Enforce that #[marker] traits cannot have associated items

Author: scottmcm

src/librustc_typeck/check/wfcheck.rs

@@ -304,6 +304,17 @@ fn check_type_defn<'a, 'tcx, F>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 
 fn check_trait<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, item: &hir::Item) {
     let trait_def_id = tcx.hir.local_def_id(item.id);
+
+    let trait_def = tcx.trait_def(trait_def_id);
+    if trait_def.is_marker {
+        for associated_def_id in &*tcx.associated_item_def_ids(trait_def_id) {
+            tcx.sess.struct_span_err(
+                    tcx.def_span(*associated_def_id),
+                    "marker traits cannot have associated items",
+                ).emit();
+        }
+    }
+
     for_item(tcx, item).with_fcx(|fcx, _| {
         check_where_clauses(tcx, fcx, item.span, trait_def_id, None);
         vec![]

src/test/ui/marker_trait_attr/marker-trait-with-associated-items.rs

@@ -0,0 +1,50 @@
+// Copyright 2018 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.
+
+#![feature(marker_trait_attr)]
+#![feature(associated_type_defaults)]
+
+#[marker]
+trait MarkerConst {
+    const N: usize;
+    //~^ ERROR marker traits cannot have associated items
+}
+
+#[marker]
+trait MarkerType {
+    type Output;
+    //~^ ERROR marker traits cannot have associated items
+}
+
+#[marker]
+trait MarkerFn {
+    fn foo();
+    //~^ ERROR marker traits cannot have associated items
+}
+
+#[marker]
+trait MarkerConstWithDefault {
+    const N: usize = 43;
+    //~^ ERROR marker traits cannot have associated items
+}
+
+#[marker]
+trait MarkerTypeWithDefault {
+    type Output = ();
+    //~^ ERROR marker traits cannot have associated items
+}
+
+#[marker]
+trait MarkerFnWithDefault {
+    fn foo() {}
+    //~^ ERROR marker traits cannot have associated items
+}
+
+fn main() {}

src/test/ui/marker_trait_attr/marker-trait-with-associated-items.stderr

@@ -0,0 +1,38 @@
+error: marker traits cannot have associated items
+  --> $DIR/marker-trait-with-associated-items.rs:16:5
+   |
+LL |     const N: usize;
+   |     ^^^^^^^^^^^^^^^
+
+error: marker traits cannot have associated items
+  --> $DIR/marker-trait-with-associated-items.rs:22:5
+   |
+LL |     type Output;
+   |     ^^^^^^^^^^^^
+
+error: marker traits cannot have associated items
+  --> $DIR/marker-trait-with-associated-items.rs:28:5
+   |
+LL |     fn foo();
+   |     ^^^^^^^^^
+
+error: marker traits cannot have associated items
+  --> $DIR/marker-trait-with-associated-items.rs:34:5
+   |
+LL |     const N: usize = 43;
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: marker traits cannot have associated items
+  --> $DIR/marker-trait-with-associated-items.rs:40:5
+   |
+LL |     type Output = ();
+   |     ^^^^^^^^^^^^^^^^^
+
+error: marker traits cannot have associated items
+  --> $DIR/marker-trait-with-associated-items.rs:46:5
+   |
+LL |     fn foo() {}
+   |     ^^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+