Fix zerocopy-derive hygiene on Rust 1.58 and below

[joshlf]

Running ./cargo.sh +msrv test --package zerocopy-derive -- --skip ui with our MSRV set to 1.58 results in the following test failure:

error[E0446]: private type `Private` in public interface
  --> zerocopy-derive/tests/priv_in_pub.rs:18:10
   |
18 | #[derive(KnownLayout, IntoBytes, FromZeros, FromBytes, Unaligned)]
   |          ^^^^^^^^^^^ can't leak private type
...
24 | struct Private(());
   | ------------------- `Private` declared as private
   |
   = note: this error originates in the derive macro `KnownLayout` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0446]: private type `Private` in public interface
  --> zerocopy-derive/tests/priv_in_pub.rs:18:23
   |
18 | #[derive(KnownLayout, IntoBytes, FromZeros, FromBytes, Unaligned)]
   |                       ^^^^^^^^^ can't leak private type
...
24 | struct Private(());
   | ------------------- `Private` declared as private
   |
   = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0446]: private type `Private` in public interface
  --> zerocopy-derive/tests/priv_in_pub.rs:18:34
   |
18 | #[derive(KnownLayout, IntoBytes, FromZeros, FromBytes, Unaligned)]
   |                                  ^^^^^^^^^ can't leak private type
...
24 | struct Private(());
   | ------------------- `Private` declared as private
   |
   = note: this error originates in the derive macro `FromZeros` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0446]: private type `Private` in public interface
  --> zerocopy-derive/tests/priv_in_pub.rs:18:45
   |
18 | #[derive(KnownLayout, IntoBytes, FromZeros, FromBytes, Unaligned)]
   |                                             ^^^^^^^^^ can't leak private type
...
24 | struct Private(());
   | ------------------- `Private` declared as private
   |
   = note: this error originates in the derive macro `FromBytes` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0446]: private type `Private` in public interface
  --> zerocopy-derive/tests/priv_in_pub.rs:18:56
   |
18 | #[derive(KnownLayout, IntoBytes, FromZeros, FromBytes, Unaligned)]
   |                                                        ^^^^^^^^^ can't leak private type
...
24 | struct Private(());
   | ------------------- `Private` declared as private
   |
   = note: this error originates in the derive macro `Unaligned` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0446`.

For the time being, we've disabled this test on versions 1.58 and earlier. This isn't a regression because our published crates do not currently support 1.58. That said, we should fix this if possible before releasing 0.8.

So far, I have found the following possible culprits:

• Accessing private fields from the def site of a proc macro rust-lang/rust#46635 (comment)
• https://docs.rs/quote/latest/quote/macro.quote.html#hygiene

In particular this documentation suggests that the quote! macro itself may not be the problem:

Any interpolated tokens preserve the Span information provided by their ToTokens implementation. Tokens that originate within the quote! invocation are spanned with Span::call_site().

A different span can be provided through the quote_spanned! macro.

I have not yet tried to replace all quote! invocations with quote_spanned! (passing Span::call_site()). That's the first thing to try. Next would be to consider whether an existing token passed to quote! is carrying its own span information.

[jswrenn]

This isn't a hygiene issue, since it doesn't relate to how our derive captures names. Rather, it relates to a limitation of Rust's priv-in-pub checker being lifted in 1.59 because of rust-lang/rust#90586.

Disabling the test on versions below 1.59 is a sensible — even shippable — solution. There's no soundness issue here, just an expressivity issue, and it's not unlike all of the other aspects in which new features getting added to Rust increases the expressivity of the zerocopy crate in other ways. (The only difference is that this expressivity gain doesn't require us to explicitly gate an API based on rustc version.)

[joshlf]

Ack. I'll remove this as a 0.8 blocker.

[joshlf]

@jswrenn Any opposition to closing this as not planned?

[jswrenn]

No opposition.