# ADTs and Generic Arguments

The term `ADT` stands for "Algebraic data type", in rust this refers to a struct, enum, or union.

## ADTs Representation

Let's consider the example of a type like `MyStruct<u32>`, where `MyStruct` is defined like so:

```rust,ignore
struct MyStruct<T> { x: u8, y: T }
```

The type `MyStruct<u32>` would be an instance of `TyKind::Adt`:

```rust,ignore
Adt(&'tcx AdtDef, GenericArgs<'tcx>)
//  ------------  ---------------
//  (1)            (2)
//
// (1) represents the `MyStruct` part
// (2) represents the `<u32>`, or "substitutions" / generic arguments
```

There are two parts:

- The [`AdtDef`][adtdef] references the struct/enum/union but without the values for its type
  parameters. In our example, this is the `MyStruct` part *without* the argument `u32`.
  (Note that in the HIR, structs, enums and unions are represented differently, but in `ty::Ty`,
  they are all represented using `TyKind::Adt`.)
- The [`GenericArgs`] is a list of values that are to be substituted
for the generic parameters.  In our example of `MyStruct<u32>`, we would end up with a list like
`[u32]`. We’ll dig more into generics and substitutions in a little bit.


### **`AdtDef` and `DefId`**

For every type defined in the source code, there is a unique `DefId` (see [this
chapter](../hir.md#identifiers-in-the-hir)). This includes ADTs and generics. In the `MyStruct<T>`
definition we gave above, there are two `DefId`s: one for `MyStruct` and one for `T`.  Notice that
the code above does not generate a new `DefId` for `u32` because it is not defined in that code (it
is only referenced).

`AdtDef` is more or less a wrapper around `DefId` with lots of useful helper methods. There is
essentially a one-to-one relationship between `AdtDef` and `DefId`. You can get the `AdtDef` for a
`DefId` with the [`tcx.adt_def(def_id)` query][adtdefq]. `AdtDef`s are all interned, as shown
by the `'tcx` lifetime.


## Question: Why not substitute “inside” the `AdtDef`?

Recall that we represent a generic struct with `(AdtDef, args)`. So why bother with this scheme?

Well, the alternate way we could have chosen to represent types would be to always create a new,
fully-substituted form of the `AdtDef` where all the types are already substituted. This seems like
less of a hassle. However, the `(AdtDef, args)` scheme has some advantages over this.

First, `(AdtDef, args)` scheme has an efficiency win:

```rust,ignore
struct MyStruct<T> {
  ... 100s of fields ...
}

// Want to do: MyStruct<A> ==> MyStruct<B>
```

in an example like this, we can instantiate `MyStruct<A>` as `MyStruct<B>` (and so on) very cheaply,
by just replacing the one reference to `A` with `B`. But if we eagerly instantiated all the fields,
that could be a lot more work because we might have to go through all of the fields in the `AdtDef`