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| <h1 id="region-inference-nll"><a class="header" href="#region-inference-nll">Region inference (NLL)</a></h1> |
| <ul> |
| <li><a href="#universal-regions">Universal regions</a></li> |
| <li><a href="#region-variables">Region variables</a></li> |
| <li><a href="#constraints">Constraints</a></li> |
| <li><a href="#inference-overview">Inference Overview</a> |
| <ul> |
| <li><a href="#example">Example</a></li> |
| <li><a href="#some-details">Some details</a></li> |
| </ul> |
| </li> |
| </ul> |
| <p>The MIR-based region checking code is located in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/index.html">the <code>rustc_mir::borrow_check</code> |
| module</a>.</p> |
| <p>The MIR-based region analysis consists of two major functions:</p> |
| <ul> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/nll/fn.replace_regions_in_mir.html"><code>replace_regions_in_mir</code></a>, invoked first, has two jobs: |
| <ul> |
| <li>First, it finds the set of regions that appear within the |
| signature of the function (e.g., <code>'a</code> in <code>fn foo<'a>(&'a u32) { ... }</code>). These are called the "universal" or "free" regions – in |
| particular, they are the regions that <a href="../appendix/background.html#free-vs-bound">appear free</a> in the |
| function body.</li> |
| <li>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.</li> |
| </ul> |
| </li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/nll/fn.compute_regions.html"><code>compute_regions</code></a>, 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 <code>replace_regions_in_mir</code> introduced. |
| <ul> |
| <li>To do that, it first runs the <a href="./type_check.html">MIR type checker</a>. 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 <a href="./region_inference/constraint_propagation.html">constraints</a> between region |
| variables, indicating their potential values and relationships to |
| one another.</li> |
| <li>After this, we perform <a href="./region_inference/constraint_propagation.html">constraint propagation</a> by creating a |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html"><code>RegionInferenceContext</code></a> and invoking its <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#method.solve"><code>solve</code></a> |
| method.</li> |
| <li>The <a href="https://rust-lang.github.io/rfcs/2094-nll.html">NLL RFC</a> also includes fairly thorough (and hopefully readable) |
| coverage.</li> |
| </ul> |
| </li> |
| </ul> |
| <h2 id="universal-regions"><a class="header" href="#universal-regions">Universal regions</a></h2> |
| <p>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/universal_regions/struct.UniversalRegions.html"><code>UniversalRegions</code></a> type represents a collection of <em>universal</em> regions |
| corresponding to some MIR <code>DefId</code>. It is constructed in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/nll/fn.replace_regions_in_mir.html"><code>replace_regions_in_mir</code></a> when we replace all regions with fresh inference |
| variables. <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/universal_regions/struct.UniversalRegions.html"><code>UniversalRegions</code></a> contains indices for all the free regions in |
| the given MIR along with any relationships that are <em>known</em> to hold between |
| them (e.g. implied bounds, where clauses, etc.).</p> |
| <p>For example, given the MIR for the following function:</p> |
| <pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>fn foo<'a>(x: &'a u32) { |
| // ... |
| } |
| <span class="boring">}</span></code></pre></pre> |
| <p>we would create a universal region for <code>'a</code> and one for <code>'static</code>. There may |
| also be some complications for handling closures, but we will ignore those for |
| the moment.</p> |
| <p>TODO: write about <em>how</em> these regions are computed.</p> |
| <p><a id="region-variables"></a></p> |
| <h2 id="region-variables"><a class="header" href="#region-variables">Region variables</a></h2> |
| <p>The value of a region can be thought of as a <strong>set</strong>. 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 <code>'a: 'b</code>, then <code>end('b)</code> is in the set for |
| <code>'a</code>); we call the domain of this set a <code>RegionElement</code>. In the code, the value |
| for all regions is maintained in <a href="https://github.com/rust-lang/rust/tree/master/compiler/rustc_borrowck/src/region_infer">the <code>rustc_borrowck::region_infer</code> module</a>. |
| 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 <code>RegionElementIndex</code>, and |
| use sparse bitsets).</p> |
| <p>The kinds of region elements are as follows:</p> |
| <ul> |
| <li>Each <strong><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Location.html"><code>location</code></a></strong> in the MIR control-flow graph: a location is just |
| the pair of a basic block and an index. This identifies the point |
| <strong>on entry</strong> to the statement with that index (or the terminator, if |
| the index is equal to <code>statements.len()</code>).</li> |
| <li>There is an element <code>end('a)</code> for each universal region <code>'a</code>, |
| corresponding to some portion of the caller's (or caller's caller, |
| etc) control-flow graph.</li> |
| <li>Similarly, there is an element denoted <code>end('static)</code> corresponding |
| to the remainder of program execution after this function returns.</li> |
| <li>There is an element <code>!1</code> for each placeholder region <code>!1</code>. This |
| corresponds (intuitively) to some unknown set of other elements – |
| for details on placeholders, see the section |
| <a href="region_inference/placeholders_and_universes.html">placeholders and universes</a>.</li> |
| </ul> |
| <h2 id="constraints"><a class="header" href="#constraints">Constraints</a></h2> |
| <p>Before we can infer the value of regions, we need to collect |
| constraints on the regions. The full set of constraints is described |
| in <a href="./region_inference/constraint_propagation.html">the section on constraint propagation</a>, but the two most |
| common sorts of constraints are:</p> |
| <ol> |
| <li>Outlives constraints. These are constraints that one region outlives another |
| (e.g. <code>'a: 'b</code>). Outlives constraints are generated by the <a href="./type_check.html">MIR type |
| checker</a>.</li> |
| <li>Liveness constraints. Each region needs to be live at points where it can be |
| used.</li> |
| </ol> |
| <h2 id="inference-overview"><a class="header" href="#inference-overview">Inference Overview</a></h2> |
| <p>So how do we compute the contents of a region? This process is called <em>region |
| inference</em>. The high-level idea is pretty simple, but there are some details we |
| need to take care of.</p> |
| <p>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 <em>propagate</em> the |
| constraints: for each region <code>'a</code>, if <code>'a: 'b</code>, then we add all elements of |
| <code>'b</code> to <code>'a</code>, including <code>end('b)</code>. This all happens in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#method.propagate_constraints"><code>propagate_constraints</code></a>.</p> |
| <p>Then, we will check for errors. We first check that type tests are satisfied by |
| calling <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#method.check_type_tests"><code>check_type_tests</code></a>. This checks constraints like <code>T: 'a</code>. Second, we |
| check that universal regions are not "too big". This is done by calling |
| <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>. This checks that for each region <code>'a</code> if <code>'a</code> |
| contains the element <code>end('b)</code>, then we must already know that <code>'a: 'b</code> 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.</p> |
| <h3 id="example"><a class="header" href="#example">Example</a></h3> |
| <p>Consider the following example:</p> |
| <pre><code class="language-rust ignore">fn foo<'a, 'b>(x: &'a usize) -> &'b usize { |
| x |
| }</code></pre> |
| <p>Clearly, this should not compile because we don't know if <code>'a</code> outlives <code>'b</code> |
| (if it doesn't then the return value could be a dangling reference).</p> |
| <p>Let's back up a bit. We need to introduce some free inference variables (as is |
| done in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/nll/fn.replace_regions_in_mir.html"><code>replace_regions_in_mir</code></a>). This example doesn't use the exact regions |
| produced, but it (hopefully) is enough to get the idea across.</p> |
| <pre><code class="language-rust ignore">fn foo<'a, 'b>(x: &'a /* '#1 */ usize) -> &'b /* '#3 */ usize { |
| x // '#2, location L1 |
| }</code></pre> |
| <p>Some notation: <code>'#1</code>, <code>'#3</code>, and <code>'#2</code> represent the universal regions for the |
| argument, return value, and the expression <code>x</code>, respectively. Additionally, I |
| will call the location of the expression <code>x</code> <code>L1</code>.</p> |
| <p>So now we can use the liveness constraints to get the following starting points:</p> |
| <div class="table-wrapper"><table><thead><tr><th>Region</th><th>Contents</th></tr></thead><tbody> |
| <tr><td>'#1</td><td></td></tr> |
| <tr><td>'#2</td><td><code>L1</code></td></tr> |
| <tr><td>'#3</td><td><code>L1</code></td></tr> |
| </tbody></table> |
| </div> |
| <p>Now we use the outlives constraints to expand each region. Specifically, we |
| know that <code>'#2: '#3</code> ...</p> |
| <div class="table-wrapper"><table><thead><tr><th>Region</th><th>Contents</th></tr></thead><tbody> |
| <tr><td>'#1</td><td><code>L1</code></td></tr> |
| <tr><td>'#2</td><td><code>L1, end('#3) // add contents of '#3 and end('#3)</code></td></tr> |
| <tr><td>'#3</td><td><code>L1</code></td></tr> |
| </tbody></table> |
| </div> |
| <p>... and <code>'#1: '#2</code>, so ...</p> |
| <div class="table-wrapper"><table><thead><tr><th>Region</th><th>Contents</th></tr></thead><tbody> |
| <tr><td>'#1</td><td><code>L1, end('#2), end('#3) // add contents of '#2 and end('#2)</code></td></tr> |
| <tr><td>'#2</td><td><code>L1, end('#3)</code></td></tr> |
| <tr><td>'#3</td><td><code>L1</code></td></tr> |
| </tbody></table> |
| </div> |
| <p>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 <code>'#1</code> now contains <code>end('#3)</code>, but |
| we have no <code>where</code> clause or implied bound to say that <code>'a: 'b</code>... that's an |
| error!</p> |
| <h3 id="some-details"><a class="header" href="#some-details">Some details</a></h3> |
| <p>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html"><code>RegionInferenceContext</code></a> type contains all of the information needed to |
| do inference, including the universal regions from <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/nll/fn.replace_regions_in_mir.html"><code>replace_regions_in_mir</code></a> and |
| the constraints computed for each region. It is constructed just after we |
| compute the liveness constraints.</p> |
| <p>Here are some of the fields of the struct:</p> |
| <ul> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#structfield.constraints"><code>constraints</code></a>: contains all the outlives constraints.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#structfield.liveness_constraints"><code>liveness_constraints</code></a>: contains all the liveness constraints.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#structfield.universal_regions"><code>universal_regions</code></a>: contains the <code>UniversalRegions</code> returned by |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/nll/fn.replace_regions_in_mir.html"><code>replace_regions_in_mir</code></a>.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#structfield.universal_region_relations"><code>universal_region_relations</code></a>: contains relations known to be true about |
| universal regions. For example, if we have a where clause that <code>'a: 'b</code>, that |
| relation is assumed to be true while borrow checking the implementation (it |
| is checked at the caller), so <code>universal_region_relations</code> would contain <code>'a: 'b</code>.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#structfield.type_tests"><code>type_tests</code></a>: contains some constraints on types that we must check after |
| inference (e.g. <code>T: 'a</code>).</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#structfield.closure_bounds_mapping"><code>closure_bounds_mapping</code></a>: used for propagating region constraints from |
| closures back out to the creator of the closure.</li> |
| </ul> |
| <p>TODO: should we discuss any of the others fields? What about the SCCs?</p> |
| <p>Ok, now that we have constructed a <code>RegionInferenceContext</code>, we can do |
| inference. This is done by calling the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#method.solve"><code>solve</code></a> method on the context. This |
| is where we call <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/region_infer/struct.RegionInferenceContext.html#method.propagate_constraints"><code>propagate_constraints</code></a> and then check the resulting type |
| tests and universal regions, as discussed above.</p> |
| |
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