windows: expose cfg for 64-bit time_t

Windows has a uniform `time_t`. It's always 64-bits wide. A feature test
macro can change that. `libc` has a 32-bit `time_t`. This happens on
Windows with GNU. This is wrong. Stable needs to experiment.

The `cfg` allows experimenting. It exposes the right `time_t`.

Author: dybucc

build.rs

@@ -24,7 +24,7 @@ const ALLOWED_CFGS: &[&str] = &[
     "freebsd15",
     // Corresponds to `_FILE_OFFSET_BITS=64` in glibc
     "gnu_file_offset_bits64",
-    // Corresponds to `_TIME_BITS=64` in glibc
+    // Corresponds to `_TIME_BITS=64` in glibc. `_USE_32BIT_TIME_T` in Windows.
     "gnu_time_bits64",
     "libc_deny_warnings",
     // Corresponds to `__USE_TIME_BITS64` in UAPI
@@ -143,8 +143,10 @@ fn main() {
         set_cfg("linux_time_bits64");
     }
 
+    // FIXME(1.0): Windows GNU has 64-bit `time_t` by default. In x86 this is
+    // wrongly bound.
     if target_env == "gnu"
-        && target_os == "linux"
+        && matches!(target_os.as_str(), "linux" | "windows")
         && target_ptr_width == "32"
         && target_arch != "riscv32"
         && target_arch != "x86_64"
@@ -173,7 +175,7 @@ fn main() {
         let (timebits, filebits) = match (tb_env.as_deref(), fb_env.as_deref()) {
             (Ok(_), Ok(_)) => panic!(
                 "Do not set both libc_unstable_gnu_time_bits and \
-                libc_unstable_gnu_file_offset_bits"
+                 libc_unstable_gnu_file_offset_bits"
             ),
             (Err(_), Err(_)) => (defaultbits, defaultbits),
             (Ok(tb), Err(_)) if tb == "64" => (tb, tb),

libc-test/build.rs

@@ -784,6 +784,15 @@ fn test_windows(target: &str) {
     }
     cfg.define("_WIN32_WINNT", Some("0x8000"));
 
+    (gnu && i686).then(|| {
+        env::var("CARGO_CFG_LIBC_UNSTABLE_GNU_TIME_BITS")
+            .map_err(|_| ())
+            .and_then(|val| val.parse::<usize>().map_err(|_| ()))
+            .and_then(|v| (v == 64).then_some("gnu_time_bits64").ok_or(()))
+            .map(|op| _ = cfg.cfg(op, None))
+            .unwrap_or_default()
+    });
+
     headers!(
         cfg,
         "direct.h",

src/windows/mod.rs

@@ -19,7 +19,11 @@ pub type clock_t = i32;
 pub type errno_t = c_int;
 
 cfg_if! {
-    if #[cfg(all(target_arch = "x86", target_env = "gnu"))] {
+    if #[cfg(all(
+        target_arch = "x86",
+        target_env = "gnu",
+        not(gnu_time_bits64)
+    ))] {
         pub type time_t = i32;
     } else {
         pub type time_t = i64;