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use rustc_index::vec::IndexVec;
use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin};
use rustc_middle::mir::visit::{MutVisitor, TyContext};
use rustc_middle::mir::{Body, Location, Promoted};
use rustc_middle::mir::{Constant, ConstantKind};
use rustc_middle::ty::subst::SubstsRef;
use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable};

/// Replaces all free regions appearing in the MIR with fresh
/// inference variables, returning the number of variables created.
#[instrument(skip(infcx, body, promoted), level = "debug")]
pub fn renumber_mir<'tcx>(
    infcx: &InferCtxt<'_, 'tcx>,
    body: &mut Body<'tcx>,
    promoted: &mut IndexVec<Promoted, Body<'tcx>>,
) {
    debug!(?body.arg_count);

    let mut visitor = NllVisitor { infcx };

    for body in promoted.iter_mut() {
        visitor.visit_body(body);
    }

    visitor.visit_body(body);
}

/// Replaces all regions appearing in `value` with fresh inference
/// variables.
#[instrument(skip(infcx), level = "debug")]
pub fn renumber_regions<'tcx, T>(infcx: &InferCtxt<'_, 'tcx>, value: T) -> T
where
    T: TypeFoldable<'tcx>,
{
    infcx.tcx.fold_regions(value, |_region, _depth| {
        let origin = NllRegionVariableOrigin::Existential { from_forall: false };
        infcx.next_nll_region_var(origin)
    })
}

// FIXME(valtrees): This function is necessary because `fold_regions`
// panics for mir constants in the visitor.
//
// Once `visit_mir_constant` is removed we can also remove this function
// and just use `renumber_regions`.
fn renumber_regions_in_mir_constant<'tcx>(
    infcx: &InferCtxt<'_, 'tcx>,
    value: ConstantKind<'tcx>,
) -> ConstantKind<'tcx> {
    infcx.tcx.super_fold_regions(value, |_region, _depth| {
        let origin = NllRegionVariableOrigin::Existential { from_forall: false };
        infcx.next_nll_region_var(origin)
    })
}

struct NllVisitor<'a, 'tcx> {
    infcx: &'a InferCtxt<'a, 'tcx>,
}

impl<'a, 'tcx> NllVisitor<'a, 'tcx> {
    fn renumber_regions<T>(&mut self, value: T) -> T
    where
        T: TypeFoldable<'tcx>,
    {
        renumber_regions(self.infcx, value)
    }

    fn renumber_regions_in_mir_constant(
        &mut self,
        value: ConstantKind<'tcx>,
    ) -> ConstantKind<'tcx> {
        renumber_regions_in_mir_constant(self.infcx, value)
    }
}

impl<'a, 'tcx> MutVisitor<'tcx> for NllVisitor<'a, 'tcx> {
    fn tcx(&self) -> TyCtxt<'tcx> {
        self.infcx.tcx
    }

    #[instrument(skip(self), level = "debug")]
    fn visit_ty(&mut self, ty: &mut Ty<'tcx>, ty_context: TyContext) {
        *ty = self.renumber_regions(*ty);

        debug!(?ty);
    }

    #[instrument(skip(self), level = "debug")]
    fn visit_substs(&mut self, substs: &mut SubstsRef<'tcx>, location: Location) {
        *substs = self.renumber_regions(*substs);

        debug!(?substs);
    }

    #[instrument(skip(self), level = "debug")]
    fn visit_region(&mut self, region: &mut ty::Region<'tcx>, location: Location) {
        let old_region = *region;
        *region = self.renumber_regions(old_region);

        debug!(?region);
    }

    #[instrument(skip(self), level = "debug")]
    fn visit_constant(&mut self, constant: &mut Constant<'tcx>, _location: Location) {
        let literal = constant.literal;
        constant.literal = self.renumber_regions_in_mir_constant(literal);
        debug!("constant: {:#?}", constant);
    }
}