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]

1 files changed, 2 insertions, 1 deletions

diff --git a/src/doc/guide-unsafe.md b/src/doc/guide-unsafe.md
index 2ff63055480..b46b09781ca 100644
--- a/src/doc/guide-unsafe.md
+++ b/src/doc/guide-unsafe.md
@@ -537,11 +537,12 @@ extern crate core;
 use core::prelude::*;
 
 use core::mem;
-use core::raw::Slice;
 
 #[no_mangle]
 pub extern fn dot_product(a: *const u32, a_len: u32,
                           b: *const u32, b_len: u32) -> u32 {
+    use core::raw::Slice;
+
     // Convert the provided arrays into Rust slices.
     // The core::raw module guarantees that the Slice
     // structure has the same memory layout as a &[T]