Fix build on all platforms

This PR fixes the build on all platforms and all Rust version down to the
minimum Rust version supported by libc: Rust 1.13.0.

The `build.rs` is extended with logic to detect the newer Rust features used by
`libc` since Rust 1.13.0:

* Rust 1.19.0: `untagged_unions`. APIs using untagged unions are gated on
  `cfg(libc_unions)` and not available on older Rust versions.

* Rust 1.25.0: `repr(align)`. Because `repr(align)` cannot be parsed by older
  Rust versions, all uses of `repr(align)` are split into `align.rs` and
  `no_align.rs` modules, which are gated on the `cfg(libc_align)` at the top
  level. These modules sometimes contain macros that are expanded at the top
  level to avoid privacy issues (`pub(crate)` is not available in older Rust
  versions). Closes #1242 .

* Rust : `const` `mem::size_of`. These uses are worked around with hardcoded
  constants on older Rust versions.

Also, `repr(packed)` structs cannot automatically `derive()` some traits like
`Debug`. These have been moved into `s_no_extra_traits!` and the lint of missing
`Debug` implementations on public items is silenced for these. We can manually
implement the `extra_traits` for these in a follow up PR. This is tracked
in #1243. Also, `extra_traits` does not enable `align` manually anymore.

Since `f64::to_bits` is not available in older Rust versions, its usage
has been replaced with a `transmute` to an `u64` which is what that method
does under the hood.

Closes #1232 .

1 files changed, 46 insertions, 13 deletions

diff --git a/src/lib.rs b/src/lib.rs
index f46d6e263d..72e93aaf62 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -10,7 +10,7 @@
 
 //! Crate docs
 
-#![allow(bad_style, overflowing_literals, improper_ctypes)]
+#![allow(bad_style, overflowing_literals, improper_ctypes, unknown_lints)]
 #![crate_type = "rlib"]
 #![crate_name = "libc"]
 #![cfg_attr(cross_platform_docs, feature(no_core, lang_items, const_fn))]
@@ -151,7 +151,7 @@
 )]
 // Attributes needed when building as part of the standard library
 #![cfg_attr(feature = "rustc-dep-of-std", feature(cfg_target_vendor))]
-#![cfg_attr(feature = "rustc-dep-of-std", feature(link_cfg, repr_packed))]
+#![cfg_attr(feature = "rustc-dep-of-std", feature(link_cfg))]
 #![cfg_attr(feature = "rustc-dep-of-std", feature(no_core))]
 #![cfg_attr(feature = "rustc-dep-of-std", no_core)]
 #![cfg_attr(feature = "rustc-dep-of-std", allow(warnings))]
@@ -161,23 +161,56 @@
 )]
 // Enable lints
 #![cfg_attr(feature = "extra_traits", deny(missing_debug_implementations))]
-#![deny(missing_copy_implementations)]
-
+#![deny(missing_copy_implementations, safe_packed_borrows)]
 #[cfg(all(not(cross_platform_docs), feature = "use_std"))]
 extern crate std as core;
 
-#[cfg(feature = "rustc-dep-of-std")]
-extern crate rustc_std_workspace_core as core;
-#[cfg(feature = "rustc-dep-of-std")]
-#[allow(unused_imports)]
-use core::iter;
-#[cfg(feature = "rustc-dep-of-std")]
-#[allow(unused_imports)]
-use core::option;
-
 #[macro_use]
 mod macros;
 
+cfg_if! {
+    if #[cfg(feature = "rustc-dep-of-std")] {
+        extern crate rustc_std_workspace_core as core;
+        #[allow(unused_imports)]
+        use core::iter;
+        #[allow(unused_imports)]
+        use core::option;
+    }
+}
+
+cfg_if! {
+    if #[cfg(not(cross_platform_docs))] {
+        cfg_if! {
+            if #[cfg(libc_priv_mod_use)] {
+                #[cfg(libc_core_cvoid)]
+                #[allow(unused_imports)]
+                use core::ffi;
+                #[allow(unused_imports)]
+                use core::fmt;
+                #[allow(unused_imports)]
+                use core::hash;
+                #[allow(unused_imports)]
+                use core::num;
+                #[allow(unused_imports)]
+                use core::mem;
+            } else {
+                #[doc(hidden)]
+                #[allow(unused_imports)]
+                pub use core::fmt;
+                #[doc(hidden)]
+                #[allow(unused_imports)]
+                pub use core::hash;
+                #[doc(hidden)]
+                #[allow(unused_imports)]
+                pub use core::num;
+                #[doc(hidden)]
+                #[allow(unused_imports)]
+                pub use core::mem;
+            }
+        }
+    }
+}
+
 mod dox;
 
 cfg_if! {