rust: use bindgen --size_t-is-usize option

This will give us identical bindings for all platforms regardless of
what underlying C type they use to define size_t.

For some reason, this isn't the default:
rust-lang/rust-bindgen#1901

Previously, we generated different things on different platforms:

x86-64:     pub type size_t = ::std::os::raw::c_ulong;
32-bit arm: pub type size_t = ::std::os::raw::c_uint;

It also lets us eliminate a couple of size_t casts in the Rust wrapper
code.

Bug: None
Test: (cd rust/minijail; cargo test)
Change-Id: Ic37fc7fb2cf2ad173b4bc184a499011d299c8917

rust/minijail-sys/build.rs

@@ -94,6 +94,7 @@ fn bindings_generation() -> io::Result<()> {
         .args(&["--blacklist-type", "__uint64_t"])
         .args(&["--whitelist-function", "^minijail_.*"])
         .args(&["--whitelist-var", "^MINIJAIL_.*"])
+        .arg("--size_t-is-usize")
         .arg("--no-layout-tests")
         .arg("--disable-header-comment")
         .args(&["--output", gen_file.to_str().unwrap()])

rust/minijail-sys/libminijail.rs

@@ -10,7 +10,6 @@ pub type rlim_t = __rlim64_t;
 pub type gid_t = __gid_t;
 pub type uid_t = __uid_t;
 pub type pid_t = __pid_t;
-pub type size_t = ::std::os::raw::c_ulong;
 #[repr(C)]
 pub struct sock_filter {
     pub code: __u16,
@@ -68,7 +67,7 @@ extern "C" {
     pub fn minijail_change_gid(j: *mut minijail, gid: gid_t);
 }
 extern "C" {
-    pub fn minijail_set_supplementary_gids(j: *mut minijail, size: size_t, list: *const gid_t);
+    pub fn minijail_set_supplementary_gids(j: *mut minijail, size: usize, list: *const gid_t);
 }
 extern "C" {
     pub fn minijail_keep_supplementary_gids(j: *mut minijail);
@@ -256,7 +255,7 @@ extern "C" {
     pub fn minijail_mount_tmp(j: *mut minijail);
 }
 extern "C" {
-    pub fn minijail_mount_tmp_size(j: *mut minijail, size: size_t);
+    pub fn minijail_mount_tmp_size(j: *mut minijail, size: usize);
 }
 extern "C" {
     pub fn minijail_mount_dev(j: *mut minijail);

rust/minijail/src/lib.rs

@@ -465,7 +465,7 @@ impl Minijail {
     }
     pub fn set_supplementary_gids(&mut self, ids: &[libc::gid_t]) {
         unsafe {
-            minijail_set_supplementary_gids(self.jail, ids.len() as size_t, ids.as_ptr());
+            minijail_set_supplementary_gids(self.jail, ids.len(), ids.as_ptr());
         }
     }
     pub fn keep_supplementary_gids(&mut self) {
@@ -767,7 +767,7 @@ impl Minijail {
     }
     pub fn mount_tmp_size(&mut self, size: usize) {
         unsafe {
-            minijail_mount_tmp_size(self.jail, size as size_t);
+            minijail_mount_tmp_size(self.jail, size);
         }
     }
     pub fn mount_bind<P1: AsRef<Path>, P2: AsRef<Path>>(