Split c_ptrdiff_t.md c_size_t.md c_ssize_t.md out of primitives.rs for consistence

Author: lygstate

library/core/src/ffi/c_ptrdiff_t.md

@@ -0,0 +1,4 @@
+Equivalent to C's `ptrdiff_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.

library/core/src/ffi/c_size_t.md

@@ -0,0 +1,4 @@
+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.

library/core/src/ffi/c_ssize_t.md

@@ -0,0 +1,4 @@
+Equivalent to C's `ssize_t` (on POSIX) or `SSIZE_T` (on Windows) type.
+
+This type is currently always [`isize`], however in the future there may be
+platforms where this is not the case.

library/core/src/ffi/primitives.rs

@@ -150,26 +150,9 @@ mod c_long_definition {
     }
 }
 
-/// 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 = "88345")]
-pub type c_size_t = usize;
-
-/// Equivalent to C's `ptrdiff_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 = "88345")]
-pub type c_ptrdiff_t = isize;
-
-/// Equivalent to C's `ssize_t` (on POSIX) or `SSIZE_T` (on Windows) type.
-///
-/// 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 = "88345")]
-pub type c_ssize_t = isize;
+type_alias! { "c_size_t.md", c_size_t = usize; }
+type_alias! { "c_ptrdiff_t.md", c_ptrdiff_t = isize; }
+type_alias! { "c_ssize_t.md", c_ssize_t = isize; }
 
 mod c_int_definition {
     crate::cfg_select! {