Remove dependency on the synchapi winapi feature

core/Cargo.toml

@@ -20,7 +20,7 @@ libc = "0.2.27"
 
 [target.'cfg(windows)'.dependencies]
 winapi = { version = "0.3", features = ["winnt", "ntstatus", "minwindef",
-    "winerror", "winbase", "synchapi", "errhandlingapi", "handleapi"] }
+    "winerror", "winbase", "errhandlingapi", "handleapi"] }
 
 [features]
 nightly = []

core/src/spinwait.rs

@@ -17,11 +17,27 @@ use std::sync::atomic::spin_loop_hint;
 #[cfg(windows)]
 #[inline]
 fn thread_yield() {
+    // Note that this is manually defined here rather than using the definition
+    // through `winapi`. The `winapi` definition comes from the `synchapi`
+    // header which enables the "synchronization.lib" library. It turns out,
+    // however that `Sleep` comes from `kernel32.dll` so this activation isn't
+    // necessary.
+    //
+    // This was originally identified in rust-lang/rust where on MinGW the
+    // libsynchronization.a library pulls in a dependency on a newer DLL not
+    // present in older versions of Windows. (see rust-lang/rust#49438)
+    //
+    // This is a bit of a hack for now and ideally we'd fix MinGW's own import
+    // libraries, but that'll probably take a lot longer than patching this here
+    // and avoiding the `synchapi` feature entirely.
+    extern "system" {
+        fn Sleep(a: winapi::shared::minwindef::DWORD);
+    }
     unsafe {
         // We don't use SwitchToThread here because it doesn't consider all
         // threads in the system and the thread we are waiting for may not get
         // selected.
-        winapi::um::synchapi::Sleep(0);
+        Sleep(0);
     }
 }
 #[cfg(unix)]