Rollup merge of #48618 - scottmcm:elaborate-exitcode, r=alexcrichton

Better docs and associated SUCCESS/FAILURE for process::ExitCode

Follow-up to https://github.com/rust-lang/rust/pull/48497#discussion_r170676525, since that PR was the minimal thing to unblock https://github.com/rust-lang/rust/issues/48453#issuecomment-368155082.

r? @nikomatsakis

src/libstd/process.rs

@@ -1080,14 +1080,45 @@ impl fmt::Display for ExitStatus {
     }
 }
 
-/// This is ridiculously unstable, as it's a completely-punted-upon part
-/// of the `?`-in-`main` RFC.  It's here only to allow experimenting with
-/// returning a code directly from main.  It will definitely change
-/// drastically before being stabilized, if it doesn't just get deleted.
-#[doc(hidden)]
+/// This type represents the status code a process can return to its
+/// parent under normal termination.
+///
+/// Numeric values used in this type don't have portable meanings, and
+/// different platforms may mask different amounts of them.
+///
+/// For the platform's canonical successful and unsuccessful codes, see
+/// the [`SUCCESS`] and [`FAILURE`] associated items.
+///
+/// [`SUCCESS`]: #associatedconstant.SUCCESS
+/// [`FAILURE`]: #associatedconstant.FAILURE
+///
+/// **Warning**: While various forms of this were discussed in [RFC #1937],
+/// it was ultimately cut from that RFC, and thus this type is more subject
+/// to change even than the usual unstable item churn.
+///
+/// [RFC #1937]: https://github.com/rust-lang/rfcs/pull/1937
 #[derive(Clone, Copy, Debug)]
-#[unstable(feature = "process_exitcode_placeholder", issue = "43301")]
-pub struct ExitCode(pub i32);
+#[unstable(feature = "process_exitcode_placeholder", issue = "48711")]
+pub struct ExitCode(imp::ExitCode);
+
+#[unstable(feature = "process_exitcode_placeholder", issue = "48711")]
+impl ExitCode {
+    /// The canonical ExitCode for successful termination on this platform.
+    ///
+    /// Note that a `()`-returning `main` implicitly results in a successful
+    /// termination, so there's no need to return this from `main` unless
+    /// you're also returning other possible codes.
+    #[unstable(feature = "process_exitcode_placeholder", issue = "48711")]
+    pub const SUCCESS: ExitCode = ExitCode(imp::ExitCode::SUCCESS);
+
+    /// The canonical ExitCode for unsuccessful termination on this platform.
+    ///
+    /// If you're only returning this and `SUCCESS` from `main`, consider
+    /// instead returning `Err(_)` and `Ok(())` respectively, which will
+    /// return the same codes (but will also `eprintln!` the error).
+    #[unstable(feature = "process_exitcode_placeholder", issue = "48711")]
+    pub const FAILURE: ExitCode = ExitCode(imp::ExitCode::FAILURE);
+}
 
 impl Child {
     /// Forces the child to exit. This is equivalent to sending a
@@ -1401,18 +1432,6 @@ pub fn id() -> u32 {
     ::sys::os::getpid()
 }
 
-#[cfg(target_arch = "wasm32")]
-mod exit {
-    pub const SUCCESS: i32 = 0;
-    pub const FAILURE: i32 = 1;
-}
-#[cfg(not(target_arch = "wasm32"))]
-mod exit {
-    use libc;
-    pub const SUCCESS: i32 = libc::EXIT_SUCCESS;
-    pub const FAILURE: i32 = libc::EXIT_FAILURE;
-}
-
 /// A trait for implementing arbitrary return types in the `main` function.
 ///
 /// The c-main function only supports to return integers as return type.
@@ -1433,18 +1452,15 @@ pub trait Termination {
 
 #[unstable(feature = "termination_trait_lib", issue = "43301")]
 impl Termination for () {
-    fn report(self) -> i32 { exit::SUCCESS }
+    fn report(self) -> i32 { ExitCode::SUCCESS.report() }
 }
 
 #[unstable(feature = "termination_trait_lib", issue = "43301")]
 impl<E: fmt::Debug> Termination for Result<(), E> {
     fn report(self) -> i32 {
         match self {
-            Ok(val) => val.report(),
-            Err(err) => {
-                eprintln!("Error: {:?}", err);
-                exit::FAILURE
-            }
+            Ok(()) => ().report(),
+            Err(err) => Err::<!, _>(err).report(),
         }
     }
 }
@@ -1459,15 +1475,14 @@ impl<E: fmt::Debug> Termination for Result<!, E> {
     fn report(self) -> i32 {
         let Err(err) = self;
         eprintln!("Error: {:?}", err);
-        exit::FAILURE
+        ExitCode::FAILURE.report()
     }
 }
 
 #[unstable(feature = "termination_trait_lib", issue = "43301")]
 impl Termination for ExitCode {
     fn report(self) -> i32 {
-        let ExitCode(code) = self;
-        code
+        self.0.as_i32()
     }
 }
 

src/libstd/sys/cloudabi/shims/process.rs

@@ -126,6 +126,18 @@ impl fmt::Display for ExitStatus {
     }
 }
 
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct ExitCode(bool);
+
+impl ExitCode {
+    pub const SUCCESS: ExitCode = ExitCode(false);
+    pub const FAILURE: ExitCode = ExitCode(true);
+
+    pub fn as_i32(&self) -> i32 {
+        self.0 as i32
+    }
+}
+
 pub struct Process(Void);
 
 impl Process {

src/libstd/sys/redox/process.rs

@@ -13,6 +13,7 @@ use ffi::OsStr;
 use os::unix::ffi::OsStrExt;
 use fmt;
 use io::{self, Error, ErrorKind};
+use libc::{EXIT_SUCCESS, EXIT_FAILURE};
 use path::{Path, PathBuf};
 use sys::fd::FileDesc;
 use sys::fs::{File, OpenOptions};
@@ -480,6 +481,18 @@ impl fmt::Display for ExitStatus {
     }
 }
 
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct ExitCode(u8);
+
+impl ExitCode {
+    pub const SUCCESS: ExitCode = ExitCode(EXIT_SUCCESS as _);
+    pub const FAILURE: ExitCode = ExitCode(EXIT_FAILURE as _);
+
+    pub fn as_i32(&self) -> i32 {
+        self.0 as i32
+    }
+}
+
 /// The unique id of the process (this should never be negative).
 pub struct Process {
     pid: usize,

src/libstd/sys/unix/process/mod.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-pub use self::process_common::{Command, ExitStatus, Stdio, StdioPipes};
+pub use self::process_common::{Command, ExitStatus, ExitCode, Stdio, StdioPipes};
 pub use self::process_inner::Process;
 
 mod process_common;

src/libstd/sys/unix/process/process_common.rs

@@ -13,7 +13,7 @@ use os::unix::prelude::*;
 use ffi::{OsString, OsStr, CString, CStr};
 use fmt;
 use io;
-use libc::{self, c_int, gid_t, uid_t, c_char};
+use libc::{self, c_int, gid_t, uid_t, c_char, EXIT_SUCCESS, EXIT_FAILURE};
 use ptr;
 use sys::fd::FileDesc;
 use sys::fs::{File, OpenOptions};
@@ -393,6 +393,18 @@ impl fmt::Display for ExitStatus {
     }
 }
 
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct ExitCode(u8);
+
+impl ExitCode {
+    pub const SUCCESS: ExitCode = ExitCode(EXIT_SUCCESS as _);
+    pub const FAILURE: ExitCode = ExitCode(EXIT_FAILURE as _);
+
+    pub fn as_i32(&self) -> i32 {
+        self.0 as i32
+    }
+}
+
 #[cfg(all(test, not(target_os = "emscripten")))]
 mod tests {
     use super::*;

src/libstd/sys/wasm/process.rs

@@ -129,6 +129,18 @@ impl fmt::Display for ExitStatus {
     }
 }
 
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct ExitCode(bool);
+
+impl ExitCode {
+    pub const SUCCESS: ExitCode = ExitCode(false);
+    pub const FAILURE: ExitCode = ExitCode(true);
+
+    pub fn as_i32(&self) -> i32 {
+        self.0 as i32
+    }
+}
+
 pub struct Process(Void);
 
 impl Process {

src/libstd/sys/windows/process.rs

@@ -18,7 +18,7 @@ use ffi::{OsString, OsStr};
 use fmt;
 use fs;
 use io::{self, Error, ErrorKind};
-use libc::c_void;
+use libc::{c_void, EXIT_SUCCESS, EXIT_FAILURE};
 use mem;
 use os::windows::ffi::OsStrExt;
 use path::Path;
@@ -408,6 +408,18 @@ impl fmt::Display for ExitStatus {
     }
 }
 
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct ExitCode(c::DWORD);
+
+impl ExitCode {
+    pub const SUCCESS: ExitCode = ExitCode(EXIT_SUCCESS as _);
+    pub const FAILURE: ExitCode = ExitCode(EXIT_FAILURE as _);
+
+    pub fn as_i32(&self) -> i32 {
+        self.0 as i32
+    }
+}
+
 fn zeroed_startupinfo() -> c::STARTUPINFO {
     c::STARTUPINFO {
         cb: 0,

src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-exitcode.rs

@@ -14,5 +14,5 @@
 use std::process::ExitCode;
 
 fn main() -> ExitCode {
-    ExitCode(0)
+    ExitCode::SUCCESS
 }