Universal regions

"Universal regions" is the name that the code uses to refer to "named lifetimes" -- e.g., lifetime parameters and 'static. The name derives from the fact that such lifetimes are "universally quantified" (i.e., we must make sure the code is true for all values of those lifetimes). It is worth spending a bit of discussing how lifetime parameters are handled during region inference. Consider this example:

fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> &'b u32 {

This example is intended not to compile, because we are returning x, which has type &'a u32, but our signature promises that we will return a &'b u32 value. But how are lifetimes like 'a and 'b integrated into region inference, and how this error wind up being detected?

Universal regions and their relationships to one another

Early on in region inference, one of the first things we do is to construct a UniversalRegions struct. This struct tracks the various universal regions in scope on a particular function. We also create a UniversalRegionRelations struct, which tracks their relationships to one another. So if you have e.g. where 'a: 'b, then the UniversalRegionRelations struct would track that 'a: 'b is known to hold (which could be tested with the outlives function.

Everything is a region variable

One important aspect of how NLL region inference works is that all lifetimes are represented as numbered variables. This means that the only variant of ty::RegionKind that we use is the ReVar variant. These region variables are broken into two major categories, based on their index:

  • 0..N: universal regions -- the ones we are discussing here. In this case, the code must be correct with respect to any value of those variables that meets the declared relationships.
  • N..M: existential regions -- inference variables where the region inferencer is tasked with finding some suitable value.

In fact, the universal regions can be further subdivided based on where they were brought into scope (see the RegionClassification type). These subdivisions are not important for the topics discussed here, but become important when we consider closure constraint propagation, so we discuss them there.

Universal lifetimes as the elements of a region's value

As noted previously, the value that we infer for each region is a set {E}. The elements of this set can be points in the control-flow graph, but they can also be an element end('a) corresponding to each universal lifetime 'a. If the value for some region R0 includes end('a), then this implies that R0 must extend until the end of 'a in the caller.

The "value" of a universal region

During region inference, we compute a value for each universal region in the same way as we compute values for other regions. This value represents, effectively, the lower bound on that universal region -- the things that it must outlive. We now describe how we use this value to check for errors.

Liveness and universal regions

All universal regions have an initial liveness constraint that includes the entire function body. This is because lifetime parameters are defined in the caller and must include the entirety of the function call that invokes this particular function. In addition, each universal region 'a includes itself (that is, end('a)) in its liveness constraint (i.e., 'a must extend until the end of itself). In the code, these liveness constraints are setup in init_free_and_bound_regions.

Propagating outlives constraints for universal regions

So, consider the first example of this section:

fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> &'b u32 {

Here, returning x requires that &'a u32 <: &'b u32, which gives rise to an outlives constraint 'a: 'b. Combined with our default liveness constraints we get:

'a live at {B, end('a)} // B represents the "function body"
'b live at {B, end('b)}
'a: 'b

When we process the 'a: 'b constraint, therefore, we will add end('b) into the value for 'a, resulting in a final value of {B, end('a), end('b)}.

Detecting errors

Once we have finished constraint propagation, we then enforce a constraint that if some universal region 'a includes an element end('b), then 'a: 'b must be declared in the function's bounds. If not, as in our example, that is an error. This check is done in the check_universal_regions function, which simply iterates over all universal regions, inspects their final value, and tests against the declared UniversalRegionRelations.