Check that traits with default impls have no methods

fixes #23080

Author: Jorge Aparicio

src/librustc_typeck/check/mod.rs

@@ -465,6 +465,29 @@ impl<'a, 'tcx> Visitor<'tcx> for CheckItemTypesVisitor<'a, 'tcx> {
     }
 }
 
+// Check that trait with default impls (`impl Trait for ..`) contain no methods
+struct DefaultedTraitVisitor<'a, 'tcx: 'a> { ccx: &'a CrateCtxt<'a, 'tcx> }
+
+impl<'a, 'tcx> Visitor<'tcx> for DefaultedTraitVisitor<'a, 'tcx> {
+    fn visit_item(&mut self, item: &ast::Item) {
+        let tcx = self.ccx.tcx;
+
+        match item.node {
+            ast::ItemTrait(_, _, _, ref trait_methods) => {
+                if ty::trait_has_default_impl(tcx, local_def(item.id)) &&
+                    !trait_methods.is_empty()
+                {
+                    tcx.sess.span_err(
+                        item.span,
+                        "traits with default impls (`e.g. impl Trait for ..`) must have no \
+                        methods")
+                }
+            },
+            _ => {},
+        }
+    }
+}
+
 pub fn check_item_types(ccx: &CrateCtxt) {
     let krate = ccx.tcx.map.krate();
     let mut visit = wf::CheckTypeWellFormedVisitor::new(ccx);
@@ -478,6 +501,11 @@ pub fn check_item_types(ccx: &CrateCtxt) {
     visit::walk_crate(&mut visit, krate);
 
     ccx.tcx.sess.abort_if_errors();
+
+    let mut visit = DefaultedTraitVisitor { ccx: ccx };
+    visit::walk_crate(&mut visit, krate);
+
+    ccx.tcx.sess.abort_if_errors();
 }
 
 fn check_bare_fn<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,

src/test/compile-fail/issue-23080.rs

@@ -0,0 +1,29 @@
+// Copyright 2015 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(optin_builtin_traits)]
+
+unsafe trait Trait {
+//~^ error: traits with default impls (`e.g. impl Trait for ..`) must have no methods
+    fn method(&self) {
+        println!("Hello");
+    }
+}
+
+unsafe impl Trait for .. {}
+
+fn call_method<T: Trait>(x: T) {
+    x.method();
+}
+
+fn main() {
+    // ICE
+    call_method(());
+}