I was talking to brson today about the possibility of moving Rust to a regions system. He pointed out that the complexity costs may be high. I was trying to make a slimmer version where explicit region names were never required. This is what I came up with. The truth is, it’s not that different from the original: adding back region names wouldn’t change much. But I’m posting it anyway because it includes a description of how to handle regions in types and I think it’s the most complete and correct proposal at the moment.

The summary

You would have four kinds of Rust pointers:

@MT --- pointers to task-local, boxed data
~MT --- pointers to unique data
&MT --- safe references
*MT --- unsafe, C-like pointers

Here the M refers to a mutability qualifier (default, mut, or const) and T refers to a type.

&MT types, called references, are the new addition. A reference is a pointer which always points at memory whose validity is guaranteed by some outer stack frame. The idea is that a caller can give a callee a reference to some memory that the callee may use but which may not escape the callee. This memory may be on the caller’s stack frame or it may be a reference into the task or exchange heaps which the caller is going to keep valid. This guarantee is upheld by the type system.

Reference types may appear anywhere. However, if they are used within another aggregate type such as a record, enum, or class, they “infect” their container so that it too is considered to be a reference. This is done by introducing a new kind into the type system, ref (actually, this is sort of a negative kind: more formally, there is a kind heap which contains all types but for those that may transitively include a reference). This kind may or may not be user visible: see the section on generics for a discussion of the options.

Coercion between pointer types

The type &MT is not a supertype of @MT and ~MT, but it is coercable. In the case of @, we could probably make it a true subtype, but at the moment a box pointer includes a header, ref count, etc and so is not binary compatible with a & pointer, which would be just a pointer to the box body. If we changed our representation so that @ pointers point directly to the box and the header is stored at a negative offset, then we could allow @T to be a subtype of &T.

The type ~MT, however, can never be a subtype. ~ is not a region. Rather, the data at the other end of the pointer logically belongs to a region of its own. So we can allow ~MT to be coerced to &MT, but the region will be a fresh region, and access to the ~MT pointer must be prevented for the scope of that fresh region. This is called “borrowing” a unique pointer. It is only possible for “unique paths”, where a “unique path” is a path of identifiers a.b.c...z that is the only path by which the unique variable can be reached (in practice, this means that a must be a local variable and all of the fields b...z must have unique or interior type). All of the prefixes of the unique path must be considered borrowed as well. I am not going into great detail on the handling of uniques here: it should be quite similar to what we have today in practice.

Tracking validity of references

Although the user never needs to write it explicitly, each instance of a reference type is internally associated with a region. There is one region for every block in the code. In addition, each function/method has a special region called caller. For simplicity I do not consider classes nor impls; it is relatively straightforward to extend the system to such cases.

Regions are arranged into a tree derived from the structure of the blocks in the source code. The region caller is a superregion of all the internal regions to a function.

In the implementation / formal version of the type system, these regions are represented explicitly. So a user-written type &MT expands to a type r&MT where r is the node id of the block or of the function itself (in the case of the caller region). The region r is derived from the position where &MT appears and by inference:

  • if &MT appears within a parameter list, r is the caller region.
  • if &MT appears on the type of a local variable, inference is used.
  • if &MT appears in a type declaration, see section below.

In general, the type a&T is a subtype of b&T if b is a subregion a. The reason is that, because a is a superregion of b, the pointer a&T is always valid whenever the region b is valid.

References in type declarations

The rules for which region is assigned when the user writes &MT omitted one important case: what happens when this type appears in a type declaration? Consider the following example:

type crate_ctxt = {
    mut_map: &map<...>,
    node_map: &map<...>,
    another_map: &map<...>,
    yet_another_map: &map<...>

In such cases, the region for the internal references will be assigned when the type is used. For example:

fn trans_foo(ccx: &crate_ctxt) {...}

Here, the type of ccx will be expanded to:

caller&{mut_map: caller&map<...>, node_map: caller&map<...>, ... }

In effect, types which contain references (transitively) are implicitly parameterized by a region parameter. There is only one such parameter. When the type is instantiated in a specific context, the value for that parameter is provided based on the context.

Taking the address of variables and so forth

The unary operator &M can be be used with both lvalues and rvalues. When used with an lvalue, it takes the address of the lvalue. The mutability qualifier provided must agree with the mutability of the lvalue. When used with an rvalue, it creates temporary space on the stack and copies the rvalue into it.

Here is an example of taking the address of lvalues:

fn foo() { // region for this block is "r"
    let x = 3;
    let mut y = 4;
    let px1 = &x;       // OK: yields type r&int
    let px2 = &const x; // OK: yields type r&const int
    let px2 = &mut x;   // Error: x is immutable
    let py1 = &y;       // Error: y is mutable.
    let py1 = &const y; // OK: yields type r&const int
    let py1 = &mut y;   // OK: yields type r&mut int

Here is an example of taking the address of rvalues:

fn foo() { // region for this block is "r"
    let p1 = &{x: 3, y: 4}; // OK: yields type r&{x:int,y:int}
    let p2 = &mut {x: 3, y: 4}; // OK: yields type r&mut {x:int,y:int}

Limitations on references

In order to guarantee that reference types do not escape the callee, the type system imposes some limitations:

  • Reference types may not be returned.
  • Reference types may not be closed over (copied/moved into a closure or interface instance).
  • Generic type variables cannot be bound to reference types unless the generic type variable is of the ref kind.

I will cover each restriction in turn. First, though, I want to more precisely define what the type checker considers to be a reference type. The definition is inductive:

  • a reference &MT;
  • a type whose definition may contain a reference (e.g., @&T or {x: &T}, or a class with a field of reference type);
  • a generic variable with bound ref.

Reference types may not be returned

The danger here is that the callee may pass back a reference to the caller that is no longer valid. This is relatively straightforward to prevent: do not allow the return type of a function to be a reference type.

Reference types may not be closed over

It is not allowed to copy a reference type into a boxed/unique closure nor is it allowed to cast a reference type to a boxed or unique iface. The reason is that these are the points where the type system loses the ability to track the constituent types and so we cannot distinguish a fn@() that closes over a reference type from other fn@() types.

Generic types

There is of course a concern that the limitations on reference types might be circumvented through the use of generics. This is prevented through the use of a type kind ref. A generic type variable may not be bound to a reference type unless it includes the bound ref. Moreover, any generic type variables bound by ref are considered reference types and therefore must obey the above restrictions.

A note on variance

In general, ptr types like &MT or @MT are covariant in T if M is not mut. This is different from references today which are always covariant in T; the current behavior is what leads to the type hole pointed out in the mailing list.