rust/src/librustc/infer

Type inference engine

This is loosely based on standard HM-type inference, but with an extension to try and accommodate subtyping. There is nothing principled about this extension; it's sound---I hope!---but it's a heuristic, ultimately, and does not guarantee that it finds a valid typing even if one exists (in fact, there are known scenarios where it fails, some of which may eventually become problematic).

Key idea

The main change is that each type variable T is associated with a lower-bound L and an upper-bound U. L and U begin as bottom and top, respectively, but gradually narrow in response to new constraints being introduced. When a variable is finally resolved to a concrete type, it can (theoretically) select any type that is a supertype of L and a subtype of U.

There are several critical invariants which we maintain:

• the upper-bound of a variable only becomes lower and the lower-bound only becomes higher over time;
• the lower-bound L is always a subtype of the upper bound U;
• the lower-bound L and upper-bound U never refer to other type variables, but only to types (though those types may contain type variables).

An aside: if the terms upper- and lower-bound confuse you, think of "supertype" and "subtype". The upper-bound is a "supertype" (super=upper in Latin, or something like that anyway) and the lower-bound is a "subtype" (sub=lower in Latin). I find it helps to visualize a simple class hierarchy, like Java minus interfaces and primitive types. The class Object is at the root (top) and other types lie in between. The bottom type is then the Null type. So the tree looks like:

        Object
        /    \
    String   Other
        \    /
        (null)

So the upper bound type is the "supertype" and the lower bound is the "subtype" (also, super and sub mean upper and lower in Latin, or something like that anyway).

Satisfying constraints

At a primitive level, there is only one form of constraint that the inference understands: a subtype relation. So the outside world can say "make type A a subtype of type B". If there are variables involved, the inferencer will adjust their upper- and lower-bounds as needed to ensure that this relation is satisfied. (We also allow "make type A equal to type B", but this is translated into "A <: B" and "B <: A")

As stated above, we always maintain the invariant that type bounds never refer to other variables. This keeps the inference relatively simple, avoiding the scenario of having a kind of graph where we have to pump constraints along and reach a fixed point, but it does impose some heuristics in the case where the user is relating two type variables A <: B.

Combining two variables such that variable A will forever be a subtype of variable B is the trickiest part of the algorithm because there is often no right choice---that is, the right choice will depend on future constraints which we do not yet know. The problem comes about because both A and B have bounds that can be adjusted in the future. Let's look at some of the cases that can come up.

Imagine, to start, the best case, where both A and B have an upper and lower bound (that is, the bounds are not top nor bot respectively). In that case, if we're lucky, A.ub <: B.lb, and so we know that whatever A and B should become, they will forever have the desired subtyping relation. We can just leave things as they are.

Option 1: Unify

However, suppose that A.ub is not a subtype of B.lb. In that case, we must make a decision. One option is to unify A and B so that they are one variable whose bounds are:

UB = GLB(A.ub, B.ub)
LB = LUB(A.lb, B.lb)

(Note that we will have to verify that LB <: UB; if it does not, the types are not intersecting and there is an error) In that case, A <: B holds trivially because A==B. However, we have now lost some flexibility, because perhaps the user intended for A and B to end up as different types and not the same type.

Pictorally, what this does is to take two distinct variables with (hopefully not completely) distinct type ranges and produce one with the intersection.

                  B.ub                  B.ub
                   /\                    /
           A.ub   /  \           A.ub   /
           /   \ /    \              \ /
          /     X      \              UB
         /     / \      \            / \
        /     /   /      \          /   /
        \     \  /       /          \  /
         \      X       /             LB
          \    / \     /             / \
           \  /   \   /             /   \
           A.lb    B.lb          A.lb    B.lb

Option 2: Relate UB/LB

Another option is to keep A and B as distinct variables but set their bounds in such a way that, whatever happens, we know that A <: B will hold. This can be achieved by ensuring that A.ub <: B.lb. In practice there are two ways to do that, depicted pictorially here:

    Before                Option #1            Option #2

             B.ub                B.ub                B.ub
              /\                 /  \                /  \
      A.ub   /  \        A.ub   /(B')\       A.ub   /(B')\
      /   \ /    \           \ /     /           \ /     /
     /     X      \         __UB____/             UB    /
    /     / \      \       /  |                   |    /
   /     /   /      \     /   |                   |   /
   \     \  /       /    /(A')|                   |  /
    \      X       /    /     LB            ______LB/
     \    / \     /    /     / \           / (A')/ \
      \  /   \   /     \    /   \          \    /   \
      A.lb    B.lb       A.lb    B.lb        A.lb    B.lb

In these diagrams, UB and LB are defined as before. As you can see, the new ranges A' and B' are quite different from the range that would be produced by unifying the variables.

What we do now

Our current technique is to try (transactionally) to relate the existing bounds of A and B, if there are any (i.e., if UB(A) != top && LB(B) != bot). If that succeeds, we're done. If it fails, then we merge A and B into same variable.

This is not clearly the correct course. For example, if UB(A) != top but LB(B) == bot, we could conceivably set LB(B) to UB(A) and leave the variables unmerged. This is sometimes the better course, it depends on the program.

The main case which fails today that I would like to support is:

fn foo<T>(x: T, y: T) { ... }

fn bar() {
    let x: @mut int = @mut 3;
    let y: @int = @3;
    foo(x, y);
}

In principle, the inferencer ought to find that the parameter T to foo(x, y) is @const int. Today, however, it does not; this is because the type variable T is merged with the type variable for X, and thus inherits its UB/LB of @mut int. This leaves no flexibility for T to later adjust to accommodate @int.

What to do when not all bounds are present

In the prior discussion we assumed that A.ub was not top and B.lb was not bot. Unfortunately this is rarely the case. Often type variables have "lopsided" bounds. For example, if a variable in the program has been initialized but has not been used, then its corresponding type variable will have a lower bound but no upper bound. When that variable is then used, we would like to know its upper bound---but we don't have one! In this case we'll do different things depending on how the variable is being used.

Transactional support

Whenever we adjust merge variables or adjust their bounds, we always keep a record of the old value. This allows the changes to be undone.

Regions

I've only talked about type variables here, but region variables follow the same principle. They have upper- and lower-bounds. A region A is a subregion of a region B if A being valid implies that B is valid. This basically corresponds to the block nesting structure: the regions for outer block scopes are superregions of those for inner block scopes.

Integral and floating-point type variables

There is a third variety of type variable that we use only for inferring the types of unsuffixed integer literals. Integral type variables differ from general-purpose type variables in that there's no subtyping relationship among the various integral types, so instead of associating each variable with an upper and lower bound, we just use simple unification. Each integer variable is associated with at most one integer type. Floating point types are handled similarly to integral types.

GLB/LUB

Computing the greatest-lower-bound and least-upper-bound of two types/regions is generally straightforward except when type variables are involved. In that case, we follow a similar "try to use the bounds when possible but otherwise merge the variables" strategy. In other words, GLB(A, B) where A and B are variables will often result in A and B being merged and the result being A.

Type coercion

We have a notion of assignability which differs somewhat from subtyping; in particular it may cause region borrowing to occur. See the big comment later in this file on Type Coercion for specifics.

In conclusion

I showed you three ways to relate A and B. There are also more, of course, though I'm not sure if there are any more sensible options. The main point is that there are various options, each of which produce a distinct range of types for A and B. Depending on what the correct values for A and B are, one of these options will be the right choice: but of course we don't know the right values for A and B yet, that's what we're trying to find! In our code, we opt to unify (Option #1).

Implementation details

We make use of a trait-like implementation strategy to consolidate duplicated code between subtypes, GLB, and LUB computations. See the section on "Type Combining" below for details.