Make GCC backend explicitly fail on tail call attempts

Author: semtexzv

compiler/rustc_codegen_gcc/src/builder.rs

@@ -1685,10 +1685,12 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
     fn apply_attrs_to_cleanup_callsite(&mut self, _llret: RValue<'gcc>) {
         // FIXME(bjorn3): implement
     }
-    
+
     fn set_tail_call(&mut self, _call_inst: RValue<'gcc>) {
-        // TODO: Implement proper tail call optimization for GCC backend
-        // For now, this is a no-op to make compilation work
+        // Explicitly fail when this method is called
+        bug!(
+            "Guaranteed tail calls with the 'become' keyword are not implemented in the GCC backend"
+        );
     }
 
     fn set_span(&mut self, _span: Span) {}