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| <h1 id="proof-trees"><a class="header" href="#proof-trees">Proof trees</a></h1> |
| <p>While the trait solver itself only returns whether a goal holds and the necessary |
| constraints, we sometimes also want to know what happened while trying to prove |
| it. While the trait solver should generally be treated as a black box by the rest |
| of the compiler, we cannot completely ignore its internals and provide "proof trees" |
| as an interface for this. To use them you implement the <a href="https://github.com/rust-lang/rust/blob/d6c8169c186ab16a3404cd0d0866674018e8a19e/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs#L403"><code>ProofTreeVisitor</code></a> trait, |
| see its existing implementations for examples. The most notable uses are to compute |
| the <a href="https://github.com/rust-lang/rust/blob/d6c8169c186ab16a3404cd0d0866674018e8a19e/compiler/rustc_trait_selection/src/traits/coherence.rs#L742-L748">intercrate ambiguity causes for coherence errors</a>, |
| <a href="https://github.com/rust-lang/rust/blob/d6c8169c186ab16a3404cd0d0866674018e8a19e/compiler/rustc_trait_selection/src/solve/fulfill.rs#L343-L356">improving trait solver errors</a>, and |
| <a href="https://github.com/rust-lang/rust/blob/d6c8169c186ab16a3404cd0d0866674018e8a19e/compiler/rustc_hir_typeck/src/closure.rs#L333-L339">eagerly inferring closure signatures</a>.</p> |
| <h2 id="computing-proof-trees"><a class="header" href="#computing-proof-trees">Computing proof trees</a></h2> |
| <p>The trait solver uses <a href="./canonicalization.html">Canonicalization</a> and uses completely separate <code>InferCtxt</code> for |
| each nested goal. Both diagnostics and auto-traits in rustdoc need to correctly |
| handle "looking into nested goals". Given a goal like <code>Vec<Vec<?x>>: Debug</code>, we |
| canonicalize to <code>exists<T0> Vec<Vec<T0>>: Debug</code>, instantiate that goal as |
| <code>Vec<Vec<?0>>: Debug</code>, get a nested goal <code>Vec<?0>: Debug</code>, canonicalize this to get |
| <code>exists<T0> Vec<T0>: Debug</code>, instantiate this as <code>Vec<?0>: Debug</code> which then results |
| in a nested <code>?0: Debug</code> goal which is ambiguous.</p> |
| <p>We compute proof trees by passing a <a href="https://github.com/rust-lang/rust/blob/d6c8169c186ab16a3404cd0d0866674018e8a19e/compiler/rustc_next_trait_solver/src/solve/inspect/build.rs#L40"><code>ProofTreeBuilder</code></a> to the search graph which is |
| converting the evaluation steps of the trait solver into a tree. When storing any |
| data using inference variables or placeholders, the data is canonicalized together |
| with the list of all unconstrained inference variables created during this computation. |
| This <a href="https://github.com/rust-lang/rust/blob/d6c8169c186ab16a3404cd0d0866674018e8a19e/compiler/rustc_type_ir/src/solve/inspect.rs#L31-L47"><code>CanonicalState</code></a> is then instantiated in the parent inference context while |
| walking the proof tree, using the list of inference variables to connect all the |
| canonicalized values created during this evaluation.</p> |
| <h2 id="debugging-the-solver"><a class="header" href="#debugging-the-solver">Debugging the solver</a></h2> |
| <p>We previously also tried to use proof trees to debug the solver implementation. This |
| has different design requirements than analyzing it programmatically. The recommended |
| way to debug the trait solver is by using <code>tracing</code>. The trait solver only uses the |
| <code>debug</code> tracing level for its general 'shape' and <code>trace</code> for additional detail. |
| <code>RUSTC_LOG=rustc_next_trait_solver=debug</code> therefore gives you a general outline |
| and <code>RUSTC_LOG=rustc_next_trait_solver=trace</code> can then be used if more precise |
| information is required.</p> |
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