In the Rust SDK, the B512 definition matches the Sway standard library type with the same name and will be converted accordingly when interacting with contracts:
pub struct B512 {
pub bytes: [Bits256; 2],
}Here's an example:
let hi_bits = Bits256::from_hex_str(
"0xbd0c9b8792876713afa8bff383eebf31c43437823ed761cc3600d0016de5110c",
)?;
let lo_bits = Bits256::from_hex_str(
"0x44ac566bd156b4fc71a4a4cb2655d3dd360c695edb17dc3b64d611e122fea23d",
)?;
let b512 = B512::from((hi_bits, lo_bits));