wip

src/librustc/ty/sty.rs

@@ -1299,6 +1299,13 @@ impl<'a, 'gcx, 'tcx> TyS<'tcx> {
         }
     }
 
+    pub fn is_ty_anon(&self) -> bool {
+        match self.sty {
+            TyAnon(..) => true,
+            _ => false,
+        }
+    }
+
     pub fn is_ty_infer(&self) -> bool {
         match self.sty {
             TyInfer(_) => true,

src/librustc_typeck/check/mod.rs

@@ -1132,9 +1132,17 @@ fn check_fn<'a, 'gcx, 'tcx>(inherited: &'a Inherited<'a, 'gcx, 'tcx>,
             if id == fn_id {
                 match entry_type {
                     config::EntryMain => {
+                        let return_ty_span = decl.output.span();
+                        if ret_ty.is_ty_anon() {
+                            struct_span_err!(fcx.sess(),
+                                        return_ty_span,
+                                        E0912,
+                                        "impl Trait is not allowed in the return type of `main`")
+                                        .help("consider using `()`, or a `Result`")
+                                        .emit();
+                        }
                         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);
 

src/librustc_typeck/diagnostics.rs

@@ -4770,4 +4770,5 @@ register_diagnostics! {
     E0641, // cannot cast to/from a pointer with an unknown kind
     E0645, // trait aliases not finished
     E0907, // type inside generator must be known in this context
+    E0912, // impl Trait cannot be returned from main
 }