rustc: disallow duplicate methods in trait impls

Closes #8153

src/librustc/middle/typeck/collect.rs

@@ -48,6 +48,8 @@ use util::ppaux::Repr;
 use std::rc::Rc;
 use std::vec_ng::Vec;
 use std::vec_ng;
+use collections::HashSet;
+
 use syntax::abi::AbiSet;
 use syntax::ast::{RegionTyParamBound, TraitTyParamBound};
 use syntax::ast;
@@ -478,7 +480,12 @@ fn convert_methods(ccx: &CrateCtxt,
                    rcvr_visibility: ast::Visibility)
 {
     let tcx = ccx.tcx;
+    let mut seen_methods = HashSet::new();
     for m in ms.iter() {
+        if !seen_methods.insert(m.ident.repr(ccx.tcx)) {
+            tcx.sess.span_err(m.span, "duplicate method in trait impl");
+        }
+
         let num_rcvr_ty_params = rcvr_ty_generics.type_param_defs().len();
         let m_ty_generics = ty_generics_for_fn_or_method(ccx, &m.generics,
                                                          num_rcvr_ty_params);

src/test/compile-fail/issue-8153.rs

@@ -0,0 +1,26 @@
+// Copyright 2013 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.
+
+// Test that duplicate methods in impls are not allowed
+
+struct Foo;
+
+trait Bar {
+    fn bar(&self) -> int;
+}
+
+impl Bar for Foo {
+    fn bar(&self) -> int {1}
+    fn bar(&self) -> int {2} //~ ERROR duplicate method
+}
+
+fn main() {
+    println!("{}", Foo.bar());
+}