Borrow Checker 1 @WaffleLapkin @BoxyUwU

Warning: this quiz is still "work-in-progress", some questions might not have good explanations (or any at all), formatting/structure/titles/etc are not final, and so on. You might want to return here on a later date.

fn func(a: &'static u32) {
    dbg!(a);
}

fn accepts_func(f: fn(&u32), data: &u32) {
    f(data);
}

fn main() {
    //  &'static u32 can be shortened to any
    // lifetime borrow so func should be fine
    accepts_func(func, &274);
}

Solution

error[E0308]: mismatched types
  --> examples/borrowck_1.rs:12:18
   |
12 |     accepts_func(func, &274);
   |     ------------ ^^^^ one type is more general than the other
   |     |
   |     arguments to this function are incorrect
   |
   = note: expected fn pointer `for<'a> fn(&'a _)`
                 found fn item `fn(&'static _) {func}`
note: function defined here
  --> examples/borrowck_1.rs:5:4
   |
5  | fn accepts_func(f: fn(&u32), data: &u32) {
   |    ^^^^^^^^^^^^ -----------

For more information about this error, try `rustc --explain E0308`.
error: could not compile `code` (example "borrowck_1") due to 1 previous error

Elided lifetimes in function pointers use "for all", so fn(&u32) in accepts_func is the same as for<'a> fn(&'a u32). In other words accepts_func requires a function that can be called with a reference with any lifetime.

Notably func can't be called with any lifetime — it can only be called with 'static, causing the error.

Note that normally it's okay to shorten lifetimes, i.e. coerce &'big _ to &'small _. However, in function pointers it's different, you can coerce fn(&'small _) to fn(&'big _) but not the other way around. This is because when you call fn(&'big _) the argument can be coerced to &'small _. This is also commonly known as functions being contravariant in respect to their arguments. You can learn more about variance in the Subtyping and Variance chapter of the nomicon.

Unfair Rust Quiz

Borrow Checker 1 @WaffleLapkin @BoxyUwU