Unable to use generics after updating to 0.8 from 0.7 #2302
Description
I am attempting to update my codebase to use zerocopy 0.8 for all the wins we get! However, I'm stuck on a build break that I'm having a hard time understanding.
Here is a simplified version of the code causing the issue:
use zerocopy::{FromBytes, Immutable, KnownLayout, IntoBytes};
use zerocopy_derive::{FromBytes, Immutable, IntoBytes, KnownLayout};
#[derive(IntoBytes, FromBytes, KnownLayout, Immutable)]
#[repr(C, packed)]
struct Header {
magic: u32,
size: u32,
}
#[derive(IntoBytes, FromBytes, KnownLayout, Immutable)]
#[repr(C, packed)]
struct Packet<P: PacketPayload> {
header: Header,
payload: P
}
trait PacketPayload: IntoBytes + FromBytes + KnownLayout + Immutable {}
#[derive(IntoBytes, FromBytes, KnownLayout, Immutable)]
#[repr(C, packed)]
struct SimplePayload {
name: [u8; 32]
}
impl PacketPayload for SimplePayload {}
fn main() {
let packet = Packet {
header: Header {
magic: 0,
size: 0
},
payload: SimplePayload {
name: [0; 32]
}
};
let bytes = packet.as_bytes();
println!("Bytes: {bytes:?}");
}This is the compilation error I'm seeing:
error[E0277]: the size for values of type `<P as KnownLayout>::MaybeUninit` cannot be known at compilation time
--> src/main.rs:11:32
|
11 | #[derive(IntoBytes, FromBytes, KnownLayout, Immutable)]
| ^^^^^^^^^^^ doesn't have a size known at compile-time
|
= help: the trait `Sized` is not implemented for `<P as KnownLayout>::MaybeUninit`
= note: the last field of a packed struct may only have a dynamically sized type if it does not need drop to be run
= help: change the field's type to have a statically known size
= note: this error originates in the derive macro `KnownLayout` (in Nightly builds, run with -Z macro-backtrace for more info)
For more information about this error, try `rustc --explain E0277`.
error: could not compile `zerocopy_test` (bin "zerocopy_test") due to 1 previous error
The equivalent (in my mind) version of this code compiles in zerocopy 0.7:
use zerocopy::{FromBytes, FromZeroes, AsBytes};
use zerocopy_derive::{FromBytes, FromZeroes, AsBytes};
#[derive(AsBytes, FromBytes, FromZeroes)]
#[repr(C, packed)]
struct Header {
magic: u32,
size: u32,
}
#[derive(AsBytes, FromBytes, FromZeroes)]
#[repr(C, packed)]
struct Packet<P: PacketPayload> {
header: Header,
payload: P
}
trait PacketPayload: AsBytes + FromBytes + FromZeroes {}
#[derive(AsBytes, FromBytes, FromZeroes)]
#[repr(C, packed)]
struct SimplePayload {
name: [u8; 32]
}
impl PacketPayload for SimplePayload {}
fn main() {
let packet = Packet {
header: Header {
magic: 0,
size: 0
},
payload: SimplePayload {
name: [0; 32]
}
};
let bytes = packet.as_bytes();
println!("Bytes: {bytes:?}");
}