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|
//! In certain situations, rust automatically inserts derefs as necessary: for
//! example, field accesses `foo.bar` still work when `foo` is actually a
//! reference to a type with the field `bar`. This is an approximation of the
//! logic in rustc (which lives in librustc_typeck/check/autoderef.rs).
use std::iter::successors;
use base_db::CrateId;
use chalk_ir::cast::Cast;
use hir_def::lang_item::LangItemTarget;
use hir_expand::name::name;
use log::{info, warn};
use crate::{
db::HirDatabase,
to_assoc_type_id, to_chalk_trait_id,
traits::{InEnvironment, Solution},
utils::generics,
AliasEq, AliasTy, BoundVar, Canonical, CanonicalVarKinds, DebruijnIndex, Interner,
ProjectionTy, Substitution, TraitRef, Ty, TyKind,
};
const AUTODEREF_RECURSION_LIMIT: usize = 10;
pub fn autoderef<'a>(
db: &'a dyn HirDatabase,
krate: Option<CrateId>,
ty: InEnvironment<Canonical<Ty>>,
) -> impl Iterator<Item = Canonical<Ty>> + 'a {
let InEnvironment { goal: ty, environment } = ty;
successors(Some(ty), move |ty| {
deref(db, krate?, InEnvironment { goal: ty, environment: environment.clone() })
})
.take(AUTODEREF_RECURSION_LIMIT)
}
pub(crate) fn deref(
db: &dyn HirDatabase,
krate: CrateId,
ty: InEnvironment<&Canonical<Ty>>,
) -> Option<Canonical<Ty>> {
if let Some(derefed) = ty.goal.value.builtin_deref() {
Some(Canonical { value: derefed, binders: ty.goal.binders.clone() })
} else {
deref_by_trait(db, krate, ty)
}
}
fn deref_by_trait(
db: &dyn HirDatabase,
krate: CrateId,
ty: InEnvironment<&Canonical<Ty>>,
) -> Option<Canonical<Ty>> {
let deref_trait = match db.lang_item(krate, "deref".into())? {
LangItemTarget::TraitId(it) => it,
_ => return None,
};
let target = db.trait_data(deref_trait).associated_type_by_name(&name![Target])?;
let generic_params = generics(db.upcast(), target.into());
if generic_params.len() != 1 {
// the Target type + Deref trait should only have one generic parameter,
// namely Deref's Self type
return None;
}
// FIXME make the Canonical / bound var handling nicer
let parameters =
Substitution::build_for_generics(&generic_params).push(ty.goal.value.clone()).build();
// Check that the type implements Deref at all
let trait_ref =
TraitRef { trait_id: to_chalk_trait_id(deref_trait), substitution: parameters.clone() };
let implements_goal = Canonical {
binders: ty.goal.binders.clone(),
value: InEnvironment {
goal: trait_ref.cast(&Interner),
environment: ty.environment.clone(),
},
};
if db.trait_solve(krate, implements_goal).is_none() {
return None;
}
// Now do the assoc type projection
let projection = AliasEq {
alias: AliasTy::Projection(ProjectionTy {
associated_ty_id: to_assoc_type_id(target),
substitution: parameters,
}),
ty: TyKind::BoundVar(BoundVar::new(
DebruijnIndex::INNERMOST,
ty.goal.binders.len(&Interner),
))
.intern(&Interner),
};
let obligation = projection.cast(&Interner);
let in_env = InEnvironment { goal: obligation, environment: ty.environment };
let canonical = Canonical {
value: in_env,
binders: CanonicalVarKinds::from_iter(
&Interner,
ty.goal.binders.iter(&Interner).cloned().chain(Some(chalk_ir::WithKind::new(
chalk_ir::VariableKind::Ty(chalk_ir::TyVariableKind::General),
chalk_ir::UniverseIndex::ROOT,
))),
),
};
let solution = db.trait_solve(krate, canonical)?;
match &solution {
Solution::Unique(vars) => {
// FIXME: vars may contain solutions for any inference variables
// that happened to be inside ty. To correctly handle these, we
// would have to pass the solution up to the inference context, but
// that requires a larger refactoring (especially if the deref
// happens during method resolution). So for the moment, we just
// check that we're not in the situation we're we would actually
// need to handle the values of the additional variables, i.e.
// they're just being 'passed through'. In the 'standard' case where
// we have `impl<T> Deref for Foo<T> { Target = T }`, that should be
// the case.
// FIXME: if the trait solver decides to truncate the type, these
// assumptions will be broken. We would need to properly introduce
// new variables in that case
for i in 1..vars.0.binders.len(&Interner) {
if vars.0.value.at(&Interner, i - 1).assert_ty_ref(&Interner).kind(&Interner)
!= &TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, i - 1))
{
warn!("complex solution for derefing {:?}: {:?}, ignoring", ty.goal, solution);
return None;
}
}
Some(Canonical {
value: vars
.0
.value
.at(&Interner, vars.0.value.len(&Interner) - 1)
.assert_ty_ref(&Interner)
.clone(),
binders: vars.0.binders.clone(),
})
}
Solution::Ambig(_) => {
info!("Ambiguous solution for derefing {:?}: {:?}", ty.goal, solution);
None
}
}
}
|
