The ty module: representing types

The ty module defines how the Rust compiler represents types internally. It also defines the typing context (tcx or TyCtxt), which is the central data structure in the compiler.

The tcx and how it uses lifetimes

The tcx ("typing context") is the central data structure in the compiler. It is the context that you use to perform all manner of queries. The struct TyCtxt defines a reference to this shared context:

tcx: TyCtxt<'a, 'gcx, 'tcx>
//          --  ----  ----
//          |   |     |
//          |   |     innermost arena lifetime (if any)
//          |   "global arena" lifetime
//          lifetime of this reference

As you can see, the TyCtxt type takes three lifetime parameters. These lifetimes are perhaps the most complex thing to understand about the tcx. During Rust compilation, we allocate most of our memory in arenas, which are basically pools of memory that get freed all at once. When you see a reference with a lifetime like 'tcx or 'gcx, you know that it refers to arena-allocated data (or data that lives as long as the arenas, anyhow).

We use two distinct levels of arenas. The outer level is the "global arena". This arena lasts for the entire compilation: so anything you allocate in there is only freed once compilation is basically over (actually, when we shift to executing LLVM).

To reduce peak memory usage, when we do type inference, we also use an inner level of arena. These arenas get thrown away once type inference is over. This is done because type inference generates a lot of "throw-away" types that are not particularly interesting after type inference completes, so keeping around those allocations would be wasteful.

Often, we wish to write code that explicitly asserts that it is not taking place during inference. In that case, there is no "local" arena, and all the types that you can access are allocated in the global arena. To express this, the idea is to use the same lifetime for the 'gcx and 'tcx parameters of TyCtxt. Just to be a touch confusing, we tend to use the name 'tcx in such contexts. Here is an example:

fn not_in_inference<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) {
    //                                        ----  ----
    //                                        Using the same lifetime here asserts
    //                                        that the innermost arena accessible through
    //                                        this reference *is* the global arena.

In contrast, if we want to code that can be usable during type inference, then you need to declare a distinct 'gcx and 'tcx lifetime parameter:

fn maybe_in_inference<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>, def_id: DefId) {
    //                                                ----  ----
    //                                        Using different lifetimes here means that
    //                                        the innermost arena *may* be distinct
    //                                        from the global arena (but doesn't have to be).

Allocating and working with types

Rust types are represented using the Ty<'tcx> defined in the ty module (not to be confused with the Ty struct from the HIR). This is in fact a simple type alias for a reference with 'tcx lifetime:

pub type Ty<'tcx> = &'tcx TyS<'tcx>;

You can basically ignore the TyS struct – you will basically never access it explicitly. We always pass it by reference using the Ty<'tcx> alias – the only exception I think is to define inherent methods on types. Instances of TyS are only ever allocated in one of the rustc arenas (never e.g. on the stack).

One common operation on types is to match and see what kinds of types they are. This is done by doing match ty.sty, sort of like this:

fn test_type<'tcx>(ty: Ty<'tcx>) {
    match ty.sty {
        ty::TyArray(elem_ty, len) => { ... }

The sty field (the origin of this name is unclear to me; perhaps structural type?) is of type TyKind<'tcx>, which is an enum defining all of the different kinds of types in the compiler.

N.B. inspecting the sty field on types during type inference can be risky, as there may be inference variables and other things to consider, or sometimes types are not yet known that will become known later.).

To allocate a new type, you can use the various mk_ methods defined on the tcx. These have names that correpond mostly to the various kinds of type variants. For example:

let array_ty = tcx.mk_array(elem_ty, len * 2);

These methods all return a Ty<'tcx> – note that the lifetime you get back is the lifetime of the innermost arena that this tcx has access to. In fact, types are always canonicalized and interned (so we never allocate exactly the same type twice) and are always allocated in the outermost arena where they can be (so, if they do not contain any inference variables or other "temporary" types, they will be allocated in the global arena). However, the lifetime 'tcx is always a safe approximation, so that is what you get back.

NB. Because types are interned, it is possible to compare them for equality efficiently using == – however, this is almost never what you want to do unless you happen to be hashing and looking for duplicates. This is because often in Rust there are multiple ways to represent the same type, particularly once inference is involved. If you are going to be testing for type equality, you probably need to start looking into the inference code to do it right.

You can also find various common types in the tcx itself by accessing tcx.types.bool, tcx.types.char, etc (see CommonTypes for more).

Beyond types: other kinds of arena-allocated data structures

In addition to types, there are a number of other arena-allocated data structures that you can allocate, and which are found in this module. Here are a few examples:

  • Substs, allocated with mk_substs – this will intern a slice of types, often used to specify the values to be substituted for generics (e.g. HashMap<i32, u32> would be represented as a slice &'tcx [tcx.types.i32, tcx.types.u32]).
  • TraitRef, typically passed by value – a trait reference consists of a reference to a trait along with its various type parameters (including Self), like i32: Display (here, the def-id would reference the Display trait, and the substs would contain i32).
  • Predicate defines something the trait system has to prove (see traits module).

Import conventions

Although there is no hard and fast rule, the ty module tends to be used like so:

use ty::{self, Ty, TyCtxt};

In particular, since they are so common, the Ty and TyCtxt types are imported directly. Other types are often referenced with an explicit ty:: prefix (e.g. ty::TraitRef<'tcx>). But some modules choose to import a larger or smaller set of names explicitly.