typeck: Report main return type errors on return type span

src/librustc_typeck/check/mod.rs

@@ -1114,8 +1114,9 @@ fn check_fn<'a, 'gcx, 'tcx>(inherited: &'a Inherited<'a, 'gcx, 'tcx>,
                     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(
-                            span, fn_id, ObligationCauseCode::MainFunctionType);
+                            return_ty_span, fn_id, ObligationCauseCode::MainFunctionType);
 
                         inherited.register_predicate(
                             traits::Obligation::new(

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

@@ -8,7 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern:`main` can only return types that implement std::process::Termination, not `char
-fn main() -> char {
+fn main() -> char { //~ ERROR
     ' '
 }

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

@@ -0,0 +1,11 @@
+error[E0277]: the trait bound `char: std::process::Termination` is not satisfied
+  --> $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`
+   |
+   = help: the trait `std::process::Termination` is not implemented for `char`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.