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| <h1 id="member-constraints"><a class="header" href="#member-constraints">Member constraints</a></h1> |
| <p>A member constraint <code>'m member of ['c_1..'c_N]</code> expresses that the |
| region <code>'m</code> must be <em>equal</em> to some <strong>choice regions</strong> <code>'c_i</code> (for |
| some <code>i</code>). These constraints cannot be expressed by users, but they |
| arise from <code>impl Trait</code> due to its lifetime capture rules. Consider a |
| function such as the following:</p> |
| <pre><code class="language-rust ignore">fn make(a: &'a u32, b: &'b u32) -> impl Trait<'a, 'b> { .. }</code></pre> |
| <p>Here, the true return type (often called the “hidden type”) is only |
| permitted to capture the lifetimes <code>'a</code> or <code>'b</code>. You can kind of see |
| this more clearly by desugaring that <code>impl Trait</code> return type into its |
| more explicit form:</p> |
| <pre><code class="language-rust ignore">type MakeReturn<'x, 'y> = impl Trait<'x, 'y>; |
| fn make(a: &'a u32, b: &'b u32) -> MakeReturn<'a, 'b> { .. }</code></pre> |
| <p>Here, the idea is that the hidden type must be some type that could |
| have been written in place of the <code>impl Trait<'x, 'y></code> – but clearly |
| such a type can only reference the regions <code>'x</code> or <code>'y</code> (or |
| <code>'static</code>!), as those are the only names in scope. This limitation is |
| then translated into a restriction to only access <code>'a</code> or <code>'b</code> because |
| we are returning <code>MakeReturn<'a, 'b></code>, where <code>'x</code> and <code>'y</code> have been |
| replaced with <code>'a</code> and <code>'b</code> respectively.</p> |
| <h2 id="detailed-example"><a class="header" href="#detailed-example">Detailed example</a></h2> |
| <p>To help us explain member constraints in more detail, let’s spell out |
| the <code>make</code> example in a bit more detail. First off, let’s assume that |
| you have some dummy trait:</p> |
| <pre><code class="language-rust ignore">trait Trait<'a, 'b> { } |
| impl<T> Trait<'_, '_> for T { }</code></pre> |
| <p>and this is the <code>make</code> function (in desugared form):</p> |
| <pre><code class="language-rust ignore">type MakeReturn<'x, 'y> = impl Trait<'x, 'y>; |
| fn make(a: &'a u32, b: &'b u32) -> MakeReturn<'a, 'b> { |
| (a, b) |
| }</code></pre> |
| <p>What happens in this case is that the return type will be <code>(&'0 u32, &'1 u32)</code>, |
| where <code>'0</code> and <code>'1</code> are fresh region variables. We will have the following |
| region constraints:</p> |
| <pre><code class="language-txt">'0 live at {L} |
| '1 live at {L} |
| 'a: '0 |
| 'b: '1 |
| '0 member of ['a, 'b, 'static] |
| '1 member of ['a, 'b, 'static] |
| </code></pre> |
| <p>Here the “liveness set” <code>{L}</code> corresponds to that subset of the body |
| where <code>'0</code> and <code>'1</code> are live – basically the point from where the |
| return tuple is constructed to where it is returned (in fact, <code>'0</code> and |
| <code>'1</code> might have slightly different liveness sets, but that’s not very |
| interesting to the point we are illustrating here).</p> |
| <p>The <code>'a: '0</code> and <code>'b: '1</code> constraints arise from subtyping. When we |
| construct the <code>(a, b)</code> value, it will be assigned type <code>(&'0 u32, &'1 u32)</code> – the region variables reflect that the lifetimes of these |
| references could be made smaller. For this value to be created from |
| <code>a</code> and <code>b</code>, however, we do require that:</p> |
| <pre><code class="language-txt">(&'a u32, &'b u32) <: (&'0 u32, &'1 u32) |
| </code></pre> |
| <p>which means in turn that <code>&'a u32 <: &'0 u32</code> and hence that <code>'a: '0</code> |
| (and similarly that <code>&'b u32 <: &'1 u32</code>, <code>'b: '1</code>).</p> |
| <p>Note that if we ignore member constraints, the value of <code>'0</code> would be |
| inferred to some subset of the function body (from the liveness |
| constraints, which we did not write explicitly). It would never become |
| <code>'a</code>, because there is no need for it too – we have a constraint that |
| <code>'a: '0</code>, but that just puts a “cap” on how <em>large</em> <code>'0</code> can grow to |
| become. Since we compute the <em>minimal</em> value that we can, we are happy |
| to leave <code>'0</code> as being just equal to the liveness set. This is where |
| member constraints come in.</p> |
| <h2 id="choices-are-always-lifetime-parameters"><a class="header" href="#choices-are-always-lifetime-parameters">Choices are always lifetime parameters</a></h2> |
| <p>At present, the “choice” regions from a member constraint are always lifetime |
| parameters from the current function. As of <!-- date-check --> October 2021, |
| this falls out from the placement of impl Trait, though in the future it may not |
| be the case. We take some advantage of this fact, as it simplifies the current |
| code. In particular, we don’t have to consider a case like <code>'0 member of ['1, 'static]</code>, in which the value of both <code>'0</code> and <code>'1</code> are being inferred and hence |
| changing. See <a href="https://github.com/rust-lang/rust/issues/61773">rust-lang/rust#61773</a> for more information.</p> |
| <h2 id="applying-member-constraints"><a class="header" href="#applying-member-constraints">Applying member constraints</a></h2> |
| <p>Member constraints are a bit more complex than other forms of |
| constraints. This is because they have a “or” quality to them – that |
| is, they describe multiple choices that we must select from. E.g., in |
| our example constraint <code>'0 member of ['a, 'b, 'static]</code>, it might be |
| that <code>'0</code> is equal to <code>'a</code>, <code>'b</code>, <em>or</em> <code>'static</code>. How can we pick the |
| correct one? What we currently do is to look for a <em>minimal choice</em> |
| – if we find one, then we will grow <code>'0</code> to be equal to that minimal |
| choice. To find that minimal choice, we take two factors into |
| consideration: lower and upper bounds.</p> |
| <h3 id="lower-bounds"><a class="header" href="#lower-bounds">Lower bounds</a></h3> |
| <p>The <em>lower bounds</em> are those lifetimes that <code>'0</code> <em>must outlive</em> – |
| i.e., that <code>'0</code> must be larger than. In fact, when it comes time to |
| apply member constraints, we’ve already <em>computed</em> the lower bounds of |
| <code>'0</code> because we computed its minimal value (or at least, the lower |
| bounds considering everything but member constraints).</p> |
| <p>Let <code>LB</code> be the current value of <code>'0</code>. We know then that <code>'0: LB</code> must |
| hold, whatever the final value of <code>'0</code> is. Therefore, we can rule out |
| any choice <code>'choice</code> where <code>'choice: LB</code> does not hold.</p> |
| <p>Unfortunately, in our example, this is not very helpful. The lower |
| bound for <code>'0</code> will just be the liveness set <code>{L}</code>, and we know that |
| all the lifetime parameters outlive that set. So we are left with the |
| same set of choices here. (But in other examples, particularly those |
| with different variance, lower bound constraints may be relevant.)</p> |
| <h3 id="upper-bounds"><a class="header" href="#upper-bounds">Upper bounds</a></h3> |
| <p>The <em>upper bounds</em> are those lifetimes that <em>must outlive</em> <code>'0</code> – |
| i.e., that <code>'0</code> must be <em>smaller</em> than. In our example, this would be |
| <code>'a</code>, because we have the constraint that <code>'a: '0</code>. In more complex |
| examples, the chain may be more indirect.</p> |
| <p>We can use upper bounds to rule out members in a very similar way to |
| lower bounds. If UB is some upper bound, then we know that <code>UB: '0</code> must hold, so we can rule out any choice <code>'choice</code> where <code>UB: 'choice</code> does not hold.</p> |
| <p>In our example, we would be able to reduce our choice set from <code>['a, 'b, 'static]</code> to just <code>['a]</code>. This is because <code>'0</code> has an upper bound |
| of <code>'a</code>, and neither <code>'a: 'b</code> nor <code>'a: 'static</code> is known to hold.</p> |
| <p>(For notes on how we collect upper bounds in the implementation, see |
| <a href="#collecting">the section below</a>.)</p> |
| <h3 id="minimal-choice"><a class="header" href="#minimal-choice">Minimal choice</a></h3> |
| <p>After applying lower and upper bounds, we can still sometimes have |
| multiple possibilities. For example, imagine a variant of our example |
| using types with the opposite variance. In that case, we would have |
| the constraint <code>'0: 'a</code> instead of <code>'a: '0</code>. Hence the current value |
| of <code>'0</code> would be <code>{L, 'a}</code>. Using this as a lower bound, we would be |
| able to narrow down the member choices to <code>['a, 'static]</code> because <code>'b: 'a</code> is not known to hold (but <code>'a: 'a</code> and <code>'static: 'a</code> do hold). We |
| would not have any upper bounds, so that would be our final set of choices.</p> |
| <p>In that case, we apply the <strong>minimal choice</strong> rule – basically, if |
| one of our choices if smaller than the others, we can use that. In |
| this case, we would opt for <code>'a</code> (and not <code>'static</code>).</p> |
| <p>This choice is consistent with the general ‘flow’ of region |
| propagation, which always aims to compute a minimal value for the |
| region being inferred. However, it is somewhat arbitrary.</p> |
| <p><a id="collecting"></a></p> |
| <h3 id="collecting-upper-bounds-in-the-implementation"><a class="header" href="#collecting-upper-bounds-in-the-implementation">Collecting upper bounds in the implementation</a></h3> |
| <p>In practice, computing upper bounds is a bit inconvenient, because our |
| data structures are setup for the opposite. What we do is to compute |
| the <strong>reverse SCC graph</strong> (we do this lazily and cache the result) – |
| that is, a graph where <code>'a: 'b</code> induces an edge <code>SCC('b) -> SCC('a)</code>. Like the normal SCC graph, this is a DAG. We can then do a |
| depth-first search starting from <code>SCC('0)</code> in this graph. This will |
| take us to all the SCCs that must outlive <code>'0</code>.</p> |
| <p>One wrinkle is that, as we walk the “upper bound” SCCs, their values |
| will not yet have been fully computed. However, we <strong>have</strong> already |
| applied their liveness constraints, so we have some information about |
| their value. In particular, for any regions representing lifetime |
| parameters, their value will contain themselves (i.e., the initial |
| value for <code>'a</code> includes <code>'a</code> and the value for <code>'b</code> contains <code>'b</code>). So |
| we can collect all of the lifetime parameters that are reachable, |
| which is precisely what we are interested in.</p> |
| |
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