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.

Reproduce the difference in the knurling-rs/derive-mmio repository by checking out weird-pub-issue and running the array example with cargo +1.73 and cargo +1.74. Start by examining the generated derive_mmio::Mmio code around the offending pub fn ptr() -> PrivateType; done means explaining the changed E0446 behavior and identifying the appropriate compiler or macro-side fix.