libc: add padding struct

libc-test/test/check_style.rs

@@ -20,7 +20,7 @@ use style::{Result, StyleChecker};
 const SKIP_PREFIXES: &[&str] = &[
     // Don't run the style checker on the reorganized portion of the crate while we figure
     // out what style we want.
-    "new/",
+    "new/", "types.rs",
 ];
 
 #[test]

src/macros.rs

@@ -64,6 +64,8 @@ macro_rules! cfg_if {
 /// Create an internal crate prelude with `core` reexports and common types.
 macro_rules! prelude {
     () => {
+        mod types;
+
         /// Frequently-used types that are available on all platforms
         ///
         /// We need to reexport the core types so this works with `rust-dep-of-std`.
@@ -72,14 +74,20 @@ macro_rules! prelude {
             #[allow(unused_imports)]
             pub(crate) use ::core::clone::Clone;
             #[allow(unused_imports)]
+            pub(crate) use ::core::default::Default;
+            #[allow(unused_imports)]
             pub(crate) use ::core::marker::{Copy, Send, Sync};
             #[allow(unused_imports)]
             pub(crate) use ::core::option::Option;
             #[allow(unused_imports)]
+            pub(crate) use ::core::prelude::v1::derive;
+            #[allow(unused_imports)]
             pub(crate) use ::core::{fmt, hash, iter, mem};
             #[allow(unused_imports)]
             pub(crate) use mem::{align_of, align_of_val, size_of, size_of_val};
 
+            #[allow(unused_imports)]
+            pub(crate) use crate::types::Padding;
             // Commonly used types defined in this crate
             #[allow(unused_imports)]
             pub(crate) use crate::{

src/types.rs

@@ -0,0 +1,18 @@
+//! Platform-agnostic support types.
+
+use core::mem::MaybeUninit;
+
+use crate::prelude::*;
+
+/// A transparent wrapper over `MaybeUninit<T>` to represent uninitialized padding
+/// while providing `Default`.
+#[allow(unused)]
+#[repr(transparent)]
+#[derive(Clone, Copy)]
+pub(crate) struct Padding<T: Copy>(MaybeUninit<T>);
+
+impl<T: Copy> Default for Padding<T> {
+    fn default() -> Self {
+        Self(MaybeUninit::zeroed())
+    }
+}