termination_trait: Make error message more helpful

Author: tmandry

src/librustc/traits/error_reporting.rs

@@ -585,17 +585,15 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                                          trait_ref.to_predicate(), post_message)
                             }));
 
-                        let explanation = match obligation.cause.code {
-                            ObligationCauseCode::MainFunctionType => {
+                        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())
-                            }
-                        };
+                                        pre_message,
+                                        trait_ref,
+                                        trait_ref.self_ty())
+                            };
 
                         if let Some(ref s) = label {
                             // If it has a custom "#[rustc_on_unimplemented]"

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.

src/test/compile-fail/rfc-1937-termination-trait/termination-trait-main-i32.rs

@@ -9,7 +9,8 @@
 // except according to those terms.
 
 fn main() -> i32 {
-//~^ ERROR `i32: std::process::Termination` is not satisfied
-//~| NOTE `main` can only return types that implement std::process::Termination, not `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
 }

src/test/compile-fail/rfc-1937-termination-trait/termination-trait-not-satisfied.rs

@@ -10,6 +10,6 @@
 
 struct ReturnType {}
 
-fn main() -> ReturnType { //~ ERROR `ReturnType: std::process::Termination` is not satisfied
+fn main() -> ReturnType { //~ ERROR `main` has invalid return type `ReturnType`
     ReturnType {}
 }

src/test/ui/rfc-1937-termination-trait/termination-trait-main-wrong-type.stderr

@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `char: std::process::Termination` is not satisfied
+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, not `char`
+   |              ^^^^ `main` can only return types that implement std::process::Termination
    |
    = help: consider using `()`, or a `Result`