src/doc/rustc-dev-guide/src/generics.md

   1 # Generics and substitutions
   2
   3 Given a generic type `MyType<A, B, …>`, we may want to swap out the generics `A, B, …` for some
   4 other types (possibly other generics or concrete types). We do this a lot while doing type
   5 inference, type checking, and trait solving. Conceptually, during these routines, we may find out
   6 that one type is equal to another type and want to swap one out for the other and then swap that out
   7 for another type and so on until we eventually get some concrete types (or an error).
   8
   9 In rustc this is done using the `SubstsRef` that we mentioned above (“substs” = “substitutions”).
  10 Conceptually, you can think of `SubstsRef` of a list of types that are to be substituted for the
  11 generic type parameters of the ADT.
  12
  13 `SubstsRef` is a type alias of `List<GenericArg<'tcx>>` (see [`List` rustdocs][list]).
  14 [`GenericArg`] is essentially a space-efficient wrapper around [`GenericArgKind`], which is an enum
  15 indicating what kind of generic the type parameter is (type, lifetime, or const).  Thus, `SubstsRef`
  16 is conceptually like a `&'tcx [GenericArgKind<'tcx>]` slice (but it is actually a `List`).
  17
  18 [list]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.List.html
  19 [`GenericArg`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/subst/struct.GenericArg.html
  20 [`GenericArgKind`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/subst/enum.GenericArgKind.html
  21
  22 So why do we use this `List` type instead of making it really a slice? It has the length "inline",
  23 so `&List` is only 32 bits. As a consequence, it cannot be "subsliced" (that only works if the
  24 length is out of line).
  25
  26 This also implies that you can check two `List`s for equality via `==` (which would be not be
  27 possible for ordinary slices). This is precisely because they never represent a "sub-list", only the
  28 complete `List`, which has been hashed and interned.
  29
  30 So pulling it all together, let’s go back to our example above:
  31
  32 ```rust,ignore
  33 struct MyStruct<T>
  34 ```
  35
  36 - There would be an `AdtDef` (and corresponding `DefId`) for `MyStruct`.
  37 - There would be a `TyKind::Param` (and corresponding `DefId`) for `T` (more later).
  38 - There would be a `SubstsRef` containing the list `[GenericArgKind::Type(Ty(T))]`
  39     - The `Ty(T)` here is my shorthand for entire other `ty::Ty` that has `TyKind::Param`, which we
  40       mentioned in the previous point.
  41 - This is one `TyKind::Adt` containing the `AdtDef` of `MyStruct` with the `SubstsRef` above.
  42
  43 Finally, we will quickly mention the
  44 [`Generics`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Generics.html) type. It
  45 is used to give information about the type parameters of a type.
  46
  47 ### Unsubstituted Generics
  48
  49 So above, recall that in our example the `MyStruct` struct had a generic type `T`. When we are (for
  50 example) type checking functions that use `MyStruct`, we will need to be able to refer to this type
  51 `T` without actually knowing what it is. In general, this is true inside all generic definitions: we
  52 need to be able to work with unknown types. This is done via `TyKind::Param` (which we mentioned in
  53 the example above).
  54
  55 Each `TyKind::Param` contains two things: the name and the index. In general, the index fully
  56 defines the parameter and is used by most of the code. The name is included for debug print-outs.
  57 There are two reasons for this. First, the index is convenient, it allows you to include into the
  58 list of generic arguments when substituting. Second, the index is more robust. For example, you
  59 could in principle have two distinct type parameters that use the same name, e.g. `impl<A> Foo<A> {
  60 fn bar<A>() { .. } }`, although the rules against shadowing make this difficult (but those language
  61 rules could change in the future).
  62
  63 The index of the type parameter is an integer indicating its order in the list of the type
  64 parameters. Moreover, we consider the list to include all of the type parameters from outer scopes.
  65 Consider the following example:
  66
  67 ```rust,ignore
  68 struct Foo<A, B> {
  69   // A would have index 0
  70   // B would have index 1
  71
  72   .. // some fields
  73 }
  74 impl<X, Y> Foo<X, Y> {
  75   fn method<Z>() {
  76     // inside here, X, Y and Z are all in scope
  77     // X has index 0
  78     // Y has index 1
  79     // Z has index 2
  80   }
  81 }
  82 ```
  83
  84 When we are working inside the generic definition, we will use `TyKind::Param` just like any other
  85 `TyKind`; it is just a type after all. However, if we want to use the generic type somewhere, then
  86 we will need to do substitutions.
  87
  88 For example suppose that the `Foo<A, B>` type from the previous example has a field that is a
  89 `Vec<A>`. Observe that `Vec` is also a generic type. We want to tell the compiler that the type
  90 parameter of `Vec` should be replaced with the `A` type parameter of `Foo<A, B>`. We do that with
  91 substitutions:
  92
  93 ```rust,ignore
  94 struct Foo<A, B> { // Adt(Foo, &[Param(0), Param(1)])
  95   x: Vec<A>, // Adt(Vec, &[Param(0)])
  96   ..
  97 }
  98
  99 fn bar(foo: Foo<u32, f32>) { // Adt(Foo, &[u32, f32])
 100   let y = foo.x; // Vec<Param(0)> => Vec<u32>
 101 }
 102 ```
 103
 104 This example has a few different substitutions:
 105
 106 - In the definition of `Foo`, in the type of the field `x`, we replace `Vec`'s type parameter with
 107   `Param(0)`, the first parameter of `Foo<A, B>`, so that the type of `x` is `Vec<A>`.
 108 - In the function `bar`, we specify that we want a `Foo<u32, f32>`. This means that we will
 109   substitute `Param(0)` and `Param(1)` with `u32` and `f32`.
 110 - In the body of `bar`, we access `foo.x`, which has type `Vec<Param(0)>`, but `Param(0)` has been
 111   substituted for `u32`, so `foo.x` has type `Vec<u32>`.
 112
 113 Let’s look a bit more closely at that last substitution to see why we use indexes. If we want to
 114 find the type of `foo.x`, we can get generic type of `x`, which is `Vec<Param(0)>`. Now we can take
 115 the index `0` and use it to find the right type substitution: looking at `Foo`'s `SubstsRef`, we
 116 have the list `[u32, f32]` , since we want to replace index `0`, we take the 0-th index of this
 117 list, which is `u32`. Voila!
 118
 119 You may have a couple of followup questions…
 120
 121  **`type_of`** How do we get the “generic type of `x`"? You can get the type of pretty much anything
 122  with the   `tcx.type_of(def_id)` query. In this case, we would pass the `DefId` of the field `x`.
 123  The `type_of` query always returns the definition with the generics that are in scope of the
 124  definition. For example, `tcx.type_of(def_id_of_my_struct)` would return the “self-view” of
 125  `MyStruct`: `Adt(Foo, &[Param(0), Param(1)])`.
 126
 127 **`subst`** How do we actually do the substitutions? There is a function for that too! You use
 128 [`subst`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/subst/trait.Subst.html) to
 129 replace a `SubstRef` with another list of types.
 130
 131 [Here is an example of actually using `subst` in the compiler][substex].  The exact details are not
 132 too important, but in this piece of code, we happen to be converting from the `rustc_hir::Ty` to
 133 a real `ty::Ty`. You can see that we first get some substitutions (`substs`).  Then we call
 134 `type_of` to get a type and call `ty.subst(substs)` to get a new version of `ty` with
 135 the substitutions made.
 136
 137 [substex]: https://github.com/rust-lang/rust/blob/597f432489f12a3f33419daa039ccef11a12c4fd/src/librustc_typeck/astconv.rs#L942-L953
 138
 139 **Note on indices:** It is possible for the indices in `Param` to not match with what we expect. For
 140 example, the index could be out of bounds or it could be the index of a lifetime when we were
 141 expecting a type. These sorts of errors would be caught earlier in the compiler when translating
 142 from a `rustc_hir::Ty` to a `ty::Ty`. If they occur later, that is a compiler bug.
 143
 144