diff options
Diffstat (limited to 'compiler/rustc_trait_selection/src/solve/assembly.rs')
| -rw-r--r-- | compiler/rustc_trait_selection/src/solve/assembly.rs | 387 |
1 files changed, 387 insertions, 0 deletions
diff --git a/compiler/rustc_trait_selection/src/solve/assembly.rs b/compiler/rustc_trait_selection/src/solve/assembly.rs new file mode 100644 index 000000000..31c1bc9ec --- /dev/null +++ b/compiler/rustc_trait_selection/src/solve/assembly.rs @@ -0,0 +1,387 @@ +//! Code shared by trait and projection goals for candidate assembly. + +use super::infcx_ext::InferCtxtExt; +use super::{CanonicalResponse, Certainty, EvalCtxt, Goal, MaybeCause, QueryResult}; +use rustc_hir::def_id::DefId; +use rustc_infer::traits::query::NoSolution; +use rustc_infer::traits::util::elaborate_predicates; +use rustc_middle::ty::TypeFoldable; +use rustc_middle::ty::{self, Ty, TyCtxt}; +use std::fmt::Debug; + +/// A candidate is a possible way to prove a goal. +/// +/// It consists of both the `source`, which describes how that goal would be proven, +/// and the `result` when using the given `source`. +#[derive(Debug, Clone)] +pub(super) struct Candidate<'tcx> { + pub(super) source: CandidateSource, + pub(super) result: CanonicalResponse<'tcx>, +} + +/// Possible ways the given goal can be proven. +#[derive(Debug, Clone, Copy)] +pub(super) enum CandidateSource { + /// A user written impl. + /// + /// ## Examples + /// + /// ```rust + /// fn main() { + /// let x: Vec<u32> = Vec::new(); + /// // This uses the impl from the standard library to prove `Vec<T>: Clone`. + /// let y = x.clone(); + /// } + /// ``` + Impl(DefId), + /// A builtin impl generated by the compiler. When adding a new special + /// trait, try to use actual impls whenever possible. Builtin impls should + /// only be used in cases where the impl cannot be manually be written. + /// + /// Notable examples are auto traits, `Sized`, and `DiscriminantKind`. + /// For a list of all traits with builtin impls, check out the + /// [`EvalCtxt::assemble_builtin_impl_candidates`] method. Not + BuiltinImpl, + /// An assumption from the environment. + /// + /// More precicely we've used the `n-th` assumption in the `param_env`. + /// + /// ## Examples + /// + /// ```rust + /// fn is_clone<T: Clone>(x: T) -> (T, T) { + /// // This uses the assumption `T: Clone` from the `where`-bounds + /// // to prove `T: Clone`. + /// (x.clone(), x) + /// } + /// ``` + ParamEnv(usize), + /// If the self type is an alias type, e.g. an opaque type or a projection, + /// we know the bounds on that alias to hold even without knowing its concrete + /// underlying type. + /// + /// More precisely this candidate is using the `n-th` bound in the `item_bounds` of + /// the self type. + /// + /// ## Examples + /// + /// ```rust + /// trait Trait { + /// type Assoc: Clone; + /// } + /// + /// fn foo<T: Trait>(x: <T as Trait>::Assoc) { + /// // We prove `<T as Trait>::Assoc` by looking at the bounds on `Assoc` in + /// // in the trait definition. + /// let _y = x.clone(); + /// } + /// ``` + AliasBound(usize), +} + +pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy + Eq { + fn self_ty(self) -> Ty<'tcx>; + + fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self; + + fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId; + + fn consider_impl_candidate( + ecx: &mut EvalCtxt<'_, 'tcx>, + goal: Goal<'tcx, Self>, + impl_def_id: DefId, + ) -> QueryResult<'tcx>; + + fn consider_assumption( + ecx: &mut EvalCtxt<'_, 'tcx>, + goal: Goal<'tcx, Self>, + assumption: ty::Predicate<'tcx>, + ) -> QueryResult<'tcx>; + + fn consider_auto_trait_candidate( + ecx: &mut EvalCtxt<'_, 'tcx>, + goal: Goal<'tcx, Self>, + ) -> QueryResult<'tcx>; + + fn consider_trait_alias_candidate( + ecx: &mut EvalCtxt<'_, 'tcx>, + goal: Goal<'tcx, Self>, + ) -> QueryResult<'tcx>; + + fn consider_builtin_sized_candidate( + ecx: &mut EvalCtxt<'_, 'tcx>, + goal: Goal<'tcx, Self>, + ) -> QueryResult<'tcx>; + + fn consider_builtin_copy_clone_candidate( + ecx: &mut EvalCtxt<'_, 'tcx>, + goal: Goal<'tcx, Self>, + ) -> QueryResult<'tcx>; + + fn consider_builtin_pointer_sized_candidate( + ecx: &mut EvalCtxt<'_, 'tcx>, + goal: Goal<'tcx, Self>, + ) -> QueryResult<'tcx>; + + fn consider_builtin_fn_trait_candidates( + ecx: &mut EvalCtxt<'_, 'tcx>, + goal: Goal<'tcx, Self>, + kind: ty::ClosureKind, + ) -> QueryResult<'tcx>; + + fn consider_builtin_tuple_candidate( + ecx: &mut EvalCtxt<'_, 'tcx>, + goal: Goal<'tcx, Self>, + ) -> QueryResult<'tcx>; +} + +impl<'tcx> EvalCtxt<'_, 'tcx> { + pub(super) fn assemble_and_evaluate_candidates<G: GoalKind<'tcx>>( + &mut self, + goal: Goal<'tcx, G>, + ) -> Vec<Candidate<'tcx>> { + debug_assert_eq!(goal, self.infcx.resolve_vars_if_possible(goal)); + + // HACK: `_: Trait` is ambiguous, because it may be satisfied via a builtin rule, + // object bound, alias bound, etc. We are unable to determine this until we can at + // least structually resolve the type one layer. + if goal.predicate.self_ty().is_ty_var() { + return vec![Candidate { + source: CandidateSource::BuiltinImpl, + result: self + .make_canonical_response(Certainty::Maybe(MaybeCause::Ambiguity)) + .unwrap(), + }]; + } + + let mut candidates = Vec::new(); + + self.assemble_candidates_after_normalizing_self_ty(goal, &mut candidates); + + self.assemble_impl_candidates(goal, &mut candidates); + + self.assemble_builtin_impl_candidates(goal, &mut candidates); + + self.assemble_param_env_candidates(goal, &mut candidates); + + self.assemble_alias_bound_candidates(goal, &mut candidates); + + self.assemble_object_bound_candidates(goal, &mut candidates); + + candidates + } + + /// If the self type of a goal is a projection, computing the relevant candidates is difficult. + /// + /// To deal with this, we first try to normalize the self type and add the candidates for the normalized + /// self type to the list of candidates in case that succeeds. Note that we can't just eagerly return in + /// this case as projections as self types add ` + fn assemble_candidates_after_normalizing_self_ty<G: GoalKind<'tcx>>( + &mut self, + goal: Goal<'tcx, G>, + candidates: &mut Vec<Candidate<'tcx>>, + ) { + let tcx = self.tcx(); + // FIXME: We also have to normalize opaque types, not sure where to best fit that in. + let &ty::Alias(ty::Projection, projection_ty) = goal.predicate.self_ty().kind() else { + return + }; + self.infcx.probe(|_| { + let normalized_ty = self.infcx.next_ty_infer(); + let normalizes_to_goal = goal.with( + tcx, + ty::Binder::dummy(ty::ProjectionPredicate { + projection_ty, + term: normalized_ty.into(), + }), + ); + let normalization_certainty = match self.evaluate_goal(normalizes_to_goal) { + Ok((_, certainty)) => certainty, + Err(NoSolution) => return, + }; + let normalized_ty = self.infcx.resolve_vars_if_possible(normalized_ty); + + // NOTE: Alternatively we could call `evaluate_goal` here and only have a `Normalized` candidate. + // This doesn't work as long as we use `CandidateSource` in winnowing. + let goal = goal.with(tcx, goal.predicate.with_self_ty(tcx, normalized_ty)); + // FIXME: This is broken if we care about the `usize` of `AliasBound` because the self type + // could be normalized to yet another projection with different item bounds. + let normalized_candidates = self.assemble_and_evaluate_candidates(goal); + for mut normalized_candidate in normalized_candidates { + normalized_candidate.result = + normalized_candidate.result.unchecked_map(|mut response| { + // FIXME: This currently hides overflow in the normalization step of the self type + // which is probably wrong. Maybe `unify_and` should actually keep overflow as + // we treat it as non-fatal anyways. + response.certainty = response.certainty.unify_and(normalization_certainty); + response + }); + candidates.push(normalized_candidate); + } + }) + } + + fn assemble_impl_candidates<G: GoalKind<'tcx>>( + &mut self, + goal: Goal<'tcx, G>, + candidates: &mut Vec<Candidate<'tcx>>, + ) { + let tcx = self.tcx(); + tcx.for_each_relevant_impl( + goal.predicate.trait_def_id(tcx), + goal.predicate.self_ty(), + |impl_def_id| match G::consider_impl_candidate(self, goal, impl_def_id) { + Ok(result) => candidates + .push(Candidate { source: CandidateSource::Impl(impl_def_id), result }), + Err(NoSolution) => (), + }, + ); + } + + fn assemble_builtin_impl_candidates<G: GoalKind<'tcx>>( + &mut self, + goal: Goal<'tcx, G>, + candidates: &mut Vec<Candidate<'tcx>>, + ) { + let lang_items = self.tcx().lang_items(); + let trait_def_id = goal.predicate.trait_def_id(self.tcx()); + let result = if self.tcx().trait_is_auto(trait_def_id) { + G::consider_auto_trait_candidate(self, goal) + } else if self.tcx().trait_is_alias(trait_def_id) { + G::consider_trait_alias_candidate(self, goal) + } else if lang_items.sized_trait() == Some(trait_def_id) { + G::consider_builtin_sized_candidate(self, goal) + } else if lang_items.copy_trait() == Some(trait_def_id) + || lang_items.clone_trait() == Some(trait_def_id) + { + G::consider_builtin_copy_clone_candidate(self, goal) + } else if lang_items.pointer_sized() == Some(trait_def_id) { + G::consider_builtin_pointer_sized_candidate(self, goal) + } else if let Some(kind) = self.tcx().fn_trait_kind_from_def_id(trait_def_id) { + G::consider_builtin_fn_trait_candidates(self, goal, kind) + } else if lang_items.tuple_trait() == Some(trait_def_id) { + G::consider_builtin_tuple_candidate(self, goal) + } else { + Err(NoSolution) + }; + + match result { + Ok(result) => { + candidates.push(Candidate { source: CandidateSource::BuiltinImpl, result }) + } + Err(NoSolution) => (), + } + } + + fn assemble_param_env_candidates<G: GoalKind<'tcx>>( + &mut self, + goal: Goal<'tcx, G>, + candidates: &mut Vec<Candidate<'tcx>>, + ) { + for (i, assumption) in goal.param_env.caller_bounds().iter().enumerate() { + match G::consider_assumption(self, goal, assumption) { + Ok(result) => { + candidates.push(Candidate { source: CandidateSource::ParamEnv(i), result }) + } + Err(NoSolution) => (), + } + } + } + + fn assemble_alias_bound_candidates<G: GoalKind<'tcx>>( + &mut self, + goal: Goal<'tcx, G>, + candidates: &mut Vec<Candidate<'tcx>>, + ) { + let alias_ty = match goal.predicate.self_ty().kind() { + ty::Bool + | ty::Char + | ty::Int(_) + | ty::Uint(_) + | ty::Float(_) + | ty::Adt(_, _) + | ty::Foreign(_) + | ty::Str + | ty::Array(_, _) + | ty::Slice(_) + | ty::RawPtr(_) + | ty::Ref(_, _, _) + | ty::FnDef(_, _) + | ty::FnPtr(_) + | ty::Dynamic(..) + | ty::Closure(..) + | ty::Generator(..) + | ty::GeneratorWitness(_) + | ty::Never + | ty::Tuple(_) + | ty::Param(_) + | ty::Placeholder(..) + | ty::Infer(_) + | ty::Error(_) => return, + ty::Bound(..) => bug!("unexpected bound type: {goal:?}"), + ty::Alias(_, alias_ty) => alias_ty, + }; + + for (i, (assumption, _)) in self + .tcx() + .bound_explicit_item_bounds(alias_ty.def_id) + .subst_iter_copied(self.tcx(), alias_ty.substs) + .enumerate() + { + match G::consider_assumption(self, goal, assumption) { + Ok(result) => { + candidates.push(Candidate { source: CandidateSource::AliasBound(i), result }) + } + Err(NoSolution) => (), + } + } + } + + fn assemble_object_bound_candidates<G: GoalKind<'tcx>>( + &mut self, + goal: Goal<'tcx, G>, + candidates: &mut Vec<Candidate<'tcx>>, + ) { + let self_ty = goal.predicate.self_ty(); + let bounds = match *self_ty.kind() { + ty::Bool + | ty::Char + | ty::Int(_) + | ty::Uint(_) + | ty::Float(_) + | ty::Adt(_, _) + | ty::Foreign(_) + | ty::Str + | ty::Array(_, _) + | ty::Slice(_) + | ty::RawPtr(_) + | ty::Ref(_, _, _) + | ty::FnDef(_, _) + | ty::FnPtr(_) + | ty::Alias(..) + | ty::Closure(..) + | ty::Generator(..) + | ty::GeneratorWitness(_) + | ty::Never + | ty::Tuple(_) + | ty::Param(_) + | ty::Placeholder(..) + | ty::Infer(_) + | ty::Error(_) => return, + ty::Bound(..) => bug!("unexpected bound type: {goal:?}"), + ty::Dynamic(bounds, ..) => bounds, + }; + + let tcx = self.tcx(); + for assumption in + elaborate_predicates(tcx, bounds.iter().map(|bound| bound.with_self_ty(tcx, self_ty))) + { + match G::consider_assumption(self, goal, assumption.predicate) { + Ok(result) => { + candidates.push(Candidate { source: CandidateSource::BuiltinImpl, result }) + } + Err(NoSolution) => (), + } + } + } +} |