Function find_file_fallback_exists 

pub fn find_file_fallback_exists(name: &str, ext_type: &str) -> bool

Perl Package.pm L2141-2210: FindFile_fallback Pure-check variant of find_file_fallback: strips the same suffix/prefix patterns and reports whether the fallback name has a registered binding, but DOES NOT eagerly invoke the binding’s load_definitions. Use this from pre-flight existence checks (e.g. class_cls_via_fallback in tex_job.rs) where firing the binding’s body has side effects (\LoadClass, \RequirePackage) that contaminate the subsequent real load. Witness: astro-ph0002213 — the \psfig cluster fix (mn1 → mn fallback was eagerly running mn.cls’s \LoadClass{article} before the \documentstyle[epsfig]{mn1} option-pass-through machinery had a chance to enqueue epsfig into opt@article.cls).