Map size_t to usize by default and check compatibility (fixes #1901, #1903)

This addresses the underlying issue identified in #1671, that size_t
(integer that can hold any object size) isn't guaranteed to match usize,
which is defined more like uintptr_t (integer that can hold any
pointer). However, on almost all platforms, this is true, and in fact
Rust already uses usize extensively in contexts where size_t would be
more appropriate, such as slice indexing. So, it's better for ergonomics
when interfacing with C code to map the C size_t type to usize. (See
also discussion in rust-lang/rust#65473 about how usize really should be
defined as size_t, not uintptr_t.)

The previous fix for #1671 removed the special case for size_t and
defaulted to binding it as a normal typedef.  This change effectively
reverts that and goes back to mapping size_t to usize (and ssize_t to
isize), but also ensures that if size_t is emitted, the typedef'd type
of size_t in fact is compatible with usize (defined by checking that the
size and alignment match the target pointer width). For (hypothetical)
platforms where this is not true, or for compatibility with the default
behavior of bindgen between 0.53 and this commit, onwards, you can
disable this mapping with --no-size_t-is-usize.

Author: geofft

src/codegen/mod.rs

@@ -836,9 +836,16 @@ 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;
                 }
 

src/lib.rs

@@ -550,8 +550,8 @@ 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 {
@@ -2183,7 +2183,7 @@ 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(),

src/options.rs

@@ -429,7 +429,12 @@ where
                 ),
             Arg::new("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::new("no-rustfmt-bindings")
                 .long("no-rustfmt-bindings")
                 .help("Do not format the generated bindings with rustfmt."),
@@ -915,8 +920,8 @@ 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");

tests/expectations/tests/blocks-signature.rs

@@ -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;