use crate::error::ConstNotUsedTraitAlias;
use crate::ty::fold::{TypeFolder, TypeSuperFoldable};
use crate::ty::subst::{GenericArg, GenericArgKind};
use crate::ty::{self, Ty, TyCtxt, TypeFoldable};
use rustc_data_structures::fx::FxHashMap;
use rustc_span::Span;

/// Converts generic params of a TypeFoldable from one
/// item's generics to another. Usually from a function's generics
/// list to the opaque type's own generics.
pub(super) struct ReverseMapper<'tcx> {
    tcx: TyCtxt<'tcx>,
    map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
    /// see call sites to fold_kind_no_missing_regions_error
    /// for an explanation of this field.
    do_not_error: bool,

    /// We do not want to emit any errors in typeck because
    /// the spans in typeck are subpar at the moment.
    /// Borrowck will do the same work again (this time with
    /// lifetime information) and thus report better errors.
    ignore_errors: bool,

    /// Span of function being checked.
    span: Span,
}

impl<'tcx> ReverseMapper<'tcx> {
    pub(super) fn new(
        tcx: TyCtxt<'tcx>,
        map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
        span: Span,
        ignore_errors: bool,
    ) -> Self {
        Self { tcx, map, do_not_error: false, ignore_errors, span }
    }

    fn fold_kind_no_missing_regions_error(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
        assert!(!self.do_not_error);
        self.do_not_error = true;
        let kind = kind.fold_with(self);
        self.do_not_error = false;
        kind
    }

    fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
        assert!(!self.do_not_error);
        kind.fold_with(self)
    }
}

impl<'tcx> TypeFolder<'tcx> for ReverseMapper<'tcx> {
    fn tcx(&self) -> TyCtxt<'tcx> {
        self.tcx
    }

    #[instrument(skip(self), level = "debug")]
    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
        match *r {
            // Ignore bound regions and `'static` regions that appear in the
            // type, we only need to remap regions that reference lifetimes
            // from the function declaration.
            // This would ignore `'r` in a type like `for<'r> fn(&'r u32)`.
            ty::ReLateBound(..) | ty::ReStatic => return r,

            // If regions have been erased (by writeback), don't try to unerase
            // them.
            ty::ReErased => return r,

            // The regions that we expect from borrow checking.
            ty::ReEarlyBound(_) | ty::ReFree(_) => {}

            ty::RePlaceholder(_) | ty::ReVar(_) => {
                // All of the regions in the type should either have been
                // erased by writeback, or mapped back to named regions by
                // borrow checking.
                bug!("unexpected region kind in opaque type: {:?}", r);
            }
        }

        match self.map.get(&r.into()).map(|k| k.unpack()) {
            Some(GenericArgKind::Lifetime(r1)) => r1,
            Some(u) => panic!("region mapped to unexpected kind: {:?}", u),
            None if self.do_not_error => self.tcx.lifetimes.re_static,
            None => {
                self.tcx
                    .sess
                    .struct_span_err(self.span, "non-defining opaque type use in defining scope")
                    .span_label(
                        self.span,
                        format!(
                            "lifetime `{}` is part of concrete type but not used in \
                                 parameter list of the `impl Trait` type alias",
                            r
                        ),
                    )
                    .emit();

                self.tcx().lifetimes.re_static
            }
        }
    }

    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
        match *ty.kind() {
            ty::Closure(def_id, substs) => {
                // I am a horrible monster and I pray for death. When
                // we encounter a closure here, it is always a closure
                // from within the function that we are currently
                // type-checking -- one that is now being encapsulated
                // in an opaque type. Ideally, we would
                // go through the types/lifetimes that it references
                // and treat them just like we would any other type,
                // which means we would error out if we find any
                // reference to a type/region that is not in the
                // "reverse map".
                //
                // **However,** in the case of closures, there is a
                // somewhat subtle (read: hacky) consideration. The
                // problem is that our closure types currently include
                // all the lifetime parameters declared on the
                // enclosing function, even if they are unused by the
                // closure itself. We can't readily filter them out,
                // so here we replace those values with `'empty`. This
                // can't really make a difference to the rest of the
                // compiler; those regions are ignored for the
                // outlives relation, and hence don't affect trait
                // selection or auto traits, and they are erased
                // during codegen.

                let generics = self.tcx.generics_of(def_id);
                let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
                    if index < generics.parent_count {
                        // Accommodate missing regions in the parent kinds...
                        self.fold_kind_no_missing_regions_error(kind)
                    } else {
                        // ...but not elsewhere.
                        self.fold_kind_normally(kind)
                    }
                }));

                self.tcx.mk_closure(def_id, substs)
            }

            ty::Generator(def_id, substs, movability) => {
                let generics = self.tcx.generics_of(def_id);
                let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
                    if index < generics.parent_count {
                        // Accommodate missing regions in the parent kinds...
                        self.fold_kind_no_missing_regions_error(kind)
                    } else {
                        // ...but not elsewhere.
                        self.fold_kind_normally(kind)
                    }
                }));

                self.tcx.mk_generator(def_id, substs, movability)
            }

            ty::Param(param) => {
                // Look it up in the substitution list.
                match self.map.get(&ty.into()).map(|k| k.unpack()) {
                    // Found it in the substitution list; replace with the parameter from the
                    // opaque type.
                    Some(GenericArgKind::Type(t1)) => t1,
                    Some(u) => panic!("type mapped to unexpected kind: {:?}", u),
                    None => {
                        debug!(?param, ?self.map);
                        if !self.ignore_errors {
                            self.tcx
                                .sess
                                .struct_span_err(
                                    self.span,
                                    &format!(
                                        "type parameter `{}` is part of concrete type but not \
                                          used in parameter list for the `impl Trait` type alias",
                                        ty
                                    ),
                                )
                                .emit();
                        }

                        self.tcx().ty_error()
                    }
                }
            }

            _ => ty.super_fold_with(self),
        }
    }

    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
        trace!("checking const {:?}", ct);
        // Find a const parameter
        match ct.kind() {
            ty::ConstKind::Param(..) => {
                // Look it up in the substitution list.
                match self.map.get(&ct.into()).map(|k| k.unpack()) {
                    // Found it in the substitution list, replace with the parameter from the
                    // opaque type.
                    Some(GenericArgKind::Const(c1)) => c1,
                    Some(u) => panic!("const mapped to unexpected kind: {:?}", u),
                    None => {
                        if !self.ignore_errors {
                            self.tcx.sess.emit_err(ConstNotUsedTraitAlias {
                                ct: ct.to_string(),
                                span: self.span,
                            });
                        }

                        self.tcx().const_error(ct.ty())
                    }
                }
            }

            _ => ct,
        }
    }
}