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]

4 files changed, 1 insertions, 9 deletions

diff --git a/src/libstd/io/net/unix.rs b/src/libstd/io/net/unix.rs
index d88af8dd30a..eb251075418 100644
--- a/src/libstd/io/net/unix.rs
+++ b/src/libstd/io/net/unix.rs
@@ -26,12 +26,7 @@ instances as clients.
 
 use prelude::*;
 
-use c_str::ToCStr;
-use clone::Clone;
-use io::{Listener, Acceptor, Reader, Writer, IoResult, IoError};
-use io::{standard_error, TimedOut};
-use kinds::Send;
-use boxed::Box;
+use io::{Listener, Acceptor, IoResult, IoError, TimedOut, standard_error};
 use rt::rtio::{IoFactory, LocalIo, RtioUnixListener};
 use rt::rtio::{RtioUnixAcceptor, RtioPipe};
 use time::Duration;
diff --git a/src/libstd/io/pipe.rs b/src/libstd/io/pipe.rs
index c476a99fee9..1304877fc14 100644
--- a/src/libstd/io/pipe.rs
+++ b/src/libstd/io/pipe.rs
@@ -20,7 +20,6 @@ use prelude::*;
 use io::{IoResult, IoError};
 use libc;
 use os;
-use boxed::Box;
 use rt::rtio::{RtioPipe, LocalIo};
 
 /// A synchronous, in-memory pipe.
diff --git a/src/libstd/io/process.rs b/src/libstd/io/process.rs
index 3dd4343c5f4..dd6c1f6016c 100644
--- a/src/libstd/io/process.rs
+++ b/src/libstd/io/process.rs
@@ -20,7 +20,6 @@ use io::{IoResult, IoError};
 use io;
 use libc;
 use mem;
-use boxed::Box;
 use rt::rtio::{RtioProcess, ProcessConfig, IoFactory, LocalIo};
 use rt::rtio;
 use c_str::CString;
diff --git a/src/libstd/io/util.rs b/src/libstd/io/util.rs
index c3cd2faf773..1fe0ba780a6 100644
--- a/src/libstd/io/util.rs
+++ b/src/libstd/io/util.rs
@@ -13,7 +13,6 @@
 use prelude::*;
 use cmp;
 use io;
-use boxed::Box;
 use slice::bytes::MutableByteVector;
 
 /// Wraps a `Reader`, limiting the number of bytes that can be read from it.