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]

Author: pcwalton

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]

src/libcollections/bitv.rs

@@ -68,12 +68,11 @@ use core::default::Default;
 use core::fmt;
 use core::iter::Take;
 use core::iter;
-use core::ops::Index;
 use core::slice;
 use core::uint;
 use std::hash;
 
-use {Collection, Mutable, Set, MutableSet, MutableSeq};
+use {Mutable, Set, MutableSet, MutableSeq};
 use vec::Vec;
 
 

src/libcollections/btree.rs

@@ -24,7 +24,7 @@ use alloc::boxed::Box;
 use core::fmt;
 use core::fmt::Show;
 
-use {Collection, MutableSeq};
+use MutableSeq;
 use vec::Vec;
 
 #[allow(missing_doc)]

src/libcollections/dlist.rs

@@ -31,7 +31,7 @@ use core::mem;
 use core::ptr;
 use std::hash::{Writer, Hash};
 
-use {Collection, Mutable, Deque, MutableSeq};
+use {Mutable, Deque, MutableSeq};
 
 /// A doubly-linked list.
 pub struct DList<T> {

src/libcollections/lib.rs

@@ -22,9 +22,12 @@
        html_playground_url = "http://play.rust-lang.org/")]
 
 #![feature(macro_rules, managed_boxes, default_type_params, phase, globs)]
-#![feature(unsafe_destructor)]
+#![feature(unsafe_destructor, import_shadowing)]
 #![no_std]
 
+// NOTE(stage0, pcwalton): Remove after snapshot.
+#![allow(unknown_features)]
+
 #[phase(plugin, link)] extern crate core;
 extern crate unicode;
 extern crate alloc;
@@ -36,11 +39,11 @@ extern crate alloc;
 #[cfg(test)] #[phase(plugin, link)] extern crate std;
 #[cfg(test)] #[phase(plugin, link)] extern crate log;
 
-use core::prelude::*;
+use core::prelude::Option;
 
-pub use core::collections::Collection;
 pub use bitv::{Bitv, BitvSet};
 pub use btree::BTree;
+pub use core::prelude::Collection;
 pub use dlist::DList;
 pub use enum_set::EnumSet;
 pub use priority_queue::PriorityQueue;

src/libcollections/priority_queue.rs

@@ -154,7 +154,7 @@ use core::default::Default;
 use core::mem::{zeroed, replace, swap};
 use core::ptr;
 
-use {Collection, Mutable, MutableSeq};
+use {Mutable, MutableSeq};
 use slice;
 use vec::Vec;
 

src/libcollections/ringbuf.rs

@@ -18,11 +18,10 @@ use core::prelude::*;
 use core::cmp;
 use core::default::Default;
 use core::fmt;
-use core::iter::RandomAccessIterator;
 use core::iter;
 use std::hash::{Writer, Hash};
 
-use {Deque, Collection, Mutable, MutableSeq};
+use {Deque, Mutable, MutableSeq};
 use vec::Vec;
 
 static INITIAL_CAPACITY: uint = 8u; // 2^3

src/libcollections/slice.rs

@@ -86,23 +86,22 @@ for &x in numbers.iter() {
 
 #![doc(primitive = "slice")]
 
-use core::prelude::*;
-
 use core::cmp;
 use core::mem::size_of;
 use core::mem;
+use core::prelude::{Clone, Collection, Greater, Iterator, Less, None, Option};
+use core::prelude::{Ord, Ordering, RawPtr, Some, range};
 use core::ptr;
 use core::iter::{range_step, MultiplicativeIterator};
 
-use {Collection, MutableSeq};
+use MutableSeq;
 use vec::Vec;
 
-pub use core::slice::{ref_slice, mut_ref_slice, Splits, Windows};
 pub use core::slice::{Chunks, Slice, ImmutableSlice, ImmutablePartialEqSlice};
 pub use core::slice::{ImmutableOrdSlice, MutableSlice, Items, MutItems};
-pub use core::slice::{MutSplits, MutChunks};
-pub use core::slice::{bytes, MutableCloneableSlice};
-pub use core::slice::{BinarySearchResult, Found, NotFound};
+pub use core::slice::{MutSplits, MutChunks, Splits};
+pub use core::slice::{bytes, ref_slice, MutableCloneableSlice};
+pub use core::slice::{Found, NotFound};
 
 // Functional utilities
 

src/libcollections/smallintmap.rs

@@ -21,7 +21,7 @@ use core::iter;
 use core::iter::{Enumerate, FilterMap};
 use core::mem::replace;
 
-use {Collection, Mutable, Map, MutableMap, MutableSeq};
+use {Mutable, Map, MutableMap, MutableSeq};
 use {vec, slice};
 use vec::Vec;
 use hash;

src/libcollections/str.rs

@@ -69,15 +69,17 @@ is the same as `&[u8]`.
 
 #![doc(primitive = "str")]
 
-use core::prelude::*;
-
 use core::default::Default;
 use core::fmt;
 use core::cmp;
 use core::iter::AdditiveIterator;
 use core::mem;
+use core::prelude::{Char, Clone, Collection, Eq, Equiv, ImmutableSlice};
+use core::prelude::{Iterator, MutableSlice, None, Option, Ord, Ordering};
+use core::prelude::{PartialEq, PartialOrd, Result, Slice, Some, Tuple2};
+use core::prelude::{range};
 
-use {Collection, Deque, MutableSeq};
+use {Deque, MutableSeq};
 use hash;
 use ringbuf::RingBuf;
 use string::String;