Rollup merge of #82411 - ijackson:fix-exitstatus, r=dtolnay

Fixes to ExitStatus and its docs

* On Unix, properly display every possible wait status (and don't panic on weird values)
* In the documentation, be clear and consistent about "exit status" vs "wait status".

library/std/src/sys/unix/process/process_unix.rs

@@ -527,9 +527,22 @@ impl fmt::Display for ExitStatus {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         if let Some(code) = self.code() {
             write!(f, "exit code: {}", code)
+        } else if let Some(signal) = self.signal() {
+            if self.core_dumped() {
+                write!(f, "signal: {} (core dumped)", signal)
+            } else {
+                write!(f, "signal: {}", signal)
+            }
+        } else if let Some(signal) = self.stopped_signal() {
+            write!(f, "stopped (not terminated) by signal: {}", signal)
+        } else if self.continued() {
+            write!(f, "continued (WIFCONTINUED)")
         } else {
-            let signal = self.signal().unwrap();
-            write!(f, "signal: {}", signal)
+            write!(f, "unrecognised wait status: {} {:#x}", self.0, self.0)
         }
     }
 }
+
+#[cfg(test)]
+#[path = "process_unix/tests.rs"]
+mod tests;

library/std/src/sys/unix/process/process_unix/tests.rs

@@ -0,0 +1,30 @@
+#[test]
+fn exitstatus_display_tests() {
+    // In practice this is the same on every Unix.
+    // If some weird platform turns out to be different, and this test fails, use #[cfg].
+    use crate::os::unix::process::ExitStatusExt;
+    use crate::process::ExitStatus;
+
+    let t = |v, s| assert_eq!(s, format!("{}", <ExitStatus as ExitStatusExt>::from_raw(v)));
+
+    t(0x0000f, "signal: 15");
+    t(0x0008b, "signal: 11 (core dumped)");
+    t(0x00000, "exit code: 0");
+    t(0x0ff00, "exit code: 255");
+
+    // On MacOS, 0x0137f is WIFCONTINUED, not WIFSTOPPED.  Probably *BSD is similar.
+    //   https://github.com/rust-lang/rust/pull/82749#issuecomment-790525956
+    // The purpose of this test is to test our string formatting, not our understanding of the wait
+    // status magic numbers.  So restrict these to Linux.
+    if cfg!(target_os = "linux") {
+        t(0x0137f, "stopped (not terminated) by signal: 19");
+        t(0x0ffff, "continued (WIFCONTINUED)");
+    }
+
+    // Testing "unrecognised wait status" is hard because the wait.h macros typically
+    // assume that the value came from wait and isn't mad.  With the glibc I have here
+    // this works:
+    if cfg!(all(target_os = "linux", target_env = "gnu")) {
+        t(0x000ff, "unrecognised wait status: 255 0xff");
+    }
+}