Rollup merge of #49162 - tmandry:stabilize-termination-trait, r=nikomatsakis

Stabilize termination_trait, split out termination_trait_test

For #48453.

First time contribution, so I'd really appreciate any feedback on how this PR can be better.

Not sure exactly what kind of documentation update is needed. If there is no PR to update the reference, I can try doing that this week as I have time.

20 files changed, 85 insertions, 61 deletions

diff --git a/src/librustc/diagnostics.rs b/src/librustc/diagnostics.rs
index 2fd875c3447..e22e2b55703 100644
--- a/src/librustc/diagnostics.rs
+++ b/src/librustc/diagnostics.rs
@@ -1764,12 +1764,12 @@ The `main` function was incorrectly declared.
 Erroneous code example:
 
 ```compile_fail,E0580
-fn main() -> i32 { // error: main function has wrong type
-    0
+fn main(x: i32) { // error: main function has wrong type
+    println!("{}", x);
 }
 ```
 
-The `main` function prototype should never take arguments or return type.
+The `main` function prototype should never take arguments.
 Example:
 
 ```
diff --git a/src/librustc/traits/error_reporting.rs b/src/librustc/traits/error_reporting.rs
index 79d5cf79359..e640cc315c4 100644
--- a/src/librustc/traits/error_reporting.rs
+++ b/src/librustc/traits/error_reporting.rs
@@ -595,20 +595,23 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                                          trait_ref.to_predicate(), post_message)
                             }));
 
+                        let explanation =
+                            if obligation.cause.code == ObligationCauseCode::MainFunctionType {
+                                "consider using `()`, or a `Result`".to_owned()
+                            } else {
+                                format!("{}the trait `{}` is not implemented for `{}`",
+                                        pre_message,
+                                        trait_ref,
+                                        trait_ref.self_ty())
+                            };
+
                         if let Some(ref s) = label {
                             // If it has a custom "#[rustc_on_unimplemented]"
                             // error message, let's display it as the label!
                             err.span_label(span, s.as_str());
-                            err.help(&format!("{}the trait `{}` is not implemented for `{}`",
-                                              pre_message,
-                                              trait_ref,
-                                              trait_ref.self_ty()));
+                            err.help(&explanation);
                         } else {
-                            err.span_label(span,
-                                           &*format!("{}the trait `{}` is not implemented for `{}`",
-                                                     pre_message,
-                                                     trait_ref,
-                                                     trait_ref.self_ty()));
+                            err.span_label(span, explanation);
                         }
                         if let Some(ref s) = note {
                             // If it has a custom "#[rustc_on_unimplemented]" note, let's display it
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 9afa2d42fa4..f820b0ba16b 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -1130,25 +1130,23 @@ fn check_fn<'a, 'gcx, 'tcx>(inherited: &'a Inherited<'a, 'gcx, 'tcx>,
     }
     fcx.demand_suptype(span, ret_ty, actual_return_ty);
 
-    if fcx.tcx.features().termination_trait {
-        // If the termination trait language item is activated, check that the main return type
-        // implements the termination trait.
-        if let Some(term_id) = fcx.tcx.lang_items().termination() {
-            if let Some((id, _)) = *fcx.tcx.sess.entry_fn.borrow() {
-                if id == fn_id {
-                    match fcx.sess().entry_type.get() {
-                        Some(config::EntryMain) => {
-                            let substs = fcx.tcx.mk_substs(iter::once(Kind::from(ret_ty)));
-                            let trait_ref = ty::TraitRef::new(term_id, substs);
-                            let cause = traits::ObligationCause::new(
-                                span, fn_id, ObligationCauseCode::MainFunctionType);
-
-                            inherited.register_predicate(
-                                traits::Obligation::new(
-                                    cause, param_env, trait_ref.to_predicate()));
-                        },
-                        _ => {},
-                    }
+    // Check that the main return type implements the termination trait.
+    if let Some(term_id) = fcx.tcx.lang_items().termination() {
+        if let Some((id, _)) = *fcx.tcx.sess.entry_fn.borrow() {
+            if id == fn_id {
+                match fcx.sess().entry_type.get() {
+                    Some(config::EntryMain) => {
+                        let substs = fcx.tcx.mk_substs(iter::once(Kind::from(ret_ty)));
+                        let trait_ref = ty::TraitRef::new(term_id, substs);
+                        let return_ty_span = decl.output.span();
+                        let cause = traits::ObligationCause::new(
+                            return_ty_span, fn_id, ObligationCauseCode::MainFunctionType);
+
+                        inherited.register_predicate(
+                            traits::Obligation::new(
+                                cause, param_env, trait_ref.to_predicate()));
+                    },
+                    _ => {},
                 }
             }
         }
diff --git a/src/librustc_typeck/lib.rs b/src/librustc_typeck/lib.rs
index 9f98932f24b..cd21d53b837 100644
--- a/src/librustc_typeck/lib.rs
+++ b/src/librustc_typeck/lib.rs
@@ -208,8 +208,7 @@ fn check_main_fn_ty<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             }
 
             let actual = tcx.fn_sig(main_def_id);
-            let expected_return_type = if tcx.lang_items().termination().is_some()
-                && tcx.features().termination_trait {
+            let expected_return_type = if tcx.lang_items().termination().is_some() {
                 // we take the return type of the given main function, the real check is done
                 // in `check_fn`
                 actual.output().skip_binder()
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index 36eb7291822..0b06c5d4d65 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -308,7 +308,6 @@
 #![feature(str_char)]
 #![feature(str_internals)]
 #![feature(str_utf16)]
-#![feature(termination_trait)]
 #![feature(test, rustc_private)]
 #![feature(thread_local)]
 #![feature(toowned_clone_into)]
@@ -325,6 +324,7 @@
 #![cfg_attr(test, feature(update_panic_count))]
 #![cfg_attr(windows, feature(used))]
 #![cfg_attr(stage0, feature(never_type))]
+#![cfg_attr(stage0, feature(termination_trait))]
 
 #![default_lib_allocator]
 
diff --git a/src/libstd/process.rs b/src/libstd/process.rs
index d5ac2d19e83..c877bf6aa35 100644
--- a/src/libstd/process.rs
+++ b/src/libstd/process.rs
@@ -1442,8 +1442,9 @@ pub fn id() -> u32 {
 /// a successful execution. In case of a failure, `libc::EXIT_FAILURE` is returned.
 #[cfg_attr(not(test), lang = "termination")]
 #[unstable(feature = "termination_trait_lib", issue = "43301")]
-#[rustc_on_unimplemented =
-  "`main` can only return types that implement {Termination}, not `{Self}`"]
+#[rustc_on_unimplemented(
+  message="`main` has invalid return type `{Self}`",
+  label="`main` can only return types that implement {Termination}")]
 pub trait Termination {
     /// Is called to get the representation of the value as status code.
     /// This status code is returned to the operating system.
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 69612054ae3..35642312e6d 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -428,8 +428,8 @@ declare_features! (
     // `foo.rs` as an alternative to `foo/mod.rs`
     (active, non_modrs_mods, "1.24.0", Some(44660), None),
 
-    // Termination trait in main (RFC 1937)
-    (active, termination_trait, "1.24.0", Some(43301), None),
+    // Termination trait in tests (RFC 1937)
+    (active, termination_trait_test, "1.24.0", Some(48854), None),
 
     // Allows use of the :lifetime macro fragment specifier
     (active, macro_lifetime_matcher, "1.24.0", Some(46895), None),
@@ -565,6 +565,8 @@ declare_features! (
     (accepted, inclusive_range_syntax, "1.26.0", Some(28237), None),
     // allow `..=` in patterns (RFC 1192)
     (accepted, dotdoteq_in_patterns, "1.26.0", Some(28237), None),
+    // Termination trait in main (RFC 1937)
+    (accepted, termination_trait, "1.26.0", Some(43301), None),
 );
 
 // If you change this, please modify src/doc/unstable-book as well. You must
diff --git a/src/libsyntax/test.rs b/src/libsyntax/test.rs
index 5264b627e96..6c01171d100 100644
--- a/src/libsyntax/test.rs
+++ b/src/libsyntax/test.rs
@@ -332,7 +332,7 @@ fn is_test_fn(cx: &TestCtxt, i: &ast::Item) -> bool {
             ast::ItemKind::Fn(ref decl, _, _, _, ref generics, _) => {
                 // If the termination trait is active, the compiler will check that the output
                 // type implements the `Termination` trait as `libtest` enforces that.
-                let output_matches = if cx.features.termination_trait {
+                let output_matches = if cx.features.termination_trait_test {
                     true
                 } else {
                     let no_output = match decl.output {
@@ -359,7 +359,7 @@ fn is_test_fn(cx: &TestCtxt, i: &ast::Item) -> bool {
         match has_test_signature(cx, i) {
             Yes => true,
             No => {
-                if cx.features.termination_trait {
+                if cx.features.termination_trait_test {
                     diag.span_err(i.span, "functions used as tests can not have any arguments");
                 } else {
                     diag.span_err(i.span, "functions used as tests must have signature fn() -> ()");
@@ -388,7 +388,7 @@ fn is_bench_fn(cx: &TestCtxt, i: &ast::Item) -> bool {
 
                 // If the termination trait is active, the compiler will check that the output
                 // type implements the `Termination` trait as `libtest` enforces that.
-                let output_matches = if cx.features.termination_trait {
+                let output_matches = if cx.features.termination_trait_test {
                     true
                 } else {
                     let no_output = match decl.output {
@@ -416,7 +416,7 @@ fn is_bench_fn(cx: &TestCtxt, i: &ast::Item) -> bool {
     if has_bench_attr && !has_bench_signature {
         let diag = cx.span_diagnostic;
 
-        if cx.features.termination_trait {
+        if cx.features.termination_trait_test {
             diag.span_err(i.span, "functions used as benches must have signature \
                                    `fn(&mut Bencher) -> impl Termination`");
         } else {
diff --git a/src/test/compile-fail/feature-gate-termination_trait_test.rs b/src/test/compile-fail/feature-gate-termination_trait_test.rs
new file mode 100644
index 00000000000..4af7e946716
--- /dev/null
+++ b/src/test/compile-fail/feature-gate-termination_trait_test.rs
@@ -0,0 +1,22 @@
+// 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.
+
+// compile-flags: --test
+
+fn main() {}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn it_works() -> Result<(), ()> {
+    //~^ ERROR functions used as tests must have signature fn() -> ()
+        Ok(())
+    }
+}
diff --git a/src/test/compile-fail/feature-gate-termination_trait.rs b/src/test/compile-fail/rfc-1937-termination-trait/termination-trait-main-i32.rs
index 5a56445b64e..0e6ddf7c92f 100644
--- a/src/test/compile-fail/feature-gate-termination_trait.rs
+++ b/src/test/compile-fail/rfc-1937-termination-trait/termination-trait-main-i32.rs
@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-fn main() -> i32 { //~ ERROR main function has wrong type [E0580]
+fn main() -> i32 {
+//~^ ERROR `main` has invalid return type `i32`
+//~| NOTE `main` can only return types that implement std::process::Termination
+//~| HELP consider using `()`, or a `Result`
     0
 }
diff --git a/src/test/compile-fail/rfc-1937-termination-trait/termination-trait-not-satisfied.rs b/src/test/compile-fail/rfc-1937-termination-trait/termination-trait-not-satisfied.rs
index e87e0ceebf1..b5f5472b492 100644
--- a/src/test/compile-fail/rfc-1937-termination-trait/termination-trait-not-satisfied.rs
+++ b/src/test/compile-fail/rfc-1937-termination-trait/termination-trait-not-satisfied.rs
@@ -8,10 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(termination_trait)]
-
 struct ReturnType {}
 
-fn main() -> ReturnType { //~ ERROR `ReturnType: std::process::Termination` is not satisfied
+fn main() -> ReturnType { //~ ERROR `main` has invalid return type `ReturnType`
     ReturnType {}
 }
diff --git a/src/test/run-fail/rfc-1937-termination-trait/termination-trait-for-never.rs b/src/test/run-fail/rfc-1937-termination-trait/termination-trait-for-never.rs
index c1dd44a9176..863de85af88 100644
--- a/src/test/run-fail/rfc-1937-termination-trait/termination-trait-for-never.rs
+++ b/src/test/run-fail/rfc-1937-termination-trait/termination-trait-for-never.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(termination_trait)]
-
 // error-pattern:oh, dear
 
 fn main() -> ! {
diff --git a/src/test/run-fail/rfc-1937-termination-trait/termination-trait-for-result-box-error_err.rs b/src/test/run-fail/rfc-1937-termination-trait/termination-trait-for-result-box-error_err.rs
index 8ce27c0a062..0c6cb4de956 100644
--- a/src/test/run-fail/rfc-1937-termination-trait/termination-trait-for-result-box-error_err.rs
+++ b/src/test/run-fail/rfc-1937-termination-trait/termination-trait-for-result-box-error_err.rs
@@ -11,8 +11,6 @@
 // must-compile-successfully
 // failure-status: 1
 
-#![feature(termination_trait)]
-
 use std::io::{Error, ErrorKind};
 
 fn main() -> Result<(), Box<Error>> {
diff --git a/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-empty.rs b/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-empty.rs
index 5e534da0128..046d27a9f0f 100644
--- a/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-empty.rs
+++ b/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-empty.rs
@@ -8,6 +8,4 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(termination_trait)]
-
 fn main() {}
diff --git a/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-exitcode.rs b/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-exitcode.rs
index 80fa4d17b61..4aa7d8c3a77 100644
--- a/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-exitcode.rs
+++ b/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-exitcode.rs
@@ -8,7 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(termination_trait)]
 #![feature(process_exitcode_placeholder)]
 
 use std::process::ExitCode;
diff --git a/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-result-box-error_ok.rs b/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-result-box-error_ok.rs
index 269ac451cf4..33686ed0b8f 100644
--- a/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-result-box-error_ok.rs
+++ b/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-result-box-error_ok.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(termination_trait)]
-
 use std::io::Error;
 
 fn main() -> Result<(), Box<Error>> {
diff --git a/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-result.rs b/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-result.rs
index 751db0fb500..1c87e31e763 100644
--- a/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-result.rs
+++ b/src/test/run-pass/rfc-1937-termination-trait/termination-trait-for-result.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(termination_trait)]
-
 use std::io::Error;
 
 fn main() -> Result<(), Error> {
diff --git a/src/test/run-pass/rfc-1937-termination-trait/termination-trait-in-test.rs b/src/test/run-pass/rfc-1937-termination-trait/termination-trait-in-test.rs
index 494500d522a..11997eb6917 100644
--- a/src/test/run-pass/rfc-1937-termination-trait/termination-trait-in-test.rs
+++ b/src/test/run-pass/rfc-1937-termination-trait/termination-trait-in-test.rs
@@ -10,7 +10,7 @@
 
 // compile-flags: --test
 
-#![feature(termination_trait)]
+#![feature(termination_trait_test)]
 #![feature(test)]
 
 extern crate test;
diff --git a/src/test/compile-fail/rfc-1937-termination-trait/termination-trait-main-wrong-type.rs b/src/test/ui/rfc-1937-termination-trait/termination-trait-main-wrong-type.rs
index 93e2561adf7..425f51ca2fb 100644
--- a/src/test/compile-fail/rfc-1937-termination-trait/termination-trait-main-wrong-type.rs
+++ b/src/test/ui/rfc-1937-termination-trait/termination-trait-main-wrong-type.rs
@@ -7,9 +7,7 @@
 // <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.
-#![feature(termination_trait)]
 
-fn main() -> char {
-//~^ ERROR: the trait bound `char: std::process::Termination` is not satisfied
+fn main() -> char { //~ ERROR
     ' '
 }
diff --git a/src/test/ui/rfc-1937-termination-trait/termination-trait-main-wrong-type.stderr b/src/test/ui/rfc-1937-termination-trait/termination-trait-main-wrong-type.stderr
new file mode 100644
index 00000000000..5109d9275c5
--- /dev/null
+++ b/src/test/ui/rfc-1937-termination-trait/termination-trait-main-wrong-type.stderr
@@ -0,0 +1,11 @@
+error[E0277]: `main` has invalid return type `char`
+  --> $DIR/termination-trait-main-wrong-type.rs:11:14
+   |
+LL | fn main() -> char { //~ ERROR
+   |              ^^^^ `main` can only return types that implement std::process::Termination
+   |
+   = help: consider using `()`, or a `Result`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.