codegen: support repr(align).

Fixes rust-lang#917

Author: emilio

src/codegen/mod.rs

@@ -1527,13 +1527,16 @@ impl CodeGenerator for CompInfo {
             });
         }
 
+        let mut explicit_align = None;
         if is_opaque {
             // Opaque item should not have generated methods, fields.
             debug_assert!(fields.is_empty());
             debug_assert!(methods.is_empty());
 
             match layout {
                 Some(l) => {
+                    explicit_align = Some(l.align);
+
                     let ty = helpers::blob(l);
                     fields.push(quote! {
                         pub _bindgen_opaque_blob: #ty ,
@@ -1555,6 +1558,7 @@ impl CodeGenerator for CompInfo {
                     if layout.align == 1 {
                         packed = true;
                     } else {
+                        explicit_align = Some(layout.align);
                         let ty = helpers::blob(Layout::new(0, layout.align));
                         fields.push(quote! {
                             pub __bindgen_align: #ty ,
@@ -1637,6 +1641,18 @@ impl CodeGenerator for CompInfo {
             attributes.push(attributes::repr("C"));
         }
 
+        if ctx.options().rust_features().repr_align() {
+            if let Some(explicit) = explicit_align {
+                // Ensure that the struct has the correct alignment even in
+                // presence of alignas.
+                let explicit = helpers::ast_ty::int_expr(explicit as i64);
+                attributes.push(quote! {
+                    #[repr(align(#explicit))]
+                });
+            }
+        }
+
+
         let mut derives = vec![];
         if item.can_derive_debug(ctx) {
             derives.push("Debug");

src/features.rs

@@ -92,6 +92,8 @@ macro_rules! rust_target_base {
             => Stable_1_19 => 1.19;
             /// Rust stable 1.21
             => Stable_1_21 => 1.21;
+            /// Rust stable 1.24
+            => Stable_1_24 => 1.24;
             /// Nightly rust
             => Nightly => nightly;
         );
@@ -144,6 +146,8 @@ rust_feature_def!(
     => thiscall_abi;
     /// builtin impls for `Clone` ([PR](https://github.com/rust-lang/rust/pull/43690))
     => builtin_clone_impls;
+    /// repr(align) https://github.com/rust-lang/rust/pull/47006
+    => repr_align;
 );
 
 impl From<RustTarget> for RustFeatures {
@@ -158,6 +162,10 @@ impl From<RustTarget> for RustFeatures {
             features.builtin_clone_impls = true;
         }
 
+        if rust_target >= RustTarget::Stable_1_24 {
+            features.repr_align = true;
+        }
+
         if rust_target >= RustTarget::Nightly {
             features.thiscall_abi = true;
         }

tests/expectations/tests/repr-align.rs

@@ -0,0 +1,35 @@
+/* automatically generated by rust-bindgen */
+
+#![allow(dead_code, non_snake_case, non_camel_case_types, non_upper_case_globals)]
+
+#[repr(C)]
+#[repr(align(8))]
+#[derive(Debug, Default, Copy, Clone)]
+pub struct a {
+    pub b: ::std::os::raw::c_int,
+    pub c: ::std::os::raw::c_int,
+    pub __bindgen_align: [u64; 0usize],
+}
+#[test]
+fn bindgen_test_layout_a() {
+    assert_eq!(
+        ::std::mem::size_of::<a>(),
+        8usize,
+        concat!("Size of: ", stringify!(a))
+    );
+    assert_eq!(
+        ::std::mem::align_of::<a>(),
+        8usize,
+        concat!("Alignment of ", stringify!(a))
+    );
+    assert_eq!(
+        unsafe { &(*(::std::ptr::null::<a>())).b as *const _ as usize },
+        0usize,
+        concat!("Offset of field: ", stringify!(a), "::", stringify!(b))
+    );
+    assert_eq!(
+        unsafe { &(*(::std::ptr::null::<a>())).c as *const _ as usize },
+        4usize,
+        concat!("Offset of field: ", stringify!(a), "::", stringify!(c))
+    );
+}

tests/headers/repr-align.hpp

@@ -0,0 +1,6 @@
+// bindgen-flags: --rust-target 1.24 -- -std=c++11
+
+struct alignas(8) a {
+  int b;
+  int c;
+};