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

use super::{traits::Solution, Canonical, Substs, Ty, TypeWalk};
use crate::{db::HirDatabase, generics::HasGenericParams, 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.into(), "deref".into())? {
        crate::lang_item::LangItemTarget::Trait(t) => t,
        _ => return None,
    };
    let target = deref_trait.associated_type_by_name(db, &name::TARGET_TYPE)?;

    let generic_params = target.generic_params(db);
    if generic_params.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 env = super::lower::trait_env(db, resolver);

    let parameters = Substs::build_for_generics(&generic_params)
        .push(ty.value.clone().shift_bound_vars(1))
        .build();

    let projection = super::traits::ProjectionPredicate {
        ty: Ty::Bound(0),
        projection_ty: super::ProjectionTy { associated_ty: target, parameters },
    };

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

    let in_env = super::traits::InEnvironment { value: obligation, environment: env };

    let canonical = super::Canonical { num_vars: 1 + ty.num_vars, value: in_env };

    let solution = db.trait_solve(krate.into(), 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
        }
    }
}