Clarify and test extern abi workaround for variadic function support (#3669)

Author: kennykerr

crates/tests/misc/calling_convention/Cargo.toml

@@ -25,3 +25,6 @@ features = [
     "Win32_System_SystemInformation",
     "Win32_UI_WindowsAndMessaging",
 ]
+
+[dependencies.windows-link]
+workspace = true

crates/tests/misc/calling_convention/tests/link.rs

@@ -0,0 +1,33 @@
+use windows_sys::{
+    core::{s, PCSTR, PSTR},
+    Win32::Foundation::ERROR_BUSY,
+    Win32::Networking::Ldap::LDAP_BUSY,
+};
+
+windows_link::link!("wldap32.dll" "C" fn LdapMapErrorToWin32(ldaperror: u32) -> u32);
+windows_link::link!("kernel32.dll" "system" fn GetTickCount() -> u32);
+windows_link::link!("user32.dll" "C" fn wsprintfA(param0: PSTR, param1: PCSTR, ...) -> i32);
+
+#[test]
+fn calling_convention() {
+    unsafe {
+        // This function requires cdecl on x86.
+        assert_eq!(LdapMapErrorToWin32(LDAP_BUSY as u32), ERROR_BUSY);
+
+        // This function requires stdcall on x86.
+        GetTickCount();
+    }
+}
+
+#[test]
+fn variadic() {
+    unsafe {
+        let mut buffer = vec![0u8; 1024];
+        let len = wsprintfA(buffer.as_mut_ptr(), s!("test-%d-%d!"), 123u32, 456u32);
+        let result = std::str::from_utf8_unchecked(std::slice::from_raw_parts(
+            buffer.as_ptr(),
+            len as usize,
+        ));
+        assert_eq!(result, "test-123-456!");
+    }
+}

crates/tests/misc/calling_convention/tests/ptr.rs

@@ -0,0 +1,20 @@
+// This is a test specifically for the breaking change discussed in https://github.com/microsoft/windows-rs/pull/3669
+// and https://github.com/microsoft/windows-rs/issues/3626 where a function pointer is stored. All of this can be
+// removed if the hardcoding of "C" is removed in a future breaking change.
+
+windows_link::link!("kernel32.dll" "system" fn GetTickCount() -> u32);
+
+#[cfg(target_arch = "x86")]
+type GetTickCountType = unsafe extern "system" fn() -> u32;
+
+#[cfg(not(target_arch = "x86"))]
+type GetTickCountType = unsafe extern "C" fn() -> u32;
+
+static GET_TICK_COUNT: GetTickCountType = GetTickCount;
+
+#[test]
+fn store_ptr() {
+    unsafe {
+        GET_TICK_COUNT();
+    }
+}