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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 log::{info, warn};
use super::{traits::Solution, Canonical, Ty};
use crate::{HasGenericParams, HirDatabase, Name, Resolver};
const AUTODEREF_RECURSION_LIMIT: usize = 10;
pub(crate) fn autoderef<'a>(
db: &'a impl HirDatabase,
resolver: &'a Resolver,
ty: Canonical<Ty>,
) -> impl Iterator<Item = Canonical<Ty>> + 'a {
successors(Some(ty), move |ty| deref(db, resolver, ty)).take(AUTODEREF_RECURSION_LIMIT)
}
pub(crate) fn deref(
db: &impl HirDatabase,
resolver: &Resolver,
ty: &Canonical<Ty>,
) -> Option<Canonical<Ty>> {
if let Some(derefed) = ty.value.builtin_deref() {
Some(Canonical { value: derefed, num_vars: ty.num_vars })
} else {
deref_by_trait(db, resolver, ty)
}
}
fn deref_by_trait(
db: &impl HirDatabase,
resolver: &Resolver,
ty: &Canonical<Ty>,
) -> Option<Canonical<Ty>> {
let krate = resolver.krate()?;
let deref_trait = match db.lang_item(krate, "deref".into())? {
crate::lang_item::LangItemTarget::Trait(t) => t,
_ => return None,
};
let target = deref_trait.associated_type_by_name(db, Name::target())?;
if target.generic_params(db).count_params_including_parent() != 1 {
// the Target type + Deref trait should only have one generic parameter,
// namely Deref's Self type
return None;
}
// FIXME make the Canonical handling nicer
let projection = super::traits::ProjectionPredicate {
ty: Ty::Bound(0),
projection_ty: super::ProjectionTy {
associated_ty: target,
parameters: vec![ty.value.clone().shift_bound_vars(1)].into(),
},
};
let canonical = super::Canonical { num_vars: 1 + ty.num_vars, value: projection };
let solution = db.normalize(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.
for i in 1..vars.0.num_vars {
if vars.0.value[i] != Ty::Bound((i - 1) as u32) {
warn!("complex solution for derefing {:?}: {:?}, ignoring", ty, solution);
return None;
}
}
Some(Canonical { value: vars.0.value[0].clone(), num_vars: vars.0.num_vars })
}
Solution::Ambig(_) => {
info!("Ambiguous solution for derefing {:?}: {:?}", ty, solution);
None
}
}
}
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