librustc: Forbid duplicate name bindings in the same parameter or type

parameter list.

This breaks code like:

    fn f(a: int, a: int) { ... }
    fn g<T,T>(a: T) { ... }

Change this code to not use the same name for a parameter. For example:

    fn f(a: int, b: int) { ... }
    fn g<T,U>(a: T) { ... }

Code like this is *not* affected, since `_` is not an identifier:

    fn f(_: int, _: int) { ... } // OK

Closes #17568.

[breaking-change]

src/librustc/middle/resolve.rs

@@ -4232,15 +4232,25 @@ impl<'a> Resolver<'a> {
                                type_parameters: TypeParameters,
                                f: |&mut Resolver|) {
         match type_parameters {
-            HasTypeParameters(generics, space, node_id,
-                              rib_kind) => {
-
+            HasTypeParameters(generics, space, node_id, rib_kind) => {
                 let mut function_type_rib = Rib::new(rib_kind);
-
+                let mut seen_bindings = HashSet::new();
                 for (index, type_parameter) in generics.ty_params.iter().enumerate() {
                     let ident = type_parameter.ident;
                     debug!("with_type_parameter_rib: {} {}", node_id,
                            type_parameter.id);
+
+                    if seen_bindings.contains(&ident) {
+                        self.resolve_error(type_parameter.span,
+                                           format!("the name `{}` is already \
+                                                    used for a type \
+                                                    parameter in this type \
+                                                    parameter list",
+                                                   token::get_ident(
+                                                       ident)).as_slice())
+                    }
+                    seen_bindings.insert(ident);
+
                     let def_like = DlDef(DefTyParam(space,
                                                     local_def(type_parameter.id),
                                                     index));
@@ -4313,8 +4323,8 @@ impl<'a> Resolver<'a> {
                     // Nothing to do.
                 }
                 Some(declaration) => {
+                    let mut bindings_list = HashMap::new();
                     for argument in declaration.inputs.iter() {
-                        let mut bindings_list = HashMap::new();
                         this.resolve_pattern(&*argument.pat,
                                              ArgumentIrrefutableMode,
                                              &mut bindings_list);
@@ -5056,12 +5066,24 @@ impl<'a> Resolver<'a> {
                             // must not add it if it's in the bindings list
                             // because that breaks the assumptions later
                             // passes make about or-patterns.)
-
                             if !bindings_list.contains_key(&renamed) {
                                 let this = &mut *self;
                                 let last_rib = this.value_ribs.last_mut().unwrap();
                                 last_rib.bindings.insert(renamed, DlDef(def));
                                 bindings_list.insert(renamed, pat_id);
+                            } else if mode == ArgumentIrrefutableMode &&
+                                    bindings_list.contains_key(&renamed) {
+                                // Forbid duplicate bindings in the same
+                                // parameter list.
+                                self.resolve_error(pattern.span,
+                                                   format!("identifier `{}` \
+                                                            is bound more \
+                                                            than once in \
+                                                            this parameter \
+                                                            list",
+                                                           token::get_ident(
+                                                               ident))
+                                                   .as_slice())
                             } else if bindings_list.find(&renamed) ==
                                     Some(&pat_id) {
                                 // Then this is a duplicate variable in the

src/test/compile-fail/duplicate-parameter.rs

@@ -0,0 +1,16 @@
+// Copyright 2014 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.
+
+fn f(a: int, a: int) {}
+//~^ ERROR identifier `a` is bound more than once in this parameter list
+
+fn main() {
+}
+

src/test/compile-fail/duplicate-type-parameter.rs

@@ -0,0 +1,39 @@
+// Copyright 2014 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.
+
+type Foo<T,T> = Option<T>;
+//~^ ERROR the name `T` is already used
+
+struct Bar<T,T>(T);
+//~^ ERROR the name `T` is already used
+
+struct Baz<T,T> {
+//~^ ERROR the name `T` is already used
+    x: T,
+}
+
+enum Boo<T,T> {
+//~^ ERROR the name `T` is already used
+    A(T),
+    B,
+}
+
+fn quux<T,T>(x: T) {}
+//~^ ERROR the name `T` is already used
+
+trait Qux<T,T> {}
+//~^ ERROR the name `T` is already used
+
+impl<T,T> Qux<T,T> for Option<T> {}
+//~^ ERROR the name `T` is already used
+
+fn main() {
+}
+