1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131

//! 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 hir_def::lang_item::LangItemTarget;
use hir_expand::name::name;
use log::{info, warn};

use crate::{
    db::HirDatabase,
    traits::{InEnvironment, Solution},
    utils::generics,
    BoundVar, Canonical, DebruijnIndex, Obligation, Substs, TraitRef, Ty,
};

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 { value: ty, environment } = ty;
    successors(Some(ty), move |ty| {
        deref(db, krate?, InEnvironment { value: 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.value.value.builtin_deref() {
        Some(Canonical { value: derefed, kinds: ty.value.kinds.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 =
        Substs::build_for_generics(&generic_params).push(ty.value.value.clone()).build();

    // Check that the type implements Deref at all
    let trait_ref = TraitRef { trait_: deref_trait, substs: parameters.clone() };
    let implements_goal = Canonical {
        kinds: ty.value.kinds.clone(),
        value: InEnvironment {
            value: Obligation::Trait(trait_ref),
            environment: ty.environment.clone(),
        },
    };
    if db.trait_solve(krate, implements_goal).is_none() {
        return None;
    }

    // Now do the assoc type projection
    let projection = super::traits::ProjectionPredicate {
        ty: Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, ty.value.kinds.len())),
        projection_ty: super::ProjectionTy { associated_ty: target, parameters },
    };

    let obligation = super::Obligation::Projection(projection);

    let in_env = InEnvironment { value: obligation, environment: ty.environment };

    let canonical =
        Canonical::new(in_env, ty.value.kinds.iter().copied().chain(Some(super::TyKind::General)));

    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.kinds.len() {
                if vars.0.value[i - 1] != Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, i - 1))
                {
                    warn!("complex solution for derefing {:?}: {:?}, ignoring", ty.value, solution);
                    return None;
                }
            }
            Some(Canonical {
                value: vars.0.value[vars.0.value.len() - 1].clone(),
                kinds: vars.0.kinds.clone(),
            })
        }
        Solution::Ambig(_) => {
            info!("Ambiguous solution for derefing {:?}: {:?}", ty.value, solution);
            None
        }
    }
}