Rollup merge of #139498 - alexcrichton:wasm-zst-safe, r=wesleywiser

Ignore zero-sized types in wasm future-compat warning

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

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
 }