libc: add padding struct

[mbyx]

Description

Adds a struct for padding types.

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[tgross35]

I don't think we need to export this, pub(crate) should be fine

[tgross35]

Fyi you can delete the template in the PR body if it's not relevant

[tgross35]

There shouldn't be anything to export

[tgross35]

LGTM with the last change, any reason this is marked as a draft?

[mbyx]

It seems like ctest still fails with just these changes.

[tgross35]

 error: struct `Padding` is never constructed
  --> src/types.rs:9:19
   |
 9 | pub(crate) struct Padding<T: Copy>(MaybeUninit<T>);
   |                   ^^^^^^^
   |

This bit? It just needs an allow(unused) (I don't see a ctest-specific failure)

[tgross35]

For style check, this will need an update at

libc/libc-test/test/check_style.rs

Line 20 in c0071cc

const SKIP_PREFIXES: &[&str] = &[

[tgross35]

If you need to import all of these, just add Default to the prelude macro and add use crate::prelude::* here.

This adds a bit of trickiness: there is no crate::prelude in some cases

libc/src/lib.rs

Lines 143 to 145 in c0071cc

	} else {
	// non-supported targets: empty...
	}

, so the import would fail in this module. To avoid that problem, move mod types into the prelude as well.

[tgross35]

Sigh I was thinking this was already in the prelude, sorry I keep steering you wrong here.

Add this to prelude! instead, that's consistent. Also it will need to be ::core::prelude::v1::derive like https://github.com/rust-lang/libc/blob/c0071cc9f9385da6352e40824cfd77334e85f72d/src/macros.rs#L117C15-L117C43, the 2024 prelude isn't available until way after our MSRV.

[tgross35]

Thanks for all the cycles, looks great :)