Region inference (NLL)

The MIR-based region checking code is located in the rustc_mir::borrow_check::nll module. (NLL, of course, stands for "non-lexical lifetimes", a term that will hopefully be deprecated once they become the standard kind of lifetime.)

The MIR-based region analysis consists of two major functions:

  • replace_regions_in_mir, invoked first, has two jobs:
    • First, it finds the set of regions that appear within the signature of the function (e.g., 'a in fn foo<'a>(&'a u32) { ... }). These are called the "universal" or "free" regions – in particular, they are the regions that appear free in the function body.
    • Second, it replaces all the regions from the function body with fresh inference variables. This is because (presently) those regions are the results of lexical region inference and hence are not of much interest. The intention is that – eventually – they will be "erased regions" (i.e., no information at all), since we won't be doing lexical region inference at all.
  • compute_regions, invoked second: this is given as argument the results of move analysis. It has the job of computing values for all the inference variables that replace_regions_in_mir introduced.
    • To do that, it first runs the MIR type checker. This is basically a normal type-checker but specialized to MIR, which is much simpler than full Rust, of course. Running the MIR type checker will however create various constraints between region variables, indicating their potential values and relationships to one another.
    • After this, we perform constraint propagation by creating a RegionInferenceContext and invoking its solve method.
    • The NLL RFC also includes fairly thorough (and hopefully readable) coverage.

Universal regions

The UniversalRegions type represents a collection of universal regions corresponding to some MIR DefId. It is constructed in replace_regions_in_mir when we replace all regions with fresh inference variables. UniversalRegions contains indices for all the free regions in the given MIR along with any relationships that are known to hold between them (e.g. implied bounds, where clauses, etc.).

For example, given the MIR for the following function:


# #![allow(unused_variables)]
#fn main() {
fn foo<'a>(x: &'a u32) {
    // ...
}
#}

we would create a universal region for 'a and one for 'static. There may also be some complications for handling closures, but we will ignore those for the moment.

TODO: write about how these regions are computed.

Region variables

The value of a region can be thought of as a set. This set contains all points in the MIR where the region is valid along with any regions that are outlived by this region (e.g. if 'a: 'b, then end('b) is in the set for 'a); we call the domain of this set a RegionElement. In the code, the value for all regions is maintained in the rustc_mir::borrow_check::nll::region_infer module. For each region we maintain a set storing what elements are present in its value (to make this efficient, we give each kind of element an index, the RegionElementIndex, and use sparse bitsets).

The kinds of region elements are as follows:

  • Each location in the MIR control-flow graph: a location is just the pair of a basic block and an index. This identifies the point on entry to the statement with that index (or the terminator, if the index is equal to statements.len()).
  • There is an element end('a) for each universal region 'a, corresponding to some portion of the caller's (or caller's caller, etc) control-flow graph.
  • Similarly, there is an element denoted end('static) corresponding to the remainder of program execution after this function returns.
  • There is an element !1 for each placeholder region !1. This corresponds (intuitively) to some unknown set of other elements – for details on placeholders, see the section placeholders and universes.

Constraints

Before we can infer the value of regions, we need to collect constraints on the regions. The full set of constraints is described in the section on constraint propagation, but the two most common sorts of constraints are:

  1. Outlives constraints. These are constraints that one region outlives another (e.g. 'a: 'b). Outlives constraints are generated by the MIR type checker.
  2. Liveness constraints. Each region needs to be live at points where it can be used. These constraints are collected by generate_constraints.

Inference Overview

So how do we compute the contents of a region? This process is called region inference. The high-level idea is pretty simple, but there are some details we need to take care of.

Here is the high-level idea: we start off each region with the MIR locations we know must be in it from the liveness constraints. From there, we use all of the outlives constraints computed from the type checker to propagate the constraints: for each region 'a, if 'a: 'b, then we add all elements of 'b to 'a, including end('b). This all happens in propagate_constraints.

Then, we will check for errors. We first check that type tests are satisfied by calling check_type_tests. This checks constraints like T: 'a. Second, we check that universal regions are not "too big". This is done by calling check_universal_regions. This checks that for each region 'a if 'a contains the element end('b), then we must already know that 'a: 'b holds (e.g. from a where clause). If we don't already know this, that is an error... well, almost. There is some special handling for closures that we will discuss later.

Example

Consider the following example:

fn foo<'a, 'b>(x: &'a usize) -> &'b usize {
    x
}

Clearly, this should not compile because we don't know if 'a outlives 'b (if it doesn't then the return value could be a dangling reference).

Let's back up a bit. We need to introduce some free inference variables (as is done in replace_regions_in_mir). This example doesn't use the exact regions produced, but it (hopefully) is enough to get the idea across.

fn foo<'a, 'b>(x: &'a /* '#1 */ usize) -> &'b /* '#3 */ usize {
    x // '#2, location L1
}

Some notation: '#1, '#3, and '#2 represent the universal regions for the argument, return value, and the expression x, respectively. Additionally, I will call the location of the expression x L1.

So now we can use the liveness constraints to get the following starting points:

RegionContents
'#1
'#2L1
'#3L1

Now we use the outlives constraints to expand each region. Specifically, we know that '#2: '#3 ...

RegionContents
'#1L1
'#2L1, end('#3) // add contents of '#3 and end('#3)
'#3L1

... and '#1: '#2, so ...

RegionContents
'#1L1, end('#2), end('#3) // add contents of '#2 and end('#2)
'#2L1, end('#3)
'#3L1

Now, we need to check that no regions were too big (we don't have any type tests to check in this case). Notice that '#1 now contains end('#3), but we have no where clause or implied bound to say that 'a: 'b... that's an error!

Some details

The RegionInferenceContext type contains all of the information needed to do inference, including the universal regions from replace_regions_in_mir and the constraints computed for each region. It is constructed just after we compute the liveness constraints.

Here are some of the fields of the struct:

  • constraints: contains all the outlives constraints.
  • liveness_constraints: contains all the liveness constraints.
  • universal_regions: contains the UniversalRegions returned by replace_regions_in_mir.
  • universal_region_relations: contains relations known to be true about universal regions. For example, if we have a where clause that 'a: 'b, that relation is assumed to be true while borrow checking the implementation (it is checked at the caller), so universal_region_relations would contain 'a: 'b.
  • type_tests: contains some constraints on types that we must check after inference (e.g. T: 'a).
  • closure_bounds_mapping: used for propagating region constraints from closures back out to the creator of the closure.

TODO: should we discuss any of the others fields? What about the SCCs?

Ok, now that we have constructed a RegionInferenceContext, we can do inference. This is done by calling the solve method on the context. This is where we call propagate_constraints and then check the resulting type tests and universal regions, as discussed above.