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 .

Author: gnzlbg

Cargo.toml

@@ -27,7 +27,7 @@ default = ["use_std"]
 use_std = []
 align = []
 rustc-dep-of-std = ['align', 'rustc-std-workspace-core']
-extra_traits = ["align"]
+extra_traits = []
 
 [workspace]
 members = ["libc-test"]

build.rs

@@ -3,12 +3,42 @@ use std::process::Command;
 use std::str;
 
 fn main() {
-    /*
-     * If `core::ffi::c_void` exists, libc can just re-export it. Otherwise, it
-     * must define an incompatible type to retain backwards-compatibility.
-     */
-    if rustc_minor_version().expect("Failed to get rustc version") >= 30 {
-        println!("cargo:rustc-cfg=core_cvoid");
+    let rustc_minor_ver =
+        rustc_minor_version().expect("Failed to get rustc version");
+    let rustc_dep_of_std =
+        std::env::var("CARGO_FEATURE_RUSTC_DEP_OF_STD").is_ok();
+    let align_cargo_feature = std::env::var("CARGO_FEATURE_ALIGN").is_ok();
+
+    // Rust >= 1.15 supports private module use:
+    if rustc_minor_ver >= 15 || rustc_dep_of_std {
+        println!("cargo:rustc-cfg=libc_priv_mod_use");
+    }
+
+    // Rust >= 1.19 supports unions:
+    if rustc_minor_ver >= 19 || rustc_dep_of_std {
+        println!("cargo:rustc-cfg=libc_union");
+    }
+
+    // Rust >= 1.24 supports const mem::size_of:
+    if rustc_minor_ver >= 24 || rustc_dep_of_std {
+        println!("cargo:rustc-cfg=libc_const_size_of");
+    }
+
+    // Rust >= 1.25 supports repr(align):
+    if rustc_minor_ver >= 25 || rustc_dep_of_std || align_cargo_feature {
+        println!("cargo:rustc-cfg=libc_align");
+    }
+
+    // Rust >= 1.30 supports `core::ffi::c_void`, so libc can just re-export it.
+    // Otherwise, it defines an incompatible type to retaining
+    // backwards-compatibility.
+    if rustc_minor_ver >= 30 || rustc_dep_of_std {
+        println!("cargo:rustc-cfg=libc_core_cvoid");
+    }
+
+    // Rust >= 1.33 supports repr(packed(N))
+    if rustc_minor_ver >= 33 || rustc_dep_of_std {
+        println!("cargo:rustc-cfg=libc_packedN");
     }
 }
 

src/cloudabi/mod.rs

@@ -316,14 +316,15 @@ cfg_if! {
 }
 
 cfg_if! {
-    if #[cfg(core_cvoid)] {
-        pub use core::ffi::c_void;
+    if #[cfg(libc_core_cvoid)] {
+        pub use ::ffi::c_void;
     } else {
         // Use repr(u8) as LLVM expects `void*` to be the same as `i8*` to help
         // enable more optimization opportunities around it recognizing things
         // like malloc/free.
         #[repr(u8)]
         #[allow(missing_copy_implementations)]
+        #[allow(missing_debug_implementations)]
         pub enum c_void {
             // Two dummy variants so the #[repr] attribute can be used.
             #[doc(hidden)]

src/fuchsia/align.rs

@@ -0,0 +1,75 @@
+macro_rules! expand_align {
+    () => {
+        s! {
+            #[cfg_attr(
+                any(
+                    target_pointer_width = "32",
+                    target_arch = "x86_64"
+                ),
+                repr(align(4)))]
+            #[cfg_attr(
+                not(any(
+                    target_pointer_width = "32",
+                    target_arch = "x86_64"
+                )),
+                repr(align(8)))]
+            pub struct pthread_mutexattr_t {
+                size: [u8; ::__SIZEOF_PTHREAD_MUTEXATTR_T],
+            }
+
+            #[cfg_attr(target_pointer_width = "32",
+                       repr(align(4)))]
+            #[cfg_attr(target_pointer_width = "64",
+                       repr(align(8)))]
+            pub struct pthread_rwlockattr_t {
+                size: [u8; ::__SIZEOF_PTHREAD_RWLOCKATTR_T],
+            }
+
+            #[repr(align(4))]
+            pub struct pthread_condattr_t {
+                size: [u8; ::__SIZEOF_PTHREAD_CONDATTR_T],
+            }
+        }
+
+        s_no_extra_traits! {
+            #[allow(missing_debug_implementations)]
+            #[cfg_attr(all(target_pointer_width = "32",
+                           any(target_arch = "arm",
+                               target_arch = "x86_64")),
+                       repr(align(4)))]
+            #[cfg_attr(any(target_pointer_width = "64",
+                           not(any(target_arch = "arm",
+                                   target_arch = "x86_64"))),
+                       repr(align(8)))]
+            pub struct pthread_mutex_t {
+                size: [u8; ::__SIZEOF_PTHREAD_MUTEX_T],
+            }
+
+            #[allow(missing_debug_implementations)]
+            #[cfg_attr(all(target_pointer_width = "32",
+                           any(target_arch = "arm",
+                               target_arch = "x86_64")),
+                       repr(align(4)))]
+            #[cfg_attr(any(target_pointer_width = "64",
+                           not(any(target_arch = "arm",
+                                   target_arch = "x86_64"))),
+                       repr(align(8)))]
+            pub struct pthread_rwlock_t {
+                size: [u8; ::__SIZEOF_PTHREAD_RWLOCK_T],
+            }
+
+            #[allow(missing_debug_implementations)]
+            #[cfg_attr(target_pointer_width = "32",
+                       repr(align(4)))]
+            #[cfg_attr(target_pointer_width = "64",
+                       repr(align(8)))]
+            #[cfg_attr(target_arch = "x86",
+                       repr(align(4)))]
+            #[cfg_attr(not(target_arch = "x86"),
+                       repr(align(8)))]
+            pub struct pthread_cond_t {
+                size: [u8; ::__SIZEOF_PTHREAD_COND_T],
+            }
+        }
+    }
+}