Fix expected struct sizes for nightly optimizations (no more CI warnings)

parsers/build.rs

@@ -0,0 +1,12 @@
+use rustc_version::{version_meta, Channel};
+
+fn main() {
+    let version = version_meta().unwrap();
+
+    match version.channel {
+        Channel::Nightly => {
+            println!("cargo:rustc-cfg=rustc_nightly");
+        }
+        _ => {}
+    }
+}