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, 14 insertions, 1 deletions

diff --git a/src/librustc/front/std_inject.rs b/src/librustc/front/std_inject.rs
index 5dca09faf34..ff8711c5df0 100644
--- a/src/librustc/front/std_inject.rs
+++ b/src/librustc/front/std_inject.rs
@@ -19,6 +19,7 @@ use syntax::fold::Folder;
 use syntax::fold;
 use syntax::owned_slice::OwnedSlice;
 use syntax::parse::token::InternedString;
+use syntax::parse::token::special_idents;
 use syntax::parse::token;
 use syntax::util::small_vector::SmallVector;
 
@@ -197,7 +198,19 @@ impl<'a> fold::Folder for PreludeInjector<'a> {
                                                                   ast::DUMMY_NODE_ID));
         let vi2 = ast::ViewItem {
             node: ast::ViewItemUse(vp),
-            attrs: Vec::new(),
+            attrs: vec!(ast::Attribute {
+                span: DUMMY_SP,
+                node: ast::Attribute_ {
+                    id: attr::mk_attr_id(),
+                    style: ast::AttrOuter,
+                    value: box(GC) ast::MetaItem {
+                        span: DUMMY_SP,
+                        node: ast::MetaWord(token::get_name(
+                                special_idents::prelude_import.name)),
+                    },
+                    is_sugared_doc: false,
+                },
+            }),
             vis: ast::Inherited,
             span: DUMMY_SP,
         };