Recognize windows-sys signatures as "C" or "system" depending on cfg

Author: dtolnay

src/dbghelp.rs

@@ -30,12 +30,6 @@ use core::mem;
 use core::ptr;
 use core::slice;
 
-// This is used when we're double-checking function signatures against windows-sys.
-#[inline(always)]
-fn assert_equal_types<T>(a: T, _b: T) -> T {
-    a
-}
-
 // This macro is used to define a `Dbghelp` structure which internally contains
 // all the function pointers that we might load.
 macro_rules! dbghelp {
@@ -85,14 +79,20 @@ macro_rules! dbghelp {
             // either read the cached function pointer or load it and return the
             // loaded value. Loads are asserted to succeed.
             $(pub fn $name(&mut self) -> Option<$name> {
+                // Assert that windows_sys::$name is declared to have the same
+                // argument types and return type as our declaration, although
+                // it might be either extern "C" or extern "system".
+                #[cfg(windows_raw_dylib)]
+                let _: unsafe extern "C" fn($($argty),*) -> $ret = super::windows_sys::$name;
+                #[cfg(not(windows_raw_dylib))]
+                let _: unsafe extern "system" fn($($argty),*) -> $ret = super::windows_sys::$name;
+
                 unsafe {
                     if self.$name == 0 {
                         let name = concat!(stringify!($name), "\0");
                         self.$name = self.symbol(name.as_bytes())?;
                     }
-                    let ret = mem::transmute::<usize, $name>(self.$name);
-                    assert_equal_types(ret, super::windows_sys::$name);
-                    Some(ret)
+                    Some(mem::transmute::<usize, $name>(self.$name))
                 }
             })*