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| <h1 id="type-inference"><a class="header" href="#type-inference">Type inference</a></h1> |
| <p>Type inference is the process of automatic detection of the type of an |
| expression.</p> |
| <p>It is what allows Rust to work with fewer or no type annotations, |
| making things easier for users:</p> |
| <pre class="playground"><code class="language-rust">fn main() { |
| let mut things = vec![]; |
| things.push("thing"); |
| }</code></pre> |
| <p>Here, the type of <code>things</code> is <em>inferred</em> to be <code>Vec<&str></code> because of the value |
| we push into <code>things</code>.</p> |
| <p>The type inference is based on the standard Hindley-Milner (HM) type inference |
| algorithm, but extended in various ways to accommodate subtyping, region |
| inference, and higher-ranked types.</p> |
| <h2 id="a-note-on-terminology"><a class="header" href="#a-note-on-terminology">A note on terminology</a></h2> |
| <p>We use the notation <code>?T</code> to refer to inference variables, also called |
| existential variables.</p> |
| <p>We use the terms “region” and “lifetime” interchangeably. Both refer to |
| the <code>'a</code> in <code>&'a T</code>.</p> |
| <p>The term “bound region” refers to a region that is bound in a function |
| signature, such as the <code>'a</code> in <code>for<'a> fn(&'a u32)</code>. A region is |
| “free” if it is not bound.</p> |
| <h2 id="creating-an-inference-context"><a class="header" href="#creating-an-inference-context">Creating an inference context</a></h2> |
| <p>You create an inference context by doing something like |
| the following:</p> |
| <pre><code class="language-rust ignore">let infcx = tcx.infer_ctxt().build(); |
| // Use the inference context `infcx` here.</code></pre> |
| <p><code>infcx</code> has the type <code>InferCtxt<'tcx></code>, the same <code>'tcx</code> lifetime as on |
| the <code>tcx</code> it was built from.</p> |
| <p>The <code>tcx.infer_ctxt</code> method actually returns a builder, which means |
| there are some kinds of configuration you can do before the <code>infcx</code> is |
| created. See <code>InferCtxtBuilder</code> for more information.</p> |
| <p><a id="vars"></a></p> |
| <h2 id="inference-variables"><a class="header" href="#inference-variables">Inference variables</a></h2> |
| <p>The main purpose of the inference context is to house a bunch of |
| <strong>inference variables</strong> – these represent types or regions whose precise |
| value is not yet known, but will be uncovered as we perform type-checking.</p> |
| <p>If you’re familiar with the basic ideas of unification from H-M type |
| systems, or logic languages like Prolog, this is the same concept. If |
| you’re not, you might want to read a tutorial on how H-M type |
| inference works, or perhaps this blog post on |
| <a href="http://smallcultfollowing.com/babysteps/blog/2017/03/25/unification-in-chalk-part-1/">unification in the Chalk project</a>.</p> |
| <p>All told, the inference context stores five kinds of inference variables |
| (as of <!-- date-check --> March 2023):</p> |
| <ul> |
| <li>Type variables, which come in three varieties: |
| <ul> |
| <li>General type variables (the most common). These can be unified with any |
| type.</li> |
| <li>Integral type variables, which can only be unified with an integral type, |
| and arise from an integer literal expression like <code>22</code>.</li> |
| <li>Float type variables, which can only be unified with a float type, and |
| arise from a float literal expression like <code>22.0</code>.</li> |
| </ul> |
| </li> |
| <li>Region variables, which represent lifetimes, and arise all over the place.</li> |
| <li>Const variables, which represent constants.</li> |
| </ul> |
| <p>All the type variables work in much the same way: you can create a new |
| type variable, and what you get is <code>Ty<'tcx></code> representing an |
| unresolved type <code>?T</code>. Then later you can apply the various operations |
| that the inferencer supports, such as equality or subtyping, and it |
| will possibly <strong>instantiate</strong> (or <strong>bind</strong>) that <code>?T</code> to a specific |
| value as a result.</p> |
| <p>The region variables work somewhat differently, and are described |
| below in a separate section.</p> |
| <h2 id="enforcing-equality--subtyping"><a class="header" href="#enforcing-equality--subtyping">Enforcing equality / subtyping</a></h2> |
| <p>The most basic operations you can perform in the type inferencer is |
| <strong>equality</strong>, which forces two types <code>T</code> and <code>U</code> to be the same. The |
| recommended way to add an equality constraint is to use the <code>at</code> |
| method, roughly like so:</p> |
| <pre><code class="language-rust ignore">infcx.at(...).eq(t, u);</code></pre> |
| <p>The first <code>at()</code> call provides a bit of context, i.e. why you are |
| doing this unification, and in what environment, and the <code>eq</code> method |
| performs the actual equality constraint.</p> |
| <p>When you equate things, you force them to be precisely equal. Equating |
| returns an <code>InferResult</code> – if it returns <code>Err(err)</code>, then equating |
| failed, and the enclosing <code>TypeError</code> will tell you what went wrong.</p> |
| <p>The success case is perhaps more interesting. The “primary” return |
| type of <code>eq</code> is <code>()</code> – that is, when it succeeds, it doesn’t return a |
| value of any particular interest. Rather, it is executed for its |
| side-effects of constraining type variables and so forth. However, the |
| actual return type is not <code>()</code>, but rather <code>InferOk<()></code>. The |
| <code>InferOk</code> type is used to carry extra trait obligations – your job is |
| to ensure that these are fulfilled (typically by enrolling them in a |
| fulfillment context). See the <a href="traits/resolution.html">trait chapter</a> for more background on that.</p> |
| <p>You can similarly enforce subtyping through <code>infcx.at(..).sub(..)</code>. The same |
| basic concepts as above apply.</p> |
| <h2 id="trying-equality"><a class="header" href="#trying-equality">“Trying” equality</a></h2> |
| <p>Sometimes you would like to know if it is <em>possible</em> to equate two |
| types without error. You can test that with <code>infcx.can_eq</code> (or |
| <code>infcx.can_sub</code> for subtyping). If this returns <code>Ok</code>, then equality |
| is possible – but in all cases, any side-effects are reversed.</p> |
| <p>Be aware, though, that the success or failure of these methods is always |
| <strong>modulo regions</strong>. That is, two types <code>&'a u32</code> and <code>&'b u32</code> will |
| return <code>Ok</code> for <code>can_eq</code>, even if <code>'a != 'b</code>. This falls out from the |
| “two-phase” nature of how we solve region constraints.</p> |
| <h2 id="snapshots"><a class="header" href="#snapshots">Snapshots</a></h2> |
| <p>As described in the previous section on <code>can_eq</code>, often it is useful |
| to be able to do a series of operations and then roll back their |
| side-effects. This is done for various reasons: one of them is to be |
| able to backtrack, trying out multiple possibilities before settling |
| on which path to take. Another is in order to ensure that a series of |
| smaller changes take place atomically or not at all.</p> |
| <p>To allow for this, the inference context supports a <code>snapshot</code> method. |
| When you call it, it will start recording changes that occur from the |
| operations you perform. When you are done, you can either invoke |
| <code>rollback_to</code>, which will undo those changes, or else <code>confirm</code>, which |
| will make them permanent. Snapshots can be nested as long as you follow |
| a stack-like discipline.</p> |
| <p>Rather than use snapshots directly, it is often helpful to use the |
| methods like <code>commit_if_ok</code> or <code>probe</code> that encapsulate higher-level |
| patterns.</p> |
| <h2 id="subtyping-obligations"><a class="header" href="#subtyping-obligations">Subtyping obligations</a></h2> |
| <p>One thing worth discussing is subtyping obligations. When you force |
| two types to be a subtype, like <code>?T <: i32</code>, we can often convert those |
| into equality constraints. This follows from Rust’s rather limited notion |
| of subtyping: so, in the above case, <code>?T <: i32</code> is equivalent to <code>?T = i32</code>.</p> |
| <p>However, in some cases we have to be more careful. For example, when |
| regions are involved. So if you have <code>?T <: &'a i32</code>, what we would do |
| is to first “generalize” <code>&'a i32</code> into a type with a region variable: |
| <code>&'?b i32</code>, and then unify <code>?T</code> with that (<code>?T = &'?b i32</code>). We then |
| relate this new variable with the original bound:</p> |
| <pre><code class="language-text">&'?b i32 <: &'a i32 |
| </code></pre> |
| <p>This will result in a region constraint (see below) of <code>'?b: 'a</code>.</p> |
| <p>One final interesting case is relating two unbound type variables, |
| like <code>?T <: ?U</code>. In that case, we can’t make progress, so we enqueue |
| an obligation <code>Subtype(?T, ?U)</code> and return it via the <code>InferOk</code> |
| mechanism. You’ll have to try again when more details about <code>?T</code> or |
| <code>?U</code> are known.</p> |
| <h2 id="region-constraints"><a class="header" href="#region-constraints">Region constraints</a></h2> |
| <p>Regions are inferenced somewhat differently from types. Rather than |
| eagerly unifying things, we simply collect constraints as we go, but |
| make (almost) no attempt to solve regions. These constraints have the |
| form of an “outlives” constraint:</p> |
| <pre><code class="language-text">'a: 'b |
| </code></pre> |
| <p>Actually the code tends to view them as a subregion relation, but it’s the same |
| idea:</p> |
| <pre><code class="language-text">'b <= 'a |
| </code></pre> |
| <p>(There are various other kinds of constraints, such as “verifys”; see |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/region_constraints/index.html"><code>region_constraints</code></a> module for details.)</p> |
| <p>There is one case where we do some amount of eager unification. If you have an |
| equality constraint between two regions</p> |
| <pre><code class="language-text">'a = 'b |
| </code></pre> |
| <p>we will record that fact in a unification table. You can then use |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/region_constraints/struct.RegionConstraintCollector.html#method.opportunistic_resolve_var"><code>opportunistic_resolve_var</code></a> to convert <code>'b</code> to <code>'a</code> (or vice |
| versa). This is sometimes needed to ensure termination of fixed-point |
| algorithms.</p> |
| <h2 id="solving-region-constraints"><a class="header" href="#solving-region-constraints">Solving region constraints</a></h2> |
| <p>Region constraints are only solved at the very end of |
| typechecking, once all other constraints are known and |
| all other obligations have been proven. There are two |
| ways to solve region constraints right now: lexical and |
| non-lexical. Eventually there will only be one.</p> |
| <p>An exception here is the leak-check which is used during trait solving |
| and relies on region constraints containing higher-ranked regions. Region |
| constraints in the root universe (i.e. not arising from a <code>for<'a></code>) must |
| not influence the trait system, as these regions are all erased during |
| codegen.</p> |
| <p>To solve <strong>lexical</strong> region constraints, you invoke |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_trait_selection/traits/struct.ObligationCtxt.html#method.resolve_regions_and_report_errors"><code>resolve_regions_and_report_errors</code></a>. This “closes” the region |
| constraint process and invokes the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/lexical_region_resolve/index.html"><code>lexical_region_resolve</code></a> code. Once |
| this is done, any further attempt to equate or create a subtyping |
| relationship will yield an ICE.</p> |
| <p>The NLL solver (actually, the MIR type-checker) does things slightly |
| differently. It uses canonical queries for trait solving which use |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/struct.InferCtxt.html#method.take_and_reset_region_constraints"><code>take_and_reset_region_constraints</code></a> at the end. This extracts all of the |
| outlives constraints added during the canonical query. This is required |
| as the NLL solver must not only know <em>what</em> regions outlive each other, |
| but also <em>where</em>. Finally, the NLL solver invokes <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/struct.InferCtxt.html#method.get_region_var_infos"><code>get_region_var_infos</code></a>, |
| providing all region variables to the solver.</p> |
| <h2 id="lexical-region-resolution"><a class="header" href="#lexical-region-resolution">Lexical region resolution</a></h2> |
| <p>Lexical region resolution is done by initially assigning each region |
| variable to an empty value. We then process each outlives constraint |
| repeatedly, growing region variables until a fixed-point is reached. |
| Region variables can be grown using a least-upper-bound relation on |
| the region lattice in a fairly straightforward fashion.</p> |
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