add implied bounds doc (#1915)

* add implied bounds doc

* lazy type aliases also have explicit implied bounds

Author: lcnr

Diff for: src/SUMMARY.md

@@ -120,6 +120,7 @@
         - [Interactions with turbofishing](./turbofishing-and-early-late-bound.md)
     - [Higher-ranked trait bounds](./traits/hrtb.md)
     - [Caching subtleties](./traits/caching.md)
+    - [Implied bounds](./traits/implied-bounds.md)
     - [Specialization](./traits/specialization.md)
     - [Chalk-based trait solving](./traits/chalk.md)
         - [Lowering to logic](./traits/lowering-to-logic.md)

Diff for: src/traits/implied-bounds.md

@@ -0,0 +1,84 @@
+# Implied bounds
+
+We currently add implied region bounds to avoid explicit annotations. e.g.
+`fn foo<'a, T>(x: &'a T)` can freely assume that `T: 'a` holds without specifying it.
+
+There are two kinds of implied bounds: explicit and implicit. Explicit implied bounds
+get added to the `fn predicates_of` of the relevant item while implicit ones are
+handled... well... implicitly.
+
+## explicit implied bounds
+
+The explicit implied bounds are computed in [`fn inferred_outlives_of`]. Only ADTs and
+lazy type aliases have explicit implied bounds which are computed via a fixpoint algorithm
+in the [`fn inferred_outlives_crate`] query.
+
+We use [`fn insert_required_predicates_to_be_wf`] 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.
+
+For ADTs, trait objects, and associated types the initially required predicates are
+computed in [`fn check_explicit_predicates`]. This simply uses `fn explicit_predicates_of`
+without elaborating them.
+
+Region predicates are added via [`fn insert_outlives_predicate`]. This function takes
+an outlives predicate, decomposes it and adds the components as explicit predicates only
+if the outlived region is a region parameter. [It does not add `'static` requirements][nostatic].
+
+ [`fn inferred_outlives_of`]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/mod.rs#L20
+ [`fn inferred_outlives_crate`]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/mod.rs#L83
+ [`fn insert_required_predicates_to_be_wf`]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs#L89
+ [`fn check_explicit_predicates`]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs#L238
+ [`fn insert_outlives_predicate`]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/utils.rs#L15
+ [nostatic]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/utils.rs#L159-L165
+
+## implicit implied bounds
+
+As we are unable to handle implications in binders yet, we cannot simply add the outlives
+requirements of impls and functions as explicit predicates.
+
+### using implicit implied bounds as assumptions
+
+These bounds are not added to the `ParamEnv` of the affected item itself. For lexical
+region resolution they are added using [`fn OutlivesEnvironment::with_bounds`].
+Similarly,during MIR borrowck we add them using
+[`fn UniversalRegionRelationsBuilder::add_implied_bounds`].
+
+[We add implied bounds for the function signature and impl header in MIR borrowck][mir].
+Outside of MIR borrowck we add the outlives requirements for the types returned by the
+[`fn assumed_wf_types`] query.
+
+The assumed outlives constraints for implicit bounds are computed using the
+[`fn implied_outlives_bounds`] query. This directly
+[extracts the required outlives bounds from `fn wf::obligations`][boundsfromty].
+
+MIR borrowck adds the outlives constraints for both the normalized and unnormalized types,
+lexical region resolution [only uses the unnormalized types][notnorm].
+
+[`fn OutlivesEnvironment::with_bounds`]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_infer/src/infer/outlives/env.rs#L90-L97
+[`fn UniversalRegionRelationsBuilder::add_implied_bounds`]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_borrowck/src/type_check/free_region_relations.rs#L316
+[mir]: https://github.com/rust-lang/rust/blob/91cae1dcdcf1a31bd8a92e4a63793d65cfe289bb/compiler/rustc_borrowck/src/type_check/free_region_relations.rs#L258-L332
+[`fn assumed_wf_types`]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_ty_utils/src/implied_bounds.rs#L21
+[`fn implied_outlives_bounds`]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_traits/src/implied_outlives_bounds.rs#L18C4-L18C27
+[boundsfromty]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs#L95-L96
+[notnorm]: https://github.com/rust-lang/rust/blob/91cae1dcdcf1a31bd8a92e4a63793d65cfe289bb/compiler/rustc_trait_selection/src/traits/engine.rs#L227-L250
+
+### proving implicit implied bounds
+
+As the implicit implied bounds are not included in `fn predicates_of` we have to
+separately make sure they actually hold. We generally handle this by checking that
+all used types are well formed by emitting `WellFormed` predicates.
+
+We cannot emit `WellFormed` 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.
+
+This results in multiple unsoundnesses:
+- by using subtyping: [#25860]
+- by using super trait upcasting for a higher ranked trait bound: [#84591]
+- by being able to normalize a projection when using an impl while not being able
+  to normalize it when checking the impl: [#100051]
+
+[#25860]: https://github.com/rust-lang/rust/issues/25860
+[#84591]: https://github.com/rust-lang/rust/issues/84591
+[#100051]: https://github.com/rust-lang/rust/issues/100051