Disappearing error in Rust 1.74

I tried this code:

$ git clone https://github.com/knurling-rs/derive-mmio
$ cd derive-mmio
$ git checkout weird-pub-issue
$ cargo +1.73 run --example array
...
error[E0446]: private type `Uart` in public interface
 --> examples/array.rs:1:10
  |
1 | #[derive(derive_mmio::Mmio)]
  |          ^^^^^^^^^^^^^^^^^ can't leak private type
2 | #[repr(C)]
3 | struct Uart {
  | ----------- `Uart` declared as private
  |
  = note: this error originates in the derive macro `derive_mmio::Mmio` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0446`.
error: could not compile `derive-mmio` (example "array") due to previous error
$ cargo +1.74 run --example array
    Finished dev [unoptimized + debuginfo] target(s) in 4.20s
     Running `target/debug/examples/array`

I expected to see this happen: Both 1.73 and 1.74 would report the same error?

Instead, this happened: 1.74 builds without an error.

Is this because the offending function (pub fn ptr() -> PrivateType) was macro generated?

I think the fix is to change our macro so that fn ptr() is only pub if PrivateType is pub. But it was weird that the error went away.

I believe this is expected, see #113126.

If I expand the macro (cargo expand) and write it out to a new file (and clean it up a bit), I get some warnings. Are the warnings elided because the code is macro generated?

warning: type `Uart` is more private than the item `MmioUart::<'_>::ptr`
  --> src/main.rs:39:5
   |
39 |     pub const unsafe fn ptr(&self) -> *mut Uart {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ method `MmioUart::<'_>::ptr` is reachable at visibility `pub`
   |
note: but type `Uart` is only usable at visibility `pub(crate)`

The private_interfaces lint and private_bounds lints (like most lints) are indeed silenced in macro generated code.

Do you think it would make sense to also show these two lints in macro generated code?

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Disappearing error in Rust 1.74 #138982

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Disappearing error in Rust 1.74 #138982

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Urgau commented on Mar 26, 2025

jonathanpallant commented on Mar 26, 2025

Urgau commented on Mar 26, 2025

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