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Rollup merge of #114169 - lcnr:unsize, r=compiler-errors

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refactor builtin unsize handling, extend comments

r? `@compiler-errors`
This commit is contained in:
Matthias Krüger 2023-07-31 16:57:53 +02:00 • committed by GitHub
commit b8f78fb47c
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9 changed files with 271 additions and 218 deletions
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442
compiler/rustc_trait_selection/src/solve/trait_goals.rs
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@ -7,7 +7,7 @@ use rustc_hir::def_id::DefId;
use rustc_hir::{LangItem, Movability};
use rustc_infer::traits::query::NoSolution;
use rustc_middle::traits::solve::inspect::CandidateKind;
use rustc_middle::traits::solve::{CanonicalResponse, Certainty, Goal, MaybeCause, QueryResult};
use rustc_middle::traits::solve::{CanonicalResponse, Certainty, Goal, QueryResult};
use rustc_middle::traits::{BuiltinImplSource, Reveal};
use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams, TreatProjections};
use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt};
@ -366,69 +366,6 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
)
}
fn consider_builtin_unsize_and_upcast_candidates(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> Vec<(CanonicalResponse<'tcx>, BuiltinImplSource)> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return vec![];
}
ecx.probe(|_| CandidateKind::DynUpcastingAssembly).enter(|ecx| {
let a_ty = goal.predicate.self_ty();
// We need to normalize the b_ty since it's matched structurally
// in the other functions below.
let b_ty = match ecx
.normalize_non_self_ty(goal.predicate.trait_ref.args.type_at(1), goal.param_env)
{
Ok(Some(b_ty)) => {
// If we have a type var, then bail with ambiguity.
if b_ty.is_ty_var() {
return vec![(
ecx.evaluate_added_goals_and_make_canonical_response(
Certainty::AMBIGUOUS,
)
.unwrap(),
BuiltinImplSource::Misc,
)];
} else {
b_ty
}
}
Ok(None) => {
return vec![(
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Maybe(
MaybeCause::Overflow,
))
.unwrap(),
BuiltinImplSource::Misc,
)];
}
Err(_) => return vec![],
};
let mut results = vec![];
results.extend(ecx.consider_builtin_dyn_upcast_candidates(goal.param_env, a_ty, b_ty));
results.extend(
ecx.consider_builtin_unsize_candidate(goal.with(ecx.tcx(), (a_ty, b_ty)))
.into_iter()
.map(|resp| {
// If we're unsizing from tuple -> tuple, detect
let source =
if matches!((a_ty.kind(), b_ty.kind()), (ty::Tuple(..), ty::Tuple(..)))
{
BuiltinImplSource::TupleUnsizing
} else {
BuiltinImplSource::Misc
};
(resp, source)
}),
);
results
})
}
fn consider_builtin_discriminant_kind_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
@ -486,153 +423,111 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
)?;
ecx.evaluate_added_goals_and_make_canonical_response(certainty)
}
}
impl<'tcx> EvalCtxt<'_, 'tcx> {
fn consider_builtin_unsize_candidate(
&mut self,
goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
) -> QueryResult<'tcx> {
let Goal { param_env, predicate: (a_ty, b_ty) } = goal;
self.probe_candidate("builtin unsize").enter(|ecx| {
let tcx = ecx.tcx();
fn consider_builtin_unsize_candidates(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> Vec<(CanonicalResponse<'tcx>, BuiltinImplSource)> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return vec![];
}
let misc_candidate = |ecx: &mut EvalCtxt<'_, 'tcx>, certainty| {
(
ecx.evaluate_added_goals_and_make_canonical_response(certainty).unwrap(),
BuiltinImplSource::Misc,
)
};
let result_to_single = |result, source| match result {
Ok(resp) => vec![(resp, source)],
Err(NoSolution) => vec![],
};
ecx.probe(|_| CandidateKind::DynUpcastingAssembly).enter(|ecx| {
let a_ty = goal.predicate.self_ty();
// We need to normalize the b_ty since it's matched structurally
// in the other functions below.
let b_ty = match ecx
.normalize_non_self_ty(goal.predicate.trait_ref.args.type_at(1), goal.param_env)
{
Ok(Some(b_ty)) => b_ty,
Ok(None) => return vec![misc_candidate(ecx, Certainty::OVERFLOW)],
Err(_) => return vec![],
};
let goal = goal.with(ecx.tcx(), (a_ty, b_ty));
match (a_ty.kind(), b_ty.kind()) {
(ty::Infer(ty::TyVar(_)), _) | (_, ty::Infer(ty::TyVar(_))) => {
bug!("unexpected type variable in unsize goal")
}
// Trait upcasting, or `dyn Trait + Auto + 'a` -> `dyn Trait + 'b`
(&ty::Dynamic(_, _, ty::Dyn), &ty::Dynamic(_, _, ty::Dyn)) => {
// Dyn upcasting is handled separately, since due to upcasting,
// when there are two supertraits that differ by args, we
// may return more than one query response.
Err(NoSolution)
}
(ty::Infer(ty::TyVar(..)), ..) => bug!("unexpected infer {a_ty:?} {b_ty:?}"),
(_, ty::Infer(ty::TyVar(..))) => vec![misc_candidate(ecx, Certainty::AMBIGUOUS)],
// Trait upcasting, or `dyn Trait + Auto + 'a` -> `dyn Trait + 'b`.
(
&ty::Dynamic(a_data, a_region, ty::Dyn),
&ty::Dynamic(b_data, b_region, ty::Dyn),
) => ecx.consider_builtin_dyn_upcast_candidates(
goal, a_data, a_region, b_data, b_region,
),
// `T` -> `dyn Trait` unsizing
(_, &ty::Dynamic(data, region, ty::Dyn)) => {
// Can only unsize to an object-safe type
if data
.principal_def_id()
.is_some_and(|def_id| !tcx.check_is_object_safe(def_id))
{
return Err(NoSolution);
}
(_, &ty::Dynamic(b_data, b_region, ty::Dyn)) => result_to_single(
ecx.consider_builtin_unsize_to_dyn(goal, b_data, b_region),
BuiltinImplSource::Misc,
),
let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
return Err(NoSolution);
};
// Check that the type implements all of the predicates of the def-id.
// (i.e. the principal, all of the associated types match, and any auto traits)
ecx.add_goals(
data.iter()
.map(|pred| Goal::new(tcx, param_env, pred.with_self_ty(tcx, a_ty))),
);
// The type must be Sized to be unsized.
ecx.add_goal(Goal::new(
tcx,
param_env,
ty::TraitRef::new(tcx, sized_def_id, [a_ty]),
));
// The type must outlive the lifetime of the `dyn` we're unsizing into.
ecx.add_goal(Goal::new(
tcx,
param_env,
ty::Binder::dummy(ty::OutlivesPredicate(a_ty, region)),
));
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
// `[T; n]` -> `[T]` unsizing
(&ty::Array(a_elem_ty, ..), &ty::Slice(b_elem_ty)) => {
// We just require that the element type stays the same
ecx.eq(param_env, a_elem_ty, b_elem_ty)?;
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
// Struct unsizing `Struct<T>` -> `Struct<U>` where `T: Unsize<U>`
// `[T; N]` -> `[T]` unsizing
(&ty::Array(a_elem_ty, ..), &ty::Slice(b_elem_ty)) => result_to_single(
ecx.consider_builtin_array_unsize(goal, a_elem_ty, b_elem_ty),
BuiltinImplSource::Misc,
),
// `Struct<T>` -> `Struct<U>` where `T: Unsize<U>`
(&ty::Adt(a_def, a_args), &ty::Adt(b_def, b_args))
if a_def.is_struct() && a_def.did() == b_def.did() =>
if a_def.is_struct() && a_def == b_def =>
{
let unsizing_params = tcx.unsizing_params_for_adt(a_def.did());
// We must be unsizing some type parameters. This also implies
// that the struct has a tail field.
if unsizing_params.is_empty() {
return Err(NoSolution);
}
let tail_field = a_def.non_enum_variant().tail();
let tail_field_ty = tcx.type_of(tail_field.did);
let a_tail_ty = tail_field_ty.instantiate(tcx, a_args);
let b_tail_ty = tail_field_ty.instantiate(tcx, b_args);
// Substitute just the unsizing params from B into A. The type after
// this substitution must be equal to B. This is so we don't unsize
// unrelated type parameters.
let new_a_args =
tcx.mk_args_from_iter(a_args.iter().enumerate().map(|(i, a)| {
if unsizing_params.contains(i as u32) { b_args[i] } else { a }
}));
let unsized_a_ty = Ty::new_adt(tcx, a_def, new_a_args);
// Finally, we require that `TailA: Unsize<TailB>` for the tail field
// types.
ecx.eq(param_env, unsized_a_ty, b_ty)?;
ecx.add_goal(Goal::new(
tcx,
param_env,
ty::TraitRef::new(
tcx,
tcx.lang_items().unsize_trait().unwrap(),
[a_tail_ty, b_tail_ty],
),
));
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
result_to_single(
ecx.consider_builtin_struct_unsize(goal, a_def, a_args, b_args),
BuiltinImplSource::Misc,
)
}
// Tuple unsizing `(.., T)` -> `(.., U)` where `T: Unsize<U>`
// `(A, B, T)` -> `(A, B, U)` where `T: Unsize<U>`
(&ty::Tuple(a_tys), &ty::Tuple(b_tys))
if a_tys.len() == b_tys.len() && !a_tys.is_empty() =>
{
let (a_last_ty, a_rest_tys) = a_tys.split_last().unwrap();
let b_last_ty = b_tys.last().unwrap();
// Substitute just the tail field of B., and require that they're equal.
let unsized_a_ty =
Ty::new_tup_from_iter(tcx, a_rest_tys.iter().chain([b_last_ty]).copied());
ecx.eq(param_env, unsized_a_ty, b_ty)?;
// Similar to ADTs, require that the rest of the fields are equal.
ecx.add_goal(Goal::new(
tcx,
param_env,
ty::TraitRef::new(
tcx,
tcx.lang_items().unsize_trait().unwrap(),
[*a_last_ty, *b_last_ty],
),
));
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
result_to_single(
ecx.consider_builtin_tuple_unsize(goal, a_tys, b_tys),
BuiltinImplSource::TupleUnsizing,
)
}
_ => Err(NoSolution),
_ => vec![],
}
})
}
}
impl<'tcx> EvalCtxt<'_, 'tcx> {
/// Trait upcasting allows for coercions between trait objects:
/// ```ignore (builtin impl example)
/// trait Super {}
/// trait Trait: Super {}
/// // results in builtin impls upcasting to a super trait
/// impl<'a, 'b: 'a> Unsize<dyn Super + 'a> for dyn Trait + 'b {}
/// // and impls removing auto trait bounds.
/// impl<'a, 'b: 'a> Unsize<dyn Trait + 'a> for dyn Trait + Send + 'b {}
/// ```
fn consider_builtin_dyn_upcast_candidates(
&mut self,
param_env: ty::ParamEnv<'tcx>,
a_ty: Ty<'tcx>,
b_ty: Ty<'tcx>,
goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
a_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
a_region: ty::Region<'tcx>,
b_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
b_region: ty::Region<'tcx>,
) -> Vec<(CanonicalResponse<'tcx>, BuiltinImplSource)> {
if a_ty.is_ty_var() || b_ty.is_ty_var() {
bug!("unexpected type variable in unsize goal")
}
let ty::Dynamic(a_data, a_region, ty::Dyn) = *a_ty.kind() else {
return vec![];
};
let ty::Dynamic(b_data, b_region, ty::Dyn) = *b_ty.kind() else {
return vec![];
};
let tcx = self.tcx();
let Goal { predicate: (a_ty, b_ty), .. } = goal;
// All of a's auto traits need to be in b's auto traits.
let auto_traits_compatible =
b_data.auto_traits().all(|b| a_data.auto_traits().any(|a| a == b));
@ -665,12 +560,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
let new_a_ty = Ty::new_dynamic(tcx, new_a_data, b_region, ty::Dyn);
// We also require that A's lifetime outlives B's lifetime.
ecx.eq(param_env, new_a_ty, b_ty)?;
ecx.add_goal(Goal::new(
tcx,
param_env,
ty::Binder::dummy(ty::OutlivesPredicate(a_region, b_region)),
));
ecx.eq(goal.param_env, new_a_ty, b_ty)?;
ecx.add_goal(goal.with(tcx, ty::OutlivesPredicate(a_region, b_region)));
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
},
)
@ -703,6 +594,161 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
responses
}
/// ```ignore (builtin impl example)
/// trait Trait {
/// fn foo(&self);
/// }
/// // results in the following builtin impl
/// impl<'a, T: Trait + 'a> Unsize<dyn Trait + 'a> for T {}
/// ```
fn consider_builtin_unsize_to_dyn(
&mut self,
goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
b_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
b_region: ty::Region<'tcx>,
) -> QueryResult<'tcx> {
let tcx = self.tcx();
let Goal { predicate: (a_ty, _b_ty), .. } = goal;
// Can only unsize to an object-safe trait
if b_data.principal_def_id().is_some_and(|def_id| !tcx.check_is_object_safe(def_id)) {
return Err(NoSolution);
}
// Check that the type implements all of the predicates of the trait object.
// (i.e. the principal, all of the associated types match, and any auto traits)
self.add_goals(b_data.iter().map(|pred| goal.with(tcx, pred.with_self_ty(tcx, a_ty))));
// The type must be `Sized` to be unsized.
if let Some(sized_def_id) = tcx.lang_items().sized_trait() {
self.add_goal(goal.with(tcx, ty::TraitRef::new(tcx, sized_def_id, [a_ty])));
} else {
return Err(NoSolution);
}
// The type must outlive the lifetime of the `dyn` we're unsizing into.
self.add_goal(goal.with(tcx, ty::OutlivesPredicate(a_ty, b_region)));
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
/// We have the following builtin impls for arrays:
/// ```ignore (builtin impl example)
/// impl<T: ?Sized, const N: usize> Unsize<[T]> for [T; N] {}
/// ```
/// While the impl itself could theoretically not be builtin,
/// the actual unsizing behavior is builtin. Its also easier to
/// make all impls of `Unsize` builtin as we're able to use
/// `#[rustc_deny_explicit_impl]` in this case.
fn consider_builtin_array_unsize(
&mut self,
goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
a_elem_ty: Ty<'tcx>,
b_elem_ty: Ty<'tcx>,
) -> QueryResult<'tcx> {
self.eq(goal.param_env, a_elem_ty, b_elem_ty)?;
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
/// We generate a builtin `Unsize` impls for structs with generic parameters only
/// mentioned by the last field.
/// ```ignore (builtin impl example)
/// struct Foo<T, U: ?Sized> {
/// sized_field: Vec<T>,
/// unsizable: Box<U>,
/// }
/// // results in the following builtin impl
/// impl<T: ?Sized, U: ?Sized, V: ?Sized> Unsize<Foo<T, V>> for Foo<T, U>
/// where
/// Box<U>: Unsize<Box<V>>,
/// {}
/// ```
fn consider_builtin_struct_unsize(
&mut self,
goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
def: ty::AdtDef<'tcx>,
a_args: ty::GenericArgsRef<'tcx>,
b_args: ty::GenericArgsRef<'tcx>,
) -> QueryResult<'tcx> {
let tcx = self.tcx();
let Goal { predicate: (_a_ty, b_ty), .. } = goal;
let unsizing_params = tcx.unsizing_params_for_adt(def.did());
// We must be unsizing some type parameters. This also implies
// that the struct has a tail field.
if unsizing_params.is_empty() {
return Err(NoSolution);
}
let tail_field = def.non_enum_variant().tail();
let tail_field_ty = tcx.type_of(tail_field.did);
let a_tail_ty = tail_field_ty.instantiate(tcx, a_args);
let b_tail_ty = tail_field_ty.instantiate(tcx, b_args);
// Substitute just the unsizing params from B into A. The type after
// this substitution must be equal to B. This is so we don't unsize
// unrelated type parameters.
let new_a_args = tcx.mk_args_from_iter(
a_args
.iter()
.enumerate()
.map(|(i, a)| if unsizing_params.contains(i as u32) { b_args[i] } else { a }),
);
let unsized_a_ty = Ty::new_adt(tcx, def, new_a_args);
// Finally, we require that `TailA: Unsize<TailB>` for the tail field
// types.
self.eq(goal.param_env, unsized_a_ty, b_ty)?;
self.add_goal(goal.with(
tcx,
ty::TraitRef::new(
tcx,
tcx.lang_items().unsize_trait().unwrap(),
[a_tail_ty, b_tail_ty],
),
));
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
/// We generate the following builtin impl for tuples of all sizes.
///
/// This impl is still unstable and we emit a feature error when it
/// when it is used by a coercion.
/// ```ignore (builtin impl example)
/// impl<T: ?Sized, U: ?Sized, V: ?Sized> Unsize<(T, V)> for (T, U)
/// where
/// U: Unsize<V>,
/// {}
/// ```
fn consider_builtin_tuple_unsize(
&mut self,
goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
a_tys: &'tcx ty::List<Ty<'tcx>>,
b_tys: &'tcx ty::List<Ty<'tcx>>,
) -> QueryResult<'tcx> {
let tcx = self.tcx();
let Goal { predicate: (_a_ty, b_ty), .. } = goal;
let (&a_last_ty, a_rest_tys) = a_tys.split_last().unwrap();
let &b_last_ty = b_tys.last().unwrap();
// Substitute just the tail field of B., and require that they're equal.
let unsized_a_ty =
Ty::new_tup_from_iter(tcx, a_rest_tys.iter().copied().chain([b_last_ty]));
self.eq(goal.param_env, unsized_a_ty, b_ty)?;
// Similar to ADTs, require that we can unsize the tail.
self.add_goal(goal.with(
tcx,
ty::TraitRef::new(
tcx,
tcx.lang_items().unsize_trait().unwrap(),
[a_last_ty, b_last_ty],
),
));
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
// Return `Some` if there is an impl (built-in or user provided) that may
// hold for the self type of the goal, which for coherence and soundness
// purposes must disqualify the built-in auto impl assembled by considering