Remove niche optimizations never will be stable

Author: iceiix

src/cvt.rs

@@ -1,4 +1,4 @@
-use std::io::{self, Error, ErrorKind, IoSlice, IoSliceMut, SeekFrom};
+use std::io::ErrorKind;
 
 #[doc(hidden)]
 pub trait IsMinusOne {

src/fd.rs

@@ -1,5 +1,3 @@
-#![unstable(reason = "not public", issue = "none", feature = "fd")]
-
 #[cfg(test)]
 mod tests;
 
@@ -12,11 +10,6 @@ use crate::sys_common::AsInner;
 use libc::{c_int, c_void};
 
 #[derive(Debug)]
-#[rustc_layout_scalar_valid_range_start(0)]
-// libstd/os/raw/mod.rs assures me that every libstd-supported platform has a
-// 32-bit c_int. Below is -2, in two's complement, but that only works out
-// because c_int is 32 bits.
-#[rustc_layout_scalar_valid_range_end(0xFF_FF_FF_FE)]
 pub struct FileDesc {
     fd: c_int,
 }