struct_layout: Fix field offset computation for packed(n) structs.

This can cause unnecessary padding to be computed otherwise at the end
of the struct.

With repr(packed(n)), a field can have padding to adjacent fields as
long as its alignment is less than n. So reuse the code we have to align
to a field layout, aligning to the struct layout instead.

Fixes #1934

3 files changed, 56 insertions, 1 deletions

diff --git a/tests/expectations/tests/divide-by-zero-in-struct-layout.rs b/tests/expectations/tests/divide-by-zero-in-struct-layout.rs
index 1366548e..1484719b 100644
--- a/tests/expectations/tests/divide-by-zero-in-struct-layout.rs
+++ b/tests/expectations/tests/divide-by-zero-in-struct-layout.rs
@@ -130,7 +130,6 @@ impl WithBitfieldAndAttrPacked {
 pub struct WithBitfieldAndPacked {
     pub _bitfield_1: __BindgenBitfieldUnit<[u8; 0usize], u8>,
     pub a: ::std::os::raw::c_uint,
-    pub __bindgen_padding_0: u8,
 }
 impl WithBitfieldAndPacked {
     #[inline]
diff --git a/tests/expectations/tests/packed-n-with-padding.rs b/tests/expectations/tests/packed-n-with-padding.rs
new file mode 100644
index 00000000..13cb0306
--- /dev/null
+++ b/tests/expectations/tests/packed-n-with-padding.rs
@@ -0,0 +1,48 @@
+#![allow(
+    dead_code,
+    non_snake_case,
+    non_camel_case_types,
+    non_upper_case_globals
+)]
+
+#[repr(C, packed(2))]
+#[derive(Debug, Default, Copy, Clone)]
+pub struct Packed {
+    pub a: ::std::os::raw::c_char,
+    pub b: ::std::os::raw::c_short,
+    pub c: ::std::os::raw::c_char,
+    pub d: ::std::os::raw::c_int,
+}
+#[test]
+fn bindgen_test_layout_Packed() {
+    assert_eq!(
+        ::std::mem::size_of::<Packed>(),
+        10usize,
+        concat!("Size of: ", stringify!(Packed))
+    );
+    assert_eq!(
+        ::std::mem::align_of::<Packed>(),
+        2usize,
+        concat!("Alignment of ", stringify!(Packed))
+    );
+    assert_eq!(
+        unsafe { &(*(::std::ptr::null::<Packed>())).a as *const _ as usize },
+        0usize,
+        concat!("Offset of field: ", stringify!(Packed), "::", stringify!(a))
+    );
+    assert_eq!(
+        unsafe { &(*(::std::ptr::null::<Packed>())).b as *const _ as usize },
+        2usize,
+        concat!("Offset of field: ", stringify!(Packed), "::", stringify!(b))
+    );
+    assert_eq!(
+        unsafe { &(*(::std::ptr::null::<Packed>())).c as *const _ as usize },
+        4usize,
+        concat!("Offset of field: ", stringify!(Packed), "::", stringify!(c))
+    );
+    assert_eq!(
+        unsafe { &(*(::std::ptr::null::<Packed>())).d as *const _ as usize },
+        6usize,
+        concat!("Offset of field: ", stringify!(Packed), "::", stringify!(d))
+    );
+}
diff --git a/tests/headers/packed-n-with-padding.h b/tests/headers/packed-n-with-padding.h
new file mode 100644
index 00000000..8a6233b5
--- /dev/null
+++ b/tests/headers/packed-n-with-padding.h
@@ -0,0 +1,8 @@
+#pragma pack(push, 2)
+struct Packed {
+    char a;
+    short b;
+    char c;
+    int d;
+};
+#pragma pack(pop)