Auto merge of #46479 - bkchr:termination_trait, r=arielb1

Implements RFC 1937: `?` in `main`

This is the first part of the RFC 1937 that supports new
`Termination` trait in the rust `main` function.

Thanks @nikomatsakis, @arielb1 and all other people in the gitter channel for all your help!

The support for doctest and `#[test]` is still missing, bu as @nikomatsakis said, smaller pull requests are better :)

1 files changed, 86 insertions, 0 deletions

diff --git a/src/libstd/termination.rs b/src/libstd/termination.rs
new file mode 100644
index 00000000000..61137ba4922
--- /dev/null
+++ b/src/libstd/termination.rs
@@ -0,0 +1,86 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use error::Error;
+#[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.
+/// So, every type implementing the `Termination` trait has to be converted
+/// to an integer.
+///
+/// The default implementations are returning `libc::EXIT_SUCCESS` to indicate
+/// a successful execution. In case of a failure, `libc::EXIT_FAILURE` is returned.
+#[cfg_attr(not(any(stage0, test)), lang = "termination")]
+#[unstable(feature = "termination_trait", issue = "43301")]
+#[rustc_on_unimplemented =
+  "`main` can only return types that implement {Termination}, not `{Self}`"]
+pub trait Termination {
+    /// Is called to get the representation of the value as status code.
+    /// This status code is returned to the operating system.
+    fn report(self) -> i32;
+}
+
+#[unstable(feature = "termination_trait", issue = "43301")]
+impl Termination for () {
+    fn report(self) -> i32 { exit::SUCCESS }
+}
+
+#[unstable(feature = "termination_trait", issue = "43301")]
+impl<T: Termination, E: Error> Termination for Result<T, E> {
+    fn report(self) -> i32 {
+        match self {
+            Ok(val) => val.report(),
+            Err(err) => {
+                print_error(err);
+                exit::FAILURE
+            }
+        }
+    }
+}
+
+#[unstable(feature = "termination_trait", issue = "43301")]
+fn print_error<E: Error>(err: E) {
+    eprintln!("Error: {}", err.description());
+
+    if let Some(ref err) = err.cause() {
+        eprintln!("Caused by: {}", err.description());
+    }
+}
+
+#[unstable(feature = "termination_trait", issue = "43301")]
+impl Termination for ! {
+    fn report(self) -> i32 { unreachable!(); }
+}
+
+#[unstable(feature = "termination_trait", issue = "43301")]
+impl Termination for bool {
+    fn report(self) -> i32 {
+        if self { exit::SUCCESS } else { exit::FAILURE }
+    }
+}
+
+#[unstable(feature = "termination_trait", issue = "43301")]
+impl Termination for i32 {
+    fn report(self) -> i32 {
+        self
+    }
+}