auto merge of #7113 : alexcrichton/rust/banned-warnings, r=cmr

Reopening of #7031, Closes #6963

I imagine though that this will bounce in bors once or twice... Because attributes can't be cfg(stage0)'d off, there's temporarily a lot of new stage0/stage1+ code.

src/libstd/unstable/dynamic_lib.rs

@@ -164,7 +164,6 @@ mod dl {
     use libc;
     use path;
     use ptr;
-    use str;
     use task;
     use result::*;
 
@@ -175,7 +174,7 @@ mod dl {
     }
 
     pub unsafe fn open_internal() -> *libc::c_void {
-        let mut handle = ptr::null();
+        let handle = ptr::null();
         GetModuleHandleExW(0 as libc::DWORD, ptr::null(), &handle as **libc::c_void);
         handle
     }