std: Redesign c_str and c_vec

This commit is an implementation of [RFC 494][rfc] which removes the entire
`std::c_vec` module and redesigns the `std::c_str` module as `std::ffi`.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0494-c_str-and-c_vec-stability.md

The interface of the new `CString` is outlined in the linked RFC, the primary
changes being:

* The `ToCStr` trait is gone, meaning the `with_c_str` and `to_c_str` methods
  are now gone. These two methods are replaced with a `CString::from_slice`
  method.
* The `CString` type is now just a wrapper around `Vec<u8>` with a static
  guarantee that there is a trailing nul byte with no internal nul bytes. This
  means that `CString` now implements `Deref<Target = [c_char]>`, which is where
  it gains most of its methods from. A few helper methods are added to acquire a
  slice of `u8` instead of `c_char`, as well as including a slice with the
  trailing nul byte if necessary.
* All usage of non-owned `CString` values is now done via two functions inside
  of `std::ffi`, called `c_str_to_bytes` and `c_str_to_bytes_with_nul`. These
  functions are now the one method used to convert a `*const c_char` to a Rust
  slice of `u8`.

Many more details, including newly deprecated methods, can be found linked in
the RFC. This is a:

[breaking-change]
Closes #20444

4 files changed, 7 insertions, 10 deletions

diff --git a/src/libstd/rt/args.rs b/src/libstd/rt/args.rs
index 4734a39c835..86abacb9365 100644
--- a/src/libstd/rt/args.rs
+++ b/src/libstd/rt/args.rs
@@ -46,8 +46,9 @@ pub fn clone() -> Option<Vec<Vec<u8>>> { imp::clone() }
 mod imp {
     use prelude::v1::*;
 
+    use libc;
     use mem;
-    use slice;
+    use ffi;
 
     use sync::{StaticMutex, MUTEX_INIT};
 
@@ -95,13 +96,9 @@ mod imp {
     }
 
     unsafe fn load_argc_and_argv(argc: int, argv: *const *const u8) -> Vec<Vec<u8>> {
+        let argv = argv as *const *const libc::c_char;
         range(0, argc as uint).map(|i| {
-            let arg = *argv.offset(i as int);
-            let mut len = 0u;
-            while *arg.offset(len as int) != 0 {
-                len += 1u;
-            }
-            slice::from_raw_buf(&arg, len).to_vec()
+            ffi::c_str_to_bytes(&*argv.offset(i as int)).to_vec()
         }).collect()
     }
 
diff --git a/src/libstd/rt/backtrace.rs b/src/libstd/rt/backtrace.rs
index 578239c9cc4..bb0b6fe804b 100644
--- a/src/libstd/rt/backtrace.rs
+++ b/src/libstd/rt/backtrace.rs
@@ -15,7 +15,7 @@
 use prelude::v1::*;
 
 use os;
-use sync::atomic::{mod, Ordering};
+use sync::atomic::{self, Ordering};
 
 pub use sys::backtrace::write;
 
diff --git a/src/libstd/rt/unwind.rs b/src/libstd/rt/unwind.rs
index a48a8edd82f..71169386c18 100644
--- a/src/libstd/rt/unwind.rs
+++ b/src/libstd/rt/unwind.rs
@@ -67,7 +67,7 @@ use fmt;
 use intrinsics;
 use libc::c_void;
 use mem;
-use sync::atomic::{mod, Ordering};
+use sync::atomic::{self, Ordering};
 use sync::{Once, ONCE_INIT};
 
 use rt::libunwind as uw;
diff --git a/src/libstd/rt/util.rs b/src/libstd/rt/util.rs
index 883a01fa318..bc01ce926f8 100644
--- a/src/libstd/rt/util.rs
+++ b/src/libstd/rt/util.rs
@@ -19,7 +19,7 @@ use libc::{self, uintptr_t};
 use os;
 use slice;
 use str;
-use sync::atomic::{mod, Ordering};
+use sync::atomic::{self, Ordering};
 
 /// Dynamically inquire about whether we're running under V.
 /// You should usually not use this unless your test definitely