diff --git a/src/codegen/mod.rs b/src/codegen/mod.rs
index 6f8a451a2a..421517b4e1 100644
--- a/src/codegen/mod.rs
+++ b/src/codegen/mod.rs
@@ -820,9 +820,16 @@ impl CodeGenerator for Type {
                 }
 
                 // If this is a known named type, disallow generating anything
-                // for it too.
+                // for it too. If size_t -> usize conversions are enabled, we
+                // need to check that these conversions are permissible, but
+                // nothing needs to be generated, still.
                 let spelling = self.name().expect("Unnamed alias?");
                 if utils::type_from_named(ctx, spelling).is_some() {
+                    if let "size_t" | "ssize_t" = spelling {
+                        let layout = inner_item.kind().expect_type().layout(ctx).expect("No layout?");
+                        assert!(layout.size == ctx.target_pointer_size() && layout.align == ctx.target_pointer_size(),
+                         "Target platform requires --no-size_t-is-usize");
+                    }
                     return;
                 }
 
diff --git a/src/lib.rs b/src/lib.rs
index 1ac053b7f8..acedf43dd5 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -523,8 +523,8 @@ impl Builder {
             output_vector.push("--no-record-matches".into());
         }
 
-        if self.options.size_t_is_usize {
-            output_vector.push("--size_t-is-usize".into());
+        if !self.options.size_t_is_usize {
+            output_vector.push("--no-size_t-is-usize".into());
         }
 
         if !self.options.rustfmt_bindings {
@@ -2065,7 +2065,7 @@ impl Default for BindgenOptions {
             time_phases: false,
             record_matches: true,
             rustfmt_bindings: true,
-            size_t_is_usize: false,
+            size_t_is_usize: true,
             rustfmt_configuration_file: None,
             no_partialeq_types: Default::default(),
             no_copy_types: Default::default(),
diff --git a/src/options.rs b/src/options.rs
index cbdc945ebb..3a036bd95a 100644
--- a/src/options.rs
+++ b/src/options.rs
@@ -429,7 +429,12 @@ where
                 ),
             Arg::with_name("size_t-is-usize")
                 .long("size_t-is-usize")
-                .help("Translate size_t to usize."),
+                .help("Ignored - this is enabled by default.")
+                .hidden(true),
+            Arg::with_name("no-size_t-is-usize")
+                .long("no-size_t-is-usize")
+                .help("Do not bind size_t as usize (useful on platforms \
+                       where those types are incompatible)."),
             Arg::with_name("no-rustfmt-bindings")
                 .long("no-rustfmt-bindings")
                 .help("Do not format the generated bindings with rustfmt."),
@@ -874,8 +879,8 @@ where
         builder = builder.record_matches(false);
     }
 
-    if matches.is_present("size_t-is-usize") {
-        builder = builder.size_t_is_usize(true);
+    if matches.is_present("no-size_t-is-usize") {
+        builder = builder.size_t_is_usize(false);
     }
 
     let no_rustfmt_bindings = matches.is_present("no-rustfmt-bindings");
diff --git a/tests/expectations/tests/blocks-signature.rs b/tests/expectations/tests/blocks-signature.rs
index 22136ddfb7..ed520e7273 100644
--- a/tests/expectations/tests/blocks-signature.rs
+++ b/tests/expectations/tests/blocks-signature.rs
@@ -7,7 +7,6 @@
 #![cfg(target_os = "macos")]
 
 extern crate block;
-pub type size_t = ::std::os::raw::c_ulonglong;
 extern "C" {
     #[link_name = "\u{1}_Z8atexit_bU13block_pointerFvvE"]
     pub fn atexit_b(arg1: _bindgen_ty_id_33);
@@ -85,16 +84,11 @@ impl Default for contains_block_pointers {
 }
 pub type _bindgen_ty_id_33 = *const ::block::Block<(), ()>;
 pub type _bindgen_ty_id_40 = *const ::block::Block<
-    (
-        dispatch_data_t,
-        size_t,
-        *const ::std::os::raw::c_void,
-        size_t,
-    ),
+    (dispatch_data_t, usize, *const ::std::os::raw::c_void, usize),
     bool,
 >;
-pub type _bindgen_ty_id_50 = *const ::block::Block<(size_t,), ()>;
-pub type _bindgen_ty_id_56 = *const ::block::Block<(size_t,), ()>;
+pub type _bindgen_ty_id_50 = *const ::block::Block<(usize,), ()>;
+pub type _bindgen_ty_id_56 = *const ::block::Block<(usize,), ()>;
 pub type contains_block_pointers__bindgen_ty_id_61 =
     *const ::block::Block<(::std::os::raw::c_int,), ()>;
 pub type _bindgen_ty_id_68 =
diff --git a/tests/expectations/tests/blocks.rs b/tests/expectations/tests/blocks.rs
index b2ae0b2901..661a127238 100644
--- a/tests/expectations/tests/blocks.rs
+++ b/tests/expectations/tests/blocks.rs
@@ -6,7 +6,6 @@
 )]
 #![cfg(target_os = "macos")]
 
-pub type size_t = ::std::os::raw::c_ulonglong;
 extern "C" {
     #[link_name = "\u{1}_Z8atexit_bU13block_pointerFvvE"]
     pub fn atexit_b(arg1: *mut ::std::os::raw::c_void);
diff --git a/tests/expectations/tests/issue-1498.rs b/tests/expectations/tests/issue-1498.rs
index 4f8a89305b..b972474569 100644
--- a/tests/expectations/tests/issue-1498.rs
+++ b/tests/expectations/tests/issue-1498.rs
@@ -5,7 +5,6 @@
     non_upper_case_globals
 )]
 
-pub type size_t = u64;
 #[repr(C, packed)]
 #[derive(Copy, Clone)]
 pub struct rte_memseg {
@@ -13,7 +12,7 @@ pub struct rte_memseg {
     pub phys_addr: u64,
     pub __bindgen_anon_1: rte_memseg__bindgen_ty_1,
     ///< Length of the segment.
-    pub len: size_t,
+    pub len: usize,
     ///< The pagesize of underlying memory
     pub hugepage_sz: u64,
     ///< NUMA socket ID.
diff --git a/tests/expectations/tests/jsval_layout_opaque.rs b/tests/expectations/tests/jsval_layout_opaque.rs
index 5b8427913c..4d89dd874b 100644
--- a/tests/expectations/tests/jsval_layout_opaque.rs
+++ b/tests/expectations/tests/jsval_layout_opaque.rs
@@ -94,7 +94,6 @@ where
 pub const JSVAL_TAG_SHIFT: u32 = 47;
 pub const JSVAL_PAYLOAD_MASK: u64 = 140737488355327;
 pub const JSVAL_TAG_MASK: i64 = -140737488355328;
-pub type size_t = ::std::os::raw::c_ulonglong;
 #[repr(u8)]
 #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
 pub enum JSValueType {
@@ -186,7 +185,7 @@ pub union jsval_layout {
     pub s: jsval_layout__bindgen_ty_2,
     pub asDouble: f64,
     pub asPtr: *mut ::std::os::raw::c_void,
-    pub asWord: size_t,
+    pub asWord: usize,
     pub asUIntPtr: usize,
 }
 #[repr(C)]
diff --git a/tests/expectations/tests/jsval_layout_opaque_1_0.rs b/tests/expectations/tests/jsval_layout_opaque_1_0.rs
index 260282dbba..2f5a8704b2 100644
--- a/tests/expectations/tests/jsval_layout_opaque_1_0.rs
+++ b/tests/expectations/tests/jsval_layout_opaque_1_0.rs
@@ -137,7 +137,6 @@ impl<T> ::std::cmp::Eq for __BindgenUnionField<T> {}
 pub const JSVAL_TAG_SHIFT: u32 = 47;
 pub const JSVAL_PAYLOAD_MASK: u64 = 140737488355327;
 pub const JSVAL_TAG_MASK: i64 = -140737488355328;
-pub type size_t = ::std::os::raw::c_ulonglong;
 #[repr(u8)]
 #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
 pub enum JSValueType {
@@ -229,7 +228,7 @@ pub struct jsval_layout {
     pub s: __BindgenUnionField<jsval_layout__bindgen_ty_2>,
     pub asDouble: __BindgenUnionField<f64>,
     pub asPtr: __BindgenUnionField<*mut ::std::os::raw::c_void>,
-    pub asWord: __BindgenUnionField<size_t>,
+    pub asWord: __BindgenUnionField<usize>,
     pub asUIntPtr: __BindgenUnionField<usize>,
     pub bindgen_union_field: u64,
 }
diff --git a/tests/expectations/tests/layout_array.rs b/tests/expectations/tests/layout_array.rs
index 3ba1b6d0ba..5b5f9b5de9 100644
--- a/tests/expectations/tests/layout_array.rs
+++ b/tests/expectations/tests/layout_array.rs
@@ -9,7 +9,6 @@ pub const RTE_CACHE_LINE_SIZE: u32 = 64;
 pub const RTE_MEMPOOL_OPS_NAMESIZE: u32 = 32;
 pub const RTE_MEMPOOL_MAX_OPS_IDX: u32 = 16;
 pub const RTE_HEAP_NUM_FREELISTS: u32 = 13;
-pub type size_t = ::std::os::raw::c_longlong;
 #[repr(C)]
 #[derive(Debug, Copy, Clone)]
 pub struct rte_mempool {
@@ -298,7 +297,7 @@ pub struct malloc_heap {
     pub lock: rte_spinlock_t,
     pub free_head: [malloc_heap__bindgen_ty_1; 13usize],
     pub alloc_count: ::std::os::raw::c_uint,
-    pub total_size: size_t,
+    pub total_size: usize,
 }
 #[repr(C)]
 #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
diff --git a/tests/expectations/tests/msvc-no-usr.rs b/tests/expectations/tests/msvc-no-usr.rs
index ea5a90b871..d46f0438e2 100644
--- a/tests/expectations/tests/msvc-no-usr.rs
+++ b/tests/expectations/tests/msvc-no-usr.rs
@@ -5,11 +5,10 @@
     non_upper_case_globals
 )]
 
-pub type size_t = ::std::os::raw::c_ulonglong;
 #[repr(C)]
 #[derive(Debug, Default, Copy, Clone)]
 pub struct A {
-    pub foo: size_t,
+    pub foo: usize,
 }
 #[test]
 fn bindgen_test_layout_A() {
diff --git a/tests/expectations/tests/size_t_is_usize.rs b/tests/expectations/tests/no_size_t_is_usize.rs
similarity index 90%
rename from tests/expectations/tests/size_t_is_usize.rs
rename to tests/expectations/tests/no_size_t_is_usize.rs
index 0d9ab2caba..6de96305cf 100644
--- a/tests/expectations/tests/size_t_is_usize.rs
+++ b/tests/expectations/tests/no_size_t_is_usize.rs
@@ -5,11 +5,13 @@
     non_upper_case_globals
 )]
 
+pub type size_t = ::std::os::raw::c_ulong;
+pub type ssize_t = ::std::os::raw::c_long;
 #[repr(C)]
 #[derive(Debug, Copy, Clone)]
 pub struct A {
-    pub len: usize,
-    pub offset: isize,
+    pub len: size_t,
+    pub offset: ssize_t,
     pub next: *mut A,
 }
 #[test]
diff --git a/tests/expectations/tests/nsBaseHashtable.rs b/tests/expectations/tests/nsBaseHashtable.rs
index d7607b91ab..26179b763e 100644
--- a/tests/expectations/tests/nsBaseHashtable.rs
+++ b/tests/expectations/tests/nsBaseHashtable.rs
@@ -5,7 +5,6 @@
     non_upper_case_globals
 )]
 
-pub type size_t = ::std::os::raw::c_ulonglong;
 #[repr(C)]
 #[derive(Debug, Default, Copy, Clone)]
 pub struct nsBaseHashtableET {
diff --git a/tests/headers/size_t_is_usize.h b/tests/headers/no_size_t_is_usize.h
similarity index 75%
rename from tests/headers/size_t_is_usize.h
rename to tests/headers/no_size_t_is_usize.h
index 564b486784..d4370baf9f 100644
--- a/tests/headers/size_t_is_usize.h
+++ b/tests/headers/no_size_t_is_usize.h
@@ -1,4 +1,4 @@
-// bindgen-flags: --size_t-is-usize
+// bindgen-flags: --no-size_t-is-usize
 
 typedef unsigned long size_t;
 typedef long ssize_t;