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| <h1 id="implied-bounds"><a class="header" href="#implied-bounds">Implied bounds</a></h1> |
| <p>We currently add implied region bounds to avoid explicit annotations. e.g. |
| <code>fn foo<'a, T>(x: &'a T)</code> can freely assume that <code>T: 'a</code> holds without specifying it.</p> |
| <p>There are two kinds of implied bounds: explicit and implicit. Explicit implied bounds |
| get added to the <code>fn predicates_of</code> of the relevant item while implicit ones are |
| handled... well... implicitly.</p> |
| <h2 id="explicit-implied-bounds"><a class="header" href="#explicit-implied-bounds">explicit implied bounds</a></h2> |
| <p>The explicit implied bounds are computed in <a href="https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/mod.rs#L20"><code>fn inferred_outlives_of</code></a>. Only ADTs and |
| lazy type aliases have explicit implied bounds which are computed via a fixpoint algorithm |
| in the <a href="https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/mod.rs#L83"><code>fn inferred_outlives_crate</code></a> query.</p> |
| <p>We use <a href="https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs#L89"><code>fn insert_required_predicates_to_be_wf</code></a> on all fields of all ADTs in the crate. |
| This function computes the outlives bounds for each component of the field using a |
| separate implementation.</p> |
| <p>For ADTs, trait objects, and associated types the initially required predicates are |
| computed in <a href="https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs#L238"><code>fn check_explicit_predicates</code></a>. This simply uses <code>fn explicit_predicates_of</code> |
| without elaborating them.</p> |
| <p>Region predicates are added via <a href="https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/utils.rs#L15"><code>fn insert_outlives_predicate</code></a>. This function takes |
| an outlives predicate, decomposes it and adds the components as explicit predicates only |
| if the outlived region is a region parameter. <a href="https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/utils.rs#L159-L165">It does not add <code>'static</code> requirements</a>.</p> |
| <h2 id="implicit-implied-bounds"><a class="header" href="#implicit-implied-bounds">implicit implied bounds</a></h2> |
| <p>As we are unable to handle implications in binders yet, we cannot simply add the outlives |
| requirements of impls and functions as explicit predicates.</p> |
| <h3 id="using-implicit-implied-bounds-as-assumptions"><a class="header" href="#using-implicit-implied-bounds-as-assumptions">using implicit implied bounds as assumptions</a></h3> |
| <p>These bounds are not added to the <code>ParamEnv</code> of the affected item itself. For lexical |
| region resolution they are added using <a href="https://github.com/rust-lang/rust/blob/8239a37f9c0951a037cfc51763ea52a20e71e6bd/compiler/rustc_infer/src/infer/outlives/env.rs#L50-L55"><code>fn OutlivesEnvironment::from_normalized_bounds</code></a>. |
| Similarly, during MIR borrowck we add them using |
| <a href="https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_borrowck/src/type_check/free_region_relations.rs#L316"><code>fn UniversalRegionRelationsBuilder::add_implied_bounds</code></a>.</p> |
| <p><a href="https://github.com/rust-lang/rust/blob/91cae1dcdcf1a31bd8a92e4a63793d65cfe289bb/compiler/rustc_borrowck/src/type_check/free_region_relations.rs#L258-L332">We add implied bounds for the function signature and impl header in MIR borrowck</a>. |
| Outside of MIR borrowck we add the outlives requirements for the types returned by the |
| <a href="https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_ty_utils/src/implied_bounds.rs#L21"><code>fn assumed_wf_types</code></a> query.</p> |
| <p>The assumed outlives constraints for implicit bounds are computed using the |
| <a href="https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_traits/src/implied_outlives_bounds.rs#L18C4-L18C27"><code>fn implied_outlives_bounds</code></a> query. This directly |
| <a href="https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs#L95-L96">extracts the required outlives bounds from <code>fn wf::obligations</code></a>.</p> |
| <p>MIR borrowck adds the outlives constraints for both the normalized and unnormalized types, |
| lexical region resolution <a href="https://github.com/rust-lang/rust/blob/91cae1dcdcf1a31bd8a92e4a63793d65cfe289bb/compiler/rustc_trait_selection/src/traits/engine.rs#L227-L250">only uses the unnormalized types</a>.</p> |
| <h3 id="proving-implicit-implied-bounds"><a class="header" href="#proving-implicit-implied-bounds">proving implicit implied bounds</a></h3> |
| <p>As the implicit implied bounds are not included in <code>fn predicates_of</code> we have to |
| separately make sure they actually hold. We generally handle this by checking that |
| all used types are well formed by emitting <code>WellFormed</code> predicates.</p> |
| <p>We cannot emit <code>WellFormed</code> predicates when instantiating impls, as this would result |
| in - currently often inductive - trait solver cycles. We also do not emit constraints |
| involving higher ranked regions as we're lacking the implied bounds from their binder.</p> |
| <p>This results in multiple unsoundnesses:</p> |
| <ul> |
| <li>by using subtyping: <a href="https://github.com/rust-lang/rust/issues/25860">#25860</a></li> |
| <li>by using super trait upcasting for a higher ranked trait bound: <a href="https://github.com/rust-lang/rust/issues/84591">#84591</a></li> |
| <li>by being able to normalize a projection when using an impl while not being able |
| to normalize it when checking the impl: <a href="https://github.com/rust-lang/rust/issues/100051">#100051</a></li> |
| </ul> |
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