }
+}
+
 #[cfg(test)]
 #[path = "process_unix/tests.rs"]
 mod tests;