Implement Default TyParam fallback

This patch allows type parameter defaults to influence type inference. This is a possible breaking change since it effects the way type inference works and will have different behavior when mixing defaults and literal fallback.

src/test/compile-fail/default_ty_param_conflict.rs

@@ -0,0 +1,23 @@
+// 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.
+
+use std::fmt::Debug;
+
+// Example from the RFC
+fn foo<F:Default=usize>() -> F { F::default() }
+fn bar<B:Debug=isize>(b: B) { println!("{:?}", b); }
+
+fn main() {
+    // Here, F is instantiated with $0=uint
+    let x = foo();
+
+    // Here, B is instantiated with $1=uint, and constraint $0 <: $1 is added.
+    bar(x);
+}

src/test/compile-fail/default_ty_param_type_alias.rs

@@ -0,0 +1,17 @@
+// 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.
+
+use std::collections::HashMap;
+
+type IntMap<K=usize> = HashMap<K, usize>;
+
+fn main() {
+    let x = IntMap::new();
+}

src/test/run-pass/default_ty_param_method_call_test.rs

@@ -0,0 +1,22 @@
+// 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.
+
+struct Foo;
+
+impl Foo {
+    fn method<A:Default=String>(&self) -> A {
+        A::default()
+    }
+}
+
+fn main() {
+    let f = Foo.method();
+    println!("{}", f);
+}

src/test/run-pass/default_ty_param_struct.rs

@@ -0,0 +1,21 @@
+// 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.
+
+struct Foo<A>(A);
+
+impl<A:Default=i32> Foo<A> {
+    fn new() -> Foo<A> {
+        Foo(A::default())
+    }
+}
+
+fn main() {
+    let foo = Foo::new();
+}

src/test/run-pass/default_ty_param_trait_impl.rs

@@ -0,0 +1,23 @@
+// 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.
+
+// Another example from the RFC
+trait Foo { }
+trait Bar { }
+
+impl<T:Bar=usize> Foo for Vec<T> {} // Impl 1
+impl Bar for usize { } // Impl 2
+
+fn takes_foo<F:Foo>(f: F) { }
+
+fn main() {
+    let x = Vec::new(); // x: Vec<$0>
+    takes_foo(x); // adds oblig Vec<$0> : Foo
+}

src/test/run-pass/default_ty_param_trait_impl_simple.rs

@@ -0,0 +1,26 @@
+// 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.
+
+// An example from the RFC
+trait Foo { fn takes_foo(&self); }
+trait Bar { }
+
+impl<T:Bar=usize> Foo for Vec<T> {
+    fn takes_foo(&self) {}
+} // Impl 1
+
+impl Bar for usize { } // Impl 2
+
+// fn takes_foo<F:Foo>(f: F) { }
+
+fn main() {
+    let x = Vec::new(); // x: Vec<$0>
+    x.takes_foo(); // adds oblig Vec<$0> : Foo
+}