# Instantiating `Binder`s

Much like [`EarlyBinder`], when accessing the inside of a [`Binder`] we must first discharge it by replacing the bound vars with some other value. This is for much the same reason as with `EarlyBinder`, types referencing parameters introduced by the `Binder` do not make any sense outside of that binder, for example:
```rust,ignore
fn foo<'a>(a: &'a u32) -> &'a u32 {
    a
}
fn bar<T>(a: fn(&u32) -> T) -> T {
    a(&10)
}

fn main() {
    let higher_ranked_fn_ptr = foo as for<'a> fn(&'a u32) -> &'a u32;
    let references_bound_vars = bar(higher_ranked_fn_ptr);
}
```
In this example we are providing an argument of type `for<'a> fn(&'^0 u32) -> &'^0 u32` to `bar`, we do not want to allow `T` to be inferred to the type `&'^0 u32` as it would be rather nonsensical (and likely unsound if we did not happen to ICE, `main` has no idea what `'a` is so how would the borrow checker handle a borrow with lifetime `'a`).

Unlike `EarlyBinder` we typically do not instantiate `Binder` with some concrete set of arguments from the user, i.e. `['b, 'static]` as arguments to a `for<'a1, 'a2> fn(&'a1 u32, &'a2 u32)`. Instead we usually instantiate the binder with inference variables or placeholders.

## Instantiating with inference variables

We instantiate binders with inference variables when we are trying to infer a possible instantiation of the binder, e.g. calling higher ranked function pointers or attempting to use a higher ranked where-clause to prove some bound. For example, given the `higher_ranked_fn_ptr` from the example above, if we were to call it with `&10_u32` we would: 
- Instantiate the binder with infer vars yielding a signature of `fn(&'?0 u32) -> &'?0 u32)`
- Equate the type of the provided argument `&10_u32` (&'static u32) with the type in the signature, `&'?0 u32`, inferring `'?0 = 'static`
- The provided arguments were correct as we were successfully able to unify the types of the provided arguments with the types of the arguments in fn ptr signature

As another example of instantiating with infer vars, given some `for<'a> T: Trait<'a>` where-clause, if we were attempting to prove that `T: Trait<'static>` holds we would:
- Instantiate the binder with infer vars yielding a where clause of `T: Trait<'?0>`
- Equate the goal of `T: Trait<'static>` with the instantiated where clause, inferring `'?0 = 'static`
- The goal holds because we were successfully able to unify `T: Trait<'static>` with `T: Trait<'?0>`

Instantiating binders with inference variables can be accomplished by using the [`instantiate_binder_with_fresh_vars`] method on [`InferCtxt`]. Binders should be instantiated with infer vars when we only care about one specific instantiation of the binder, if instead we wish to reason about all possible instantiations of the binder then placeholders should be used instead.

## Instantiating with placeholders

Placeholders are very similar to `Ty/ConstKind::Param`/`ReEarlyParam`, they represent some unknown type that is only equal to itself. `Ty`/`Const` and `Region` all have a [`Placeholder`] variant that is comprised of a [`Universe`] and a [`BoundVar`]. 

The `Universe` tracks which binder the placeholder originated from, and the `BoundVar` tracks which parameter on said binder that this placeholder corresponds to. Equality of placeholders is determined solely by whether the universes are equal and the `BoundVar`s are equal. See the [chapter on Placeholders and Universes][ch_placeholders_universes] for more information.

When talking with other rustc devs or seeing `Debug` formatted `Ty`/`Const`/`Region`s, `Placeholder` will often be written as `'!UNIVERSE_BOUNDVARS`. For example given some type `for<'a> fn(&'a u32, for<'b> fn(&'b &'a u32))`, after instantiating both binders (assuming the `Universe` in the current `InferCtxt` was `U0` beforehand), the type of `&'b &'a u32` would be represented as `&'!2_0 &!1_0 u32`.

When the universe of the placeholder is `0`, it will be entirely omitted from the debug output, i.e. `!0_2` would be printed as `!2`. This rarely happens in practice though as we increase the universe in the `InferCtxt` when instantiating a binder with placeholders so usually the lowest universe placeholders encounterable are ones in `U1`.