compiler/rustc_next_trait_solver/src/placeholder.rs

use core::panic;

use rustc_type_ir::data_structures::IndexMap;
use rustc_type_ir::inherent::*;
use rustc_type_ir::{
    self as ty, InferCtxtLike, Interner, TypeFoldable, TypeFolder, TypeSuperFoldable,
    TypeVisitableExt,
};

pub struct BoundVarReplacer<'a, Infcx, I = <Infcx as InferCtxtLike>::Interner>
where
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
{
    infcx: &'a Infcx,
    // These three maps track the bound variable that were replaced by placeholders. It might be
    // nice to remove these since we already have the `kind` in the placeholder; we really just need
    // the `var` (but we *could* bring that into scope if we were to track them as we pass them).
    mapped_regions: IndexMap<I::PlaceholderRegion, I::BoundRegion>,
    mapped_types: IndexMap<I::PlaceholderTy, I::BoundTy>,
    mapped_consts: IndexMap<I::PlaceholderConst, I::BoundConst>,
    // The current depth relative to *this* folding, *not* the entire normalization. In other words,
    // the depth of binders we've passed here.
    current_index: ty::DebruijnIndex,
    // The `UniverseIndex` of the binding levels above us. These are optional, since we are lazy:
    // we don't actually create a universe until we see a bound var we have to replace.
    universe_indices: &'a mut Vec<Option<ty::UniverseIndex>>,
}

impl<'a, Infcx, I> BoundVarReplacer<'a, Infcx, I>
where
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
{
    /// Returns `Some` if we *were* able to replace bound vars. If there are any bound vars that
    /// use a binding level above `universe_indices.len()`, we fail.
    pub fn replace_bound_vars<T: TypeFoldable<I>>(
        infcx: &'a Infcx,
        universe_indices: &'a mut Vec<Option<ty::UniverseIndex>>,
        value: T,
    ) -> (
        T,
        IndexMap<I::PlaceholderRegion, I::BoundRegion>,
        IndexMap<I::PlaceholderTy, I::BoundTy>,
        IndexMap<I::PlaceholderConst, I::BoundConst>,
    ) {
        let mut replacer = BoundVarReplacer {
            infcx,
            mapped_regions: Default::default(),
            mapped_types: Default::default(),
            mapped_consts: Default::default(),
            current_index: ty::INNERMOST,
            universe_indices,
        };

        let value = value.fold_with(&mut replacer);

        (value, replacer.mapped_regions, replacer.mapped_types, replacer.mapped_consts)
    }

    fn universe_for(&mut self, debruijn: ty::DebruijnIndex) -> ty::UniverseIndex {
        let infcx = self.infcx;
        let index =
            self.universe_indices.len() + self.current_index.as_usize() - debruijn.as_usize() - 1;
        let universe = self.universe_indices[index].unwrap_or_else(|| {
            for i in self.universe_indices.iter_mut().take(index + 1) {
                *i = i.or_else(|| Some(infcx.create_next_universe()))
            }
            self.universe_indices[index].unwrap()
        });
        universe
    }
}

impl<Infcx, I> TypeFolder<I> for BoundVarReplacer<'_, Infcx, I>
where
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
{
    fn cx(&self) -> I {
        self.infcx.cx()
    }

    fn fold_binder<T: TypeFoldable<I>>(&mut self, t: ty::Binder<I, T>) -> ty::Binder<I, T> {
        self.current_index.shift_in(1);
        let t = t.super_fold_with(self);
        self.current_index.shift_out(1);
        t
    }

    fn fold_region(&mut self, r: I::Region) -> I::Region {
        match r.kind() {
            ty::ReBound(debruijn, _)
                if debruijn.as_usize()
                    >= self.current_index.as_usize() + self.universe_indices.len() =>
            {
                panic!(
                    "Bound vars {r:#?} outside of `self.universe_indices`: {:#?}",
                    self.universe_indices
                );
            }
            ty::ReBound(debruijn, br) if debruijn >= self.current_index => {
                let universe = self.universe_for(debruijn);
                let p = PlaceholderLike::new(universe, br);
                self.mapped_regions.insert(p, br);
                Region::new_placeholder(self.cx(), p)
            }
            _ => r,
        }
    }

    fn fold_ty(&mut self, t: I::Ty) -> I::Ty {
        match t.kind() {
            ty::Bound(debruijn, _)
                if debruijn.as_usize() + 1
                    > self.current_index.as_usize() + self.universe_indices.len() =>
            {
                panic!(
                    "Bound vars {t:#?} outside of `self.universe_indices`: {:#?}",
                    self.universe_indices
                );
            }
            ty::Bound(debruijn, bound_ty) if debruijn >= self.current_index => {
                let universe = self.universe_for(debruijn);
                let p = PlaceholderLike::new(universe, bound_ty);
                self.mapped_types.insert(p, bound_ty);
                Ty::new_placeholder(self.cx(), p)
            }
            _ if t.has_vars_bound_at_or_above(self.current_index) => t.super_fold_with(self),
            _ => t,
        }
    }

    fn fold_const(&mut self, ct: I::Const) -> I::Const {
        match ct.kind() {
            ty::ConstKind::Bound(debruijn, _)
                if debruijn.as_usize() + 1
                    > self.current_index.as_usize() + self.universe_indices.len() =>
            {
                panic!(
                    "Bound vars {ct:#?} outside of `self.universe_indices`: {:#?}",
                    self.universe_indices
                );
            }
            ty::ConstKind::Bound(debruijn, bound_const) if debruijn >= self.current_index => {
                let universe = self.universe_for(debruijn);
                let p = PlaceholderLike::new(universe, bound_const);
                self.mapped_consts.insert(p, bound_const);
                Const::new_placeholder(self.cx(), p)
            }
            _ => ct.super_fold_with(self),
        }
    }

    fn fold_predicate(&mut self, p: I::Predicate) -> I::Predicate {
        if p.has_vars_bound_at_or_above(self.current_index) { p.super_fold_with(self) } else { p }
    }
}