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| <h1 id="universal-regions"><a class="header" href="#universal-regions">Universal regions</a></h1> |
| <p>"Universal regions" is the name that the code uses to refer to "named |
| lifetimes" -- e.g., lifetime parameters and <code>'static</code>. 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:</p> |
| <pre><code class="language-rust ignore">fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> &'b u32 { |
| x |
| }</code></pre> |
| <p>This example is intended not to compile, because we are returning <code>x</code>, |
| which has type <code>&'a u32</code>, but our signature promises that we will |
| return a <code>&'b u32</code> value. But how are lifetimes like <code>'a</code> and <code>'b</code> |
| integrated into region inference, and how this error wind up being |
| detected?</p> |
| <h2 id="universal-regions-and-their-relationships-to-one-another"><a class="header" href="#universal-regions-and-their-relationships-to-one-another">Universal regions and their relationships to one another</a></h2> |
| <p>Early on in region inference, one of the first things we do is to |
| construct a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/universal_regions/struct.UniversalRegions.html"><code>UniversalRegions</code></a> struct. This struct tracks the |
| various universal regions in scope on a particular function. We also |
| create a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/type_check/free_region_relations/struct.UniversalRegionRelations.html"><code>UniversalRegionRelations</code></a> struct, which tracks their |
| relationships to one another. So if you have e.g. <code>where 'a: 'b</code>, then |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/type_check/free_region_relations/struct.UniversalRegionRelations.html"><code>UniversalRegionRelations</code></a> struct would track that <code>'a: 'b</code> is |
| known to hold (which could be tested with the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/type_check/free_region_relations/struct.UniversalRegionRelations.html#method.outlives"><code>outlives</code></a> function).</p> |
| <h2 id="everything-is-a-region-variable"><a class="header" href="#everything-is-a-region-variable">Everything is a region variable</a></h2> |
| <p>One important aspect of how NLL region inference works is that <strong>all |
| lifetimes</strong> are represented as numbered variables. This means that the |
| only variant of <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/region_kind/enum.RegionKind.html"><code>region_kind::RegionKind</code></a> that we use is the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/region_kind/enum.RegionKind.html#variant.ReVar"><code>ReVar</code></a> |
| variant. These region variables are broken into two major categories, |
| based on their index:</p> |
| <ul> |
| <li>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.</li> |
| <li>N..M: existential regions -- inference variables where the region |
| inferencer is tasked with finding <em>some</em> suitable value.</li> |
| </ul> |
| <p>In fact, the universal regions can be further subdivided based on |
| where they were brought into scope (see the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/universal_regions/enum.RegionClassification.html#variant.Local"><code>RegionClassification</code></a> |
| type). These subdivisions are not important for the topics discussed |
| here, but become important when we consider <a href="./closure_constraints.html">closure constraint |
| propagation</a>, so we discuss them there.</p> |
| <h2 id="universal-lifetimes-as-the-elements-of-a-regions-value"><a class="header" href="#universal-lifetimes-as-the-elements-of-a-regions-value">Universal lifetimes as the elements of a region's value</a></h2> |
| <p>As noted previously, the value that we infer for each region is a set |
| <code>{E}</code>. The elements of this set can be points in the control-flow |
| graph, but they can also be an element <code>end('a)</code> corresponding to each |
| universal lifetime <code>'a</code>. If the value for some region <code>R0</code> includes |
| <code>end('a</code>), then this implies that <code>R0</code> must extend until the end of <code>'a</code> |
| in the caller.</p> |
| <h2 id="the-value-of-a-universal-region"><a class="header" href="#the-value-of-a-universal-region">The "value" of a universal region</a></h2> |
| <p>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 <strong>lower bound</strong> on that universal region |
| -- the things that it must outlive. We now describe how we use this |
| value to check for errors.</p> |
| <h2 id="liveness-and-universal-regions"><a class="header" href="#liveness-and-universal-regions">Liveness and universal regions</a></h2> |
| <p>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 <code>'a</code> includes itself (that is, <code>end('a)</code>) in its |
| liveness constraint (i.e., <code>'a</code> must extend until the end of |
| itself). In the code, these liveness constraints are setup in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#method.init_free_and_bound_regions"><code>init_free_and_bound_regions</code></a>.</p> |
| <h2 id="propagating-outlives-constraints-for-universal-regions"><a class="header" href="#propagating-outlives-constraints-for-universal-regions">Propagating outlives constraints for universal regions</a></h2> |
| <p>So, consider the first example of this section:</p> |
| <pre><code class="language-rust ignore">fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> &'b u32 { |
| x |
| }</code></pre> |
| <p>Here, returning <code>x</code> requires that <code>&'a u32 <: &'b u32</code>, which gives |
| rise to an outlives constraint <code>'a: 'b</code>. Combined with our default liveness |
| constraints we get:</p> |
| <pre><code class="language-txt">'a live at {B, end('a)} // B represents the "function body" |
| 'b live at {B, end('b)} |
| 'a: 'b |
| </code></pre> |
| <p>When we process the <code>'a: 'b</code> constraint, therefore, we will add |
| <code>end('b)</code> into the value for <code>'a</code>, resulting in a final value of <code>{B, end('a), end('b)}</code>.</p> |
| <h2 id="detecting-errors"><a class="header" href="#detecting-errors">Detecting errors</a></h2> |
| <p>Once we have finished constraint propagation, we then enforce a |
| constraint that if some universal region <code>'a</code> includes an element |
| <code>end('b)</code>, then <code>'a: 'b</code> must be declared in the function's bounds. If |
| not, as in our example, that is an error. This check is done in the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#method.check_universal_regions"><code>check_universal_regions</code></a> function, which simply iterates over all |
| universal regions, inspects their final value, and tests against the |
| declared <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/type_check/free_region_relations/struct.UniversalRegionRelations.html"><code>UniversalRegionRelations</code></a>.</p> |
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