Add c_size_t and c_ssize_t to std::os::raw.

library/std/src/os/raw/mod.rs

@@ -151,3 +151,17 @@ type_alias_no_nz! { "double.md", c_double = f64; }
 #[stable(feature = "raw_os", since = "1.1.0")]
 #[doc(no_inline)]
 pub use core::ffi::c_void;
+
+/// Equivalent to C's `size_t` type, from `stddef.h` (or `cstddef` for C++).
+///
+/// This type is currently always [`usize`], however in the future there may be
+/// platforms where this is not the case.
+#[unstable(feature = "c_size_t", issue = "none")]
+pub type c_size_t = usize;
+
+/// Equivalent to C's `ssize_t` type, from `stddef.h` (or `cstddef` for C++).
+///
+/// This type is currently always [`isize`], however in the future there may be
+/// platforms where this is not the case.
+#[unstable(feature = "c_size_t", issue = "none")]
+pub type c_ssize_t = isize;