Add From<u8> for ExitCode

This should cover a mostly cross-platform subset of supported exit codes.

library/std/src/process.rs

@@ -1691,6 +1691,14 @@ impl ExitCode {
     }
 }
 
+#[unstable(feature = "process_exitcode_placeholder", issue = "48711")]
+impl From<u8> for ExitCode {
+    /// Construct an exit code from an arbitrary u8 value.
+    fn from(code: u8) -> Self {
+        ExitCode(imp::ExitCode::from(code))
+    }
+}
+
 impl Child {
     /// Forces the child process to exit. If the child has already exited, an [`InvalidInput`]
     /// error is returned.

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

@@ -476,6 +476,12 @@ impl ExitCode {
     }
 }
 
+impl From<u8> for ExitCode {
+    fn from(code: u8) -> Self {
+        Self(code)
+    }
+}
+
 pub struct CommandArgs<'a> {
     iter: crate::slice::Iter<'a, CString>,
 }

library/std/src/sys/unsupported/process.rs

@@ -162,6 +162,15 @@ impl ExitCode {
     }
 }
 
+impl From<u8> for ExitCode {
+    fn from(code: u8) -> Self {
+        match code {
+            0 => Self::SUCCESS,
+            1..255 => Self::FAILURE,
+        }
+    }
+}
+
 pub struct Process(!);
 
 impl Process {

library/std/src/sys/windows/process.rs

@@ -666,6 +666,12 @@ impl ExitCode {
     }
 }
 
+impl From<u8> for ExitCode {
+    fn from(code: u8) -> Self {
+        ExitCode(c::DWORD::from(code))
+    }
+}
+
 fn zeroed_startupinfo() -> c::STARTUPINFO {
     c::STARTUPINFO {
         cb: 0,