librustc: Forbid external crates, imports, and/or items from being

declared with the same name in the same scope.

This breaks several common patterns. First are unused imports:

    use foo::bar;
    use baz::bar;

Change this code to the following:

    use baz::bar;

Second, this patch breaks globs that import names that are shadowed by
subsequent imports. For example:

    use foo::*; // including `bar`
    use baz::bar;

Change this code to remove the glob:

    use foo::{boo, quux};
    use baz::bar;

Or qualify all uses of `bar`:

    use foo::{boo, quux};
    use baz;

    ... baz::bar ...

Finally, this patch breaks code that, at top level, explicitly imports
`std` and doesn't disable the prelude.

    extern crate std;

Because the prelude imports `std` implicitly, there is no need to
explicitly import it; just remove such directives.

The old behavior can be opted into via the `import_shadowing` feature
gate. Use of this feature gate is discouraged.

This implements RFC #116.

Closes #16464.

[breaking-change]

src/libstd/lib.rs

@@ -107,10 +107,14 @@
 
 #![feature(macro_rules, globs, managed_boxes, linkage)]
 #![feature(default_type_params, phase, lang_items, unsafe_destructor)]
+#![feature(import_shadowing)]
 
 // Don't link to std. We are std.
 #![no_std]
 
+// NOTE(stage0, pcwalton): Remove after snapshot.
+#![allow(unknown_features)]
+
 #![allow(deprecated)]
 #![deny(missing_doc)]