Ignore zero-sized types in wasm future-compat warning

This commit fixes a false positive of the warning triggered for rust-lang#138762
and the fix is to codify that zero-sized types are "safe" in both the
old and new ABIs.

Author: alexcrichton

compiler/rustc_monomorphize/src/mono_checks/abi_check.rs

@@ -111,6 +111,11 @@ fn wasm_abi_safe<'tcx>(tcx: TyCtxt<'tcx>, arg: &ArgAbi<'tcx, Ty<'tcx>>) -> bool
         }
     }
 
+    // Zero-sized types are dropped in both ABIs, so they're safe
+    if arg.layout.is_zst() {
+        return true;
+    }
+
     false
 }
 

tests/ui/lint/wasm_c_abi_transition.rs

@@ -39,3 +39,9 @@ pub fn call_other_fun(x: MyType) {
     unsafe { other_fun(x) } //~ERROR: wasm ABI transition
     //~^WARN: previously accepted
 }
+
+// Zero-sized types are safe in both ABIs
+#[repr(C)]
+pub struct MyZstType;
+#[allow(improper_ctypes_definitions)]
+pub extern "C" fn zst_safe(_x: (), _y: MyZstType) {}