Rollup merge of rust-lang#93445 - yaahc:exitcode-constructor, r=dtolnay

Add From<u8> for ExitCode

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

We decided to stick with `u8` initially since its the common subset between all platforms that we support (excluding wasm which I think only works with `true` or `false`). Posix is supposed to take i32s, but in practice many unix platforms mask out all but the low 8 bits or in some cases the 8-15th bits. Windows takes a u32 instead of an i32. Bourne-compatible shells also report signals as exitcode 128 + `signal_no`, so there's some ambiguity there when returning exit codes > 127, but it is possible to disambiguate them on the other side so we decided against restricting the possible codes further than to `u8`.

## Related

- Detailed analysis of exit code support on various platforms: https://internals.rust-lang.org/t/mini-pre-rfc-redesigning-process-exitstatus/5426
- rust-lang#48711
- rust-lang#43301
- https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Termination.2FExit.20Status.20Stabilization

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,