Provide ExitStatusError

Closes #73125

This is in pursuance of
  Issue #73127 Consider adding #[must_use] to std::process::ExitStatus

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

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

1 files changed, 23 insertions, 3 deletions

diff --git a/library/std/src/sys/unix/process/process_fuchsia.rs b/library/std/src/sys/unix/process/process_fuchsia.rs
index b19ad4ccdc7..507abb27871 100644
--- a/library/std/src/sys/unix/process/process_fuchsia.rs
+++ b/library/std/src/sys/unix/process/process_fuchsia.rs
@@ -1,7 +1,8 @@
-use crate::convert::TryInto;
+use crate::convert::{TryFrom, TryInto};
 use crate::fmt;
 use crate::io;
 use crate::mem;
+use crate::num::{NonZeroI32, NonZeroI64};
 use crate::ptr;
 
 use crate::sys::process::process_common::*;
@@ -236,8 +237,11 @@ impl Process {
 pub struct ExitStatus(i64);
 
 impl ExitStatus {
-    pub fn success(&self) -> bool {
-        self.code() == Some(0)
+    pub fn exit_ok(&self) -> Result<(), ExitStatusError> {
+        match NonZeroI64::try_from(self.0) {
+            /* was nonzero */ Ok(failure) => Err(ExitStatusError(failure)),
+            /* was zero, couldn't convert */ Err(_) => Ok(()),
+        }
     }
 
     pub fn code(&self) -> Option<i32> {
@@ -306,3 +310,19 @@ impl fmt::Display for ExitStatus {
         write!(f, "exit code: {}", self.0)
     }
 }
+
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct ExitStatusError(NonZeroI64);
+
+impl Into<ExitStatus> for ExitStatusError {
+    fn into(self) -> ExitStatus {
+        ExitStatus(self.0.into())
+    }
+}
+
+impl ExitStatusError {
+    pub fn code(self) -> Option<NonZeroI32> {
+        // fixme: affected by the same bug as ExitStatus::code()
+        ExitStatus(self.0.into()).code().map(|st| st.try_into().unwrap())
+    }
+}