#[derive(FromBytes)] not checked for generic structs?

[peter-lyons-kehl]

Hi. I must be missing something obvious, but I can't believe that the following compiles (both with 1.85.0-nightly and 1.83.0 stable (installed with rustup on current x64 Manjaro Linux). Any tips, please?

Cargo.toml contains:

[dependencies]
zerocopy = "0.8.13"
zerocopy-derive = "0.8.13"

#![no_std]

use zerocopy::FromZeros;
use zerocopy_derive::*;

#[derive(FromZeros)]
pub struct WrapFromZeros<Repr: FromZeros> (Repr);

pub type WrapU8 = WrapFromZeros<u8>;
// Should fail to compile:
pub type WrapOptStr = WrapFromZeros<Option<&'static str>>;

// Intentionally no representation specified, surprised that we don't get a compile error:
enum NoRepr {}
pub type WrapNoRepr = WrapFromZeros<NoRepr>;

https://docs.rs/zerocopy/latest/zerocopy/derive.FromZeros.html doesn't mention anything about generics.

[peter-lyons-kehl]

I'm also perplexed that it fails even without the #[derive(FromB)] line. I thought that the bound Repr: FromB would fail for Option<&'static str> and for enum NoRepr {}.

[jswrenn]

I think your confusion stems from Rust's odd treatment of type aliases: they're essentially unchecked at the definition site. For example, this produces no error at all:

struct Wrapper<T: Sized>(T);
type UnsizedWrapper = Wrapper<[u8]>;

...until you actually use the alias; e.g., this:

impl UnsizedWrapper {}

...results in an error:

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src/lib.rs:4:6
  |
4 | impl UnsizedWrapper {
  |      ^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `[u8]`
note: required by a bound in `Wrapper`
 --> src/lib.rs:1:19
  |
1 | struct Wrapper<T: Sized>(T);
  |                   ^^^^^ required by this bound in `Wrapper`

[peter-lyons-kehl]

Wow, thank you.