Heads-up: winapi 0.2.8 will fail to build soon-ish

[RalfJung]

In rust-lang/rust#102513 we are moving ahead with finally closing the ancient soundness issue rust-lang/rust#82523 (originally tracked at rust-lang/rust#27060): we are disallowing creating references to fields of packed structs. It seems like that will make winapi 0.2.8 fail to build:

error[E0791]: `Debug` can't be derived on this `#[repr(packed)]` struct that does not derive `Copy`
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263 |           #[repr(C)] #[derive(Debug)] $(#[$attrs])*
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   ::: C:\Users\runneradmin\.cargo\registry\src\github.com-1ecc6299db9ec823\winapi-0.2.8\src\mmreg.rs:290:1
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290 | / STRUCT!{#[repr(packed)] struct WAVEFORMATEX {
291 | |     wFormatTag: ::WORD,
292 | |     nChannels: ::WORD,
293 | |     nSamplesPerSec: ::DWORD,
297 | |     cbSize: ::WORD,
298 | | }}
    | |__- in this macro invocation
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    = note: this error originates in the derive macro `Debug` which comes from the expansion of the macro `STRUCT` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0791]: `Debug` can't be derived on this `#[repr(packed)]` struct that does not derive `Copy`
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263 |           #[repr(C)] #[derive(Debug)] $(#[$attrs])*
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   ::: C:\Users\runneradmin\.cargo\registry\src\github.com-1ecc6299db9ec823\winapi-0.2.8\src\mmreg.rs:299:1
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299 | / STRUCT!{#[repr(packed)] struct WAVEFORMATEXTENSIBLE {
300 | |     Format: ::WAVEFORMATEX,
301 | |     Samples: ::WORD,
302 | |     dwChannelMask: ::DWORD,
303 | |     SubFormat: ::GUID,
304 | | }}
    |
    = note: this error originates in the derive macro `Debug` which comes from the expansion of the macro `STRUCT` (in Nightly builds, run with -Z macro-backtrace for more info)


error[E0791]: `Debug` can't be derived on this `#[repr(packed)]` struct that does not derive `Copy`
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263 |           #[repr(C)] #[derive(Debug)] $(#[$attrs])*
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   ::: C:\Users\runneradmin\.cargo\registry\src\github.com-1ecc6299db9ec823\winapi-0.2.8\src\usbspec.rs:26:1
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26  | / STRUCT!{#[repr(packed)] struct USB_CONFIGURATION_DESCRIPTOR {
27  | |     bLength: ::UCHAR,
28  | |     bDescriptorType: ::UCHAR,
29  | |     wTotalLength: ::USHORT,
...   |
34  | |     MaxPower: ::UCHAR,
35  | | }}
    |
    = note: this error originates in the derive macro `Debug` which comes from the expansion of the macro `STRUCT` (in Nightly builds, run with -Z macro-backtrace for more info)


error[E0791]: `Debug` can't be derived on this `#[repr(packed)]` struct that does not derive `Copy`
    |
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263 |           #[repr(C)] #[derive(Debug)] $(#[$attrs])*
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   ::: C:\Users\runneradmin\.cargo\registry\src\github.com-1ecc6299db9ec823\winapi-0.2.8\src\winusb.rs:8:1
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8   | / STRUCT!{#[repr(packed)] struct WINUSB_SETUP_PACKET {
9   | |     RequestType: ::UCHAR,
10  | |     Request: ::UCHAR,
11  | |     Value: ::USHORT,
12  | |     Index: ::USHORT,
13  | |     Length: ::USHORT,
14  | | }}
    |
    = note: this error originates in the derive macro `Debug` which comes from the expansion of the macro `STRUCT` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0791`.

It looks like current versions of winapi avoid that error by deriving Copy for those structs.

I am not sure if there is anything you want to do about this -- winapi 0.2.8 seems to not be supported any more. However, given how widely used this crate is, it still might make sense to do a patch release so that people can fix their build with cargo update. If you want to do that, I'm open to waiting a bit before landing the rustc PR so that you can get the patch release out first.

[RalfJung]

I wrote a patch that fixes the build and pushed it to https://github.com/RalfJung/winapi-rs/tree/0.2. I am not entirely sure, however, if there was some reason why Copy was implemented instead of derived?

[tbu-]

It might have been implemented instead of derived due to compile-duration concerns.

Is there a reason why the derived Debug implementation cannot take the implemented Copy into account? This would solve the issue in a backward compatible way.

[RalfJung]

The reason is "it's a breaking change" (given the constraints on how derive macros work), but we are probably going to do that anyway. See rust-lang/rust#104429.

[RalfJung]

We ended up changing the implementation of derive on packed types to keep winapi 0.2 working, see rust-lang/rust#104429.