Don't warn when |usize| and |isize| are used in FFI declarations.

It is safer to alias |size_t|/|ssize_t| as |usize|/|size_t| on
platforms where they are the same size, in order to reduce
unneccessary and hazardous casting.

In theory, there are platforms where |usize| and |isize| are not the
same as |size_t| and |ssize_t|, and in that case we **should** have
this warning, but we don't currently support any such platforms, so we
don't bother adding any logic to deal with that situation yet.

Author: briansmith

src/librustc_lint/builtin.rs

@@ -550,14 +550,9 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                 FfiSafe
             }
 
-            ty::TyInt(ast::TyIs) => {
-                FfiUnsafe("found Rust type `isize` in foreign module, while \
-                          `libc::c_int` or `libc::c_long` should be used")
-            }
-            ty::TyUint(ast::TyUs) => {
-                FfiUnsafe("found Rust type `usize` in foreign module, while \
-                          `libc::c_uint` or `libc::c_ulong` should be used")
-            }
+            // FIXME: We should warn about `usize`/`isize`/ if (and only if)
+            // `size_t`/'ssize_t' are not defined as aliases for them.
+
             ty::TyChar => {
                 FfiUnsafe("found Rust type `char` in foreign module, while \
                            `u32` or `libc::wchar_t` should be used")