omit unused args warnings for intrinsics without body

compiler/rustc_passes/src/liveness.rs

@@ -1521,6 +1521,18 @@ impl<'tcx> Liveness<'_, 'tcx> {
     }
 
     fn warn_about_unused_args(&self, body: &hir::Body<'_>, entry_ln: LiveNode) {
+        let fn_attrs = self.ir.tcx.hir().attrs(body.value.hir_id);
+        let is_intrinsic = fn_attrs.iter().any(|attr| attr.has_name(sym::rustc_intrinsic));
+        if is_intrinsic {
+            let has_body = match &body.value.kind {
+                rustc_hir::ExprKind::Block(block, _) => !block.stmts.is_empty() || block.expr.is_some(),
+                _ => false,
+            };
+            if has_body {
+                return;
+            }
+        }
+
         for p in body.params {
             self.check_unused_vars_in_pat(
                 p.pat,