libc: Problem in implementing siginfo_t for Linux

The definition of siginfo_t in Linux (https://github.com/torvalds/linux/blob/master/include/uapi/asm-generic/siginfo.h) uses a inlined union type as its last field. Rust did not have support for union types, I guess that was why those fields were left as a padding array in current libc implementation.

I made some attempts to use union (introduced in Rust 1.19) to represent siginfo_t. However the resulting struct is 8 bytes larger than the original one (the one currently in libc, which is the same size as the C struct type). See the code here: https://play.rust-lang.org/?gist=99630206fcc8fd44b452d22552b37793&version=stable

My guess is the 8 more bytes come from two places:

1. the union type is 8 bytes aligned, but the first 3 fields of siginfo_t are 12 bytes (3x 4-bytes integer), so the padding is needed after the first 3 fields, which makes the offset of the union type at 16 bytes (instead of 12 bytes)
2. because the union type is 8 bytes aligned, so the size of the union type cannot be 4*29 (which is not a multiple of the 8 bytes alignment), thus padding is needed at the end to make it a muliple of 8.

I am not sure why the C definition does not have issues like this (probably because it uses inlined union in the struct definition) and how to properly implement siginfo_t in Rust. Please correct me if I am wrong. For my project, I have to extract data from the padding array (which looks awful).