Detect invalid ABIs on methods and throw an error.

Trying to call methods with a non-Rust ABI on a trait object results in
an ICE. While we could try to translate them, it makes more sense to
just reject non-Rust ABIs in type-check.

This is technically a [breaking-change] due to that fact that we didn't
ICE when statically dispatching these method calls.

Fixes rust-lang#26049

Author: Aatch

src/librustc_typeck/collect.rs

@@ -622,6 +622,10 @@ fn convert_method<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
         astconv::ty_of_method(&ccx.icx(&(rcvr_ty_predicates, &sig.generics)),
                               sig, untransformed_rcvr_ty);
 
+    if !(fty.abi == abi::Rust || fty.abi == abi::RustCall) {
+
+    }
+
     let def_id = local_def(id);
     let ty_method = ty::Method::new(ident.name,
                                     ty_generics,
@@ -759,6 +763,11 @@ fn convert_methods<'a,'tcx,'i,I>(ccx: &CrateCtxt<'a, 'tcx>,
             span_err!(tcx.sess, span, E0201, "duplicate {}", fn_desc);
         }
 
+        if !(sig.abi == abi::Rust || sig.abi == abi::RustCall) {
+            span_err!(tcx.sess, span, E0394,
+                      "invalid ABI {}, methods must have a Rust ABI", sig.abi);
+        }
+
         convert_method(ccx,
                        container,
                        sig,

src/librustc_typeck/diagnostics.rs

@@ -1332,6 +1332,7 @@ register_diagnostics! {
            // `#[lang = \"{}\"]` is allowed for the `{}` primitive
     E0391, // unsupported cyclic reference between types/traits detected
     E0392, // parameter `{}` is never used
-    E0393  // the type parameter `{}` must be explicitly specified in an object
+    E0393, // the type parameter `{}` must be explicitly specified in an object
            // type because its default value `{}` references the type `Self`"
+    E0394
 }