librustc: Disallow use from reaching into impls or traits.

This can perhaps be restored in the future. For now this is a precursor to
making typedefs work as expected.

src/test/auxiliary/use_from_trait_xc.rs

@@ -0,0 +1,10 @@
+pub trait Trait {
+    fn foo();
+}
+
+struct Foo;
+
+impl Foo {
+    pub fn new() {}
+}
+

src/test/compile-fail/use-from-trait-xc.rs

@@ -0,0 +1,12 @@
+// aux-build:use_from_trait_xc.rs
+
+extern mod use_from_trait_xc;
+
+use use_from_trait_xc::Trait::foo;  //~ ERROR cannot import from a trait or type implementation
+//~^ ERROR failed to resolve import
+use use_from_trait_xc::Foo::new;    //~ ERROR cannot import from a trait or type implementation
+//~^ ERROR failed to resolve import
+
+fn main() {
+}
+

src/test/compile-fail/use-from-trait.rs

@@ -0,0 +1,17 @@
+use Trait::foo;  //~ ERROR cannot import from a trait or type implementation
+//~^ ERROR failed to resolve import
+use Foo::new;    //~ ERROR cannot import from a trait or type implementation
+//~^ ERROR failed to resolve import
+
+pub trait Trait {
+    fn foo();
+}
+
+struct Foo;
+
+impl Foo {
+    fn new() {}
+}
+
+fn main() {}
+