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% Higher-Rank Trait Bounds (HRTBs)
Rust's `Fn` traits are a little bit magic. For instance, we can write the
following code:
```rust
struct Closure<F> {
data: (u8, u16),
func: F,
}
impl<F> Closure<F>
where F: Fn(&(u8, u16)) -> &u8,
{
fn call(&self) -> &u8 {
(self.func)(&self.data)
}
}
fn do_it(data: &(u8, u16)) -> &u8 { &data.0 }
fn main() {
let clo = Closure { data: (0, 1), func: do_it };
println!("{}", clo.call());
}
```
If we try to naively desugar this code in the same way that we did in the
lifetimes section, we run into some trouble:
```rust,ignore
struct Closure<F> {
data: (u8, u16),
func: F,
}
impl<F> Closure<F>
// where F: Fn(&'??? (u8, u16)) -> &'??? u8,
{
fn call<'a>(&'a self) -> &'a u8 {
(self.func)(&self.data)
}
}
fn do_it<'b>(data: &'b (u8, u16)) -> &'b u8 { &'b data.0 }
fn main() {
'x: {
let clo = Closure { data: (0, 1), func: do_it };
println!("{}", clo.call());
}
}
```
How on earth are we supposed to express the lifetimes on `F`'s trait bound? We
need to provide some lifetime there, but the lifetime we care about can't be
named until we enter the body of `call`! Also, that isn't some fixed lifetime;
`call` works with *any* lifetime `&self` happens to have at that point.
This job requires The Magic of Higher-Rank Trait Bounds (HRTBs). The way we
desugar this is as follows:
```rust,ignore
where for<'a> F: Fn(&'a (u8, u16)) -> &'a u8,
```
(Where `Fn(a, b, c) -> d` is itself just sugar for the unstable *real* `Fn`
trait)
`for<'a>` can be read as "for all choices of `'a`", and basically produces an
*infinite list* of trait bounds that F must satisfy. Intense. There aren't many
places outside of the `Fn` traits where we encounter HRTBs, and even for
those we have a nice magic sugar for the common cases.
|
