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/libfmt_macros/lib.rs

@@ -19,7 +19,10 @@
 #![license = "MIT/ASL2"]
 #![crate_type = "rlib"]
 #![crate_type = "dylib"]
-#![feature(macro_rules, globs)]
+#![feature(macro_rules, globs, import_shadowing)]
+
+// NOTE(stage0, pcwalton): Remove after snapshot.
+#![allow(unknown_features)]
 
 use std::char;
 use std::str;