error[E0596]: cannot borrow `x` as mutable, as it is a captured variable in a `Fn` closure --> $DIR/issue-21600.rs:14:20 | LL | fn call_it(f: F) where F: Fn() { f(); } | - change this to accept `FnMut` instead of `Fn` ... LL | call_it(|| x.gen_mut()); | ------- -- ^ cannot borrow as mutable | | | | | in this closure | expects `Fn` instead of `FnMut` error[E0596]: cannot borrow `x` as mutable, as it is a captured variable in a `Fn` closure --> $DIR/issue-21600.rs:14:17 | LL | fn call_it(f: F) where F: Fn() { f(); } | - change this to accept `FnMut` instead of `Fn` ... LL | call_it(|| { | ------- -- in this closure | | | expects `Fn` instead of `FnMut` LL | call_it(|| x.gen()); LL | call_it(|| x.gen_mut()); | ^^ - mutable borrow occurs due to use of `x` in closure | | | cannot borrow as mutable error: aborting due to 2 previous errors For more information about this error, try `rustc --explain E0596`.