use hir_def::{
    expr::{Pat, PatId, RecordFieldPat},
    find_path::find_path,
    item_scope::ItemInNs,
    path::Path,
    type_ref::Mutability,
    AttrDefId, EnumVariantId, HasModule, VariantId,
};

use smallvec::{smallvec, SmallVec};

use crate::{AdtId, Interner, Scalar, Ty, TyExt, TyKind};

use super::usefulness::{MatchCheckCtx, PatCtxt};

use self::Constructor::*;

#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub(super) enum ToDo {}

#[derive(Clone, Debug, PartialEq, Eq)]
pub(super) struct IntRange {
    range: ToDo,
}

impl IntRange {
    #[inline]
    fn is_integral(ty: &Ty) -> bool {
        match ty.kind(&Interner) {
            TyKind::Scalar(Scalar::Char)
            | TyKind::Scalar(Scalar::Int(_))
            | TyKind::Scalar(Scalar::Uint(_))
            | TyKind::Scalar(Scalar::Bool) => true,
            _ => false,
        }
    }

    fn is_singleton(&self) -> bool {
        todo!()
    }

    /// See `Constructor::is_covered_by`
    fn is_covered_by(&self, other: &Self) -> bool {
        todo!()
    }
}

/// A constructor for array and slice patterns.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub(super) struct Slice {
    todo: ToDo,
}

impl Slice {
    /// See `Constructor::is_covered_by`
    fn is_covered_by(self, other: Self) -> bool {
        todo!()
    }
}

/// A value can be decomposed into a constructor applied to some fields. This struct represents
/// the constructor. See also `Fields`.
///
/// `pat_constructor` retrieves the constructor corresponding to a pattern.
/// `specialize_constructor` returns the list of fields corresponding to a pattern, given a
/// constructor. `Constructor::apply` reconstructs the pattern from a pair of `Constructor` and
/// `Fields`.
#[derive(Clone, Debug, PartialEq)]
pub(super) enum Constructor {
    /// The constructor for patterns that have a single constructor, like tuples, struct patterns
    /// and fixed-length arrays.
    Single,
    /// Enum variants.
    Variant(EnumVariantId),
    /// Ranges of integer literal values (`2`, `2..=5` or `2..5`).
    IntRange(IntRange),
    /// Ranges of floating-point literal values (`2.0..=5.2`).
    FloatRange(ToDo),
    /// String literals. Strings are not quite the same as `&[u8]` so we treat them separately.
    Str(ToDo),
    /// Array and slice patterns.
    Slice(Slice),
    /// Constants that must not be matched structurally. They are treated as black
    /// boxes for the purposes of exhaustiveness: we must not inspect them, and they
    /// don't count towards making a match exhaustive.
    Opaque,
    /// Fake extra constructor for enums that aren't allowed to be matched exhaustively. Also used
    /// for those types for which we cannot list constructors explicitly, like `f64` and `str`.
    NonExhaustive,
    /// Stands for constructors that are not seen in the matrix, as explained in the documentation
    /// for [`SplitWildcard`].
    Missing,
    /// Wildcard pattern.
    Wildcard,
}

impl Constructor {
    pub(super) fn is_wildcard(&self) -> bool {
        matches!(self, Wildcard)
    }

    fn as_int_range(&self) -> Option<&IntRange> {
        match self {
            IntRange(range) => Some(range),
            _ => None,
        }
    }

    fn as_slice(&self) -> Option<Slice> {
        match self {
            Slice(slice) => Some(*slice),
            _ => None,
        }
    }

    fn variant_id_for_adt(&self, adt: hir_def::AdtId, cx: &MatchCheckCtx<'_>) -> VariantId {
        match *self {
            Variant(id) => id.into(),
            Single => {
                assert!(!matches!(adt, hir_def::AdtId::EnumId(_)));
                match adt {
                    hir_def::AdtId::EnumId(_) => unreachable!(),
                    hir_def::AdtId::StructId(id) => id.into(),
                    hir_def::AdtId::UnionId(id) => id.into(),
                }
            }
            _ => panic!("bad constructor {:?} for adt {:?}", self, adt),
        }
    }

    /// Determines the constructor that the given pattern can be specialized to.
    pub(super) fn from_pat(cx: &MatchCheckCtx<'_>, pat: PatId) -> Self {
        let ty = cx.type_of(pat);
        match &cx.pattern_arena.borrow()[pat] {
            Pat::Bind { .. } | Pat::Wild => Wildcard,
            Pat::Tuple { .. } | Pat::Ref { .. } | Pat::Box { .. } => Single,
            Pat::Record { .. } | Pat::Path(_) | Pat::TupleStruct { .. } => {
                let variant_id =
                    cx.infer.variant_resolution_for_pat(pat).unwrap_or_else(|| todo!());
                match variant_id {
                    VariantId::EnumVariantId(id) => Variant(id),
                    VariantId::StructId(_) | VariantId::UnionId(_) => Single,
                }
            }

            Pat::Or(..) => panic!("bug: Or-pattern should have been expanded earlier on."),
            pat => todo!("Constructor::from_pat {:?}", pat),
            // Pat::Missing => {}
            // Pat::Range { start, end } => {}
            // Pat::Slice { prefix, slice, suffix } => {}
            // Pat::Lit(_) => {}
            // Pat::ConstBlock(_) => {}
        }
    }

    /// Some constructors (namely `Wildcard`, `IntRange` and `Slice`) actually stand for a set of actual
    /// constructors (like variants, integers or fixed-sized slices). When specializing for these
    /// constructors, we want to be specialising for the actual underlying constructors.
    /// Naively, we would simply return the list of constructors they correspond to. We instead are
    /// more clever: if there are constructors that we know will behave the same wrt the current
    /// matrix, we keep them grouped. For example, all slices of a sufficiently large length
    /// will either be all useful or all non-useful with a given matrix.
    ///
    /// See the branches for details on how the splitting is done.
    ///
    /// This function may discard some irrelevant constructors if this preserves behavior and
    /// diagnostics. Eg. for the `_` case, we ignore the constructors already present in the
    /// matrix, unless all of them are.
    pub(super) fn split<'a>(
        &self,
        pcx: PatCtxt<'_>,
        ctors: impl Iterator<Item = &'a Constructor> + Clone,
    ) -> SmallVec<[Self; 1]> {
        match self {
            Wildcard => {
                let mut split_wildcard = SplitWildcard::new(pcx);
                split_wildcard.split(pcx, ctors);
                split_wildcard.into_ctors(pcx)
            }
            // Fast-track if the range is trivial. In particular, we don't do the overlapping
            // ranges check.
            IntRange(ctor_range) if !ctor_range.is_singleton() => {
                todo!("Constructor::split IntRange")
            }
            Slice(_) => todo!("Constructor::split Slice"),
            // Any other constructor can be used unchanged.
            _ => smallvec![self.clone()],
        }
    }

    /// Returns whether `self` is covered by `other`, i.e. whether `self` is a subset of `other`.
    /// For the simple cases, this is simply checking for equality. For the "grouped" constructors,
    /// this checks for inclusion.
    // We inline because this has a single call site in `Matrix::specialize_constructor`.
    #[inline]
    pub(super) fn is_covered_by(&self, pcx: PatCtxt<'_>, other: &Self) -> bool {
        // This must be kept in sync with `is_covered_by_any`.
        match (self, other) {
            // Wildcards cover anything
            (_, Wildcard) => true,
            // The missing ctors are not covered by anything in the matrix except wildcards.
            (Missing, _) | (Wildcard, _) => false,

            (Single, Single) => true,
            (Variant(self_id), Variant(other_id)) => self_id == other_id,

            (IntRange(self_range), IntRange(other_range)) => self_range.is_covered_by(other_range),
            (FloatRange(..), FloatRange(..)) => {
                todo!()
            }
            (Str(self_val), Str(other_val)) => {
                todo!()
            }
            (Slice(self_slice), Slice(other_slice)) => self_slice.is_covered_by(*other_slice),

            // We are trying to inspect an opaque constant. Thus we skip the row.
            (Opaque, _) | (_, Opaque) => false,
            // Only a wildcard pattern can match the special extra constructor.
            (NonExhaustive, _) => false,

            _ => panic!(
                "bug: trying to compare incompatible constructors {:?} and {:?}",
                self, other
            ),
        }
    }

    /// Faster version of `is_covered_by` when applied to many constructors. `used_ctors` is
    /// assumed to be built from `matrix.head_ctors()` with wildcards filtered out, and `self` is
    /// assumed to have been split from a wildcard.
    fn is_covered_by_any(&self, pcx: PatCtxt<'_>, used_ctors: &[Constructor]) -> bool {
        if used_ctors.is_empty() {
            return false;
        }

        // This must be kept in sync with `is_covered_by`.
        match self {
            // If `self` is `Single`, `used_ctors` cannot contain anything else than `Single`s.
            Single => !used_ctors.is_empty(),
            Variant(_) => used_ctors.iter().any(|c| c == self),
            IntRange(range) => used_ctors
                .iter()
                .filter_map(|c| c.as_int_range())
                .any(|other| range.is_covered_by(other)),
            Slice(slice) => used_ctors
                .iter()
                .filter_map(|c| c.as_slice())
                .any(|other| slice.is_covered_by(other)),
            // This constructor is never covered by anything else
            NonExhaustive => false,
            Str(..) | FloatRange(..) | Opaque | Missing | Wildcard => {
                panic!("bug: found unexpected ctor in all_ctors: {:?}", self)
            }
        }
    }
}

/// A wildcard constructor that we split relative to the constructors in the matrix, as explained
/// at the top of the file.
///
/// A constructor that is not present in the matrix rows will only be covered by the rows that have
/// wildcards. Thus we can group all of those constructors together; we call them "missing
/// constructors". Splitting a wildcard would therefore list all present constructors individually
/// (or grouped if they are integers or slices), and then all missing constructors together as a
/// group.
///
/// However we can go further: since any constructor will match the wildcard rows, and having more
/// rows can only reduce the amount of usefulness witnesses, we can skip the present constructors
/// and only try the missing ones.
/// This will not preserve the whole list of witnesses, but will preserve whether the list is empty
/// or not. In fact this is quite natural from the point of view of diagnostics too. This is done
/// in `to_ctors`: in some cases we only return `Missing`.
#[derive(Debug)]
pub(super) struct SplitWildcard {
    /// Constructors seen in the matrix.
    matrix_ctors: Vec<Constructor>,
    /// All the constructors for this type
    all_ctors: SmallVec<[Constructor; 1]>,
}

impl SplitWildcard {
    pub(super) fn new(pcx: PatCtxt<'_>) -> Self {
        // let cx = pcx.cx;
        // let make_range = |start, end| IntRange(todo!());

        // This determines the set of all possible constructors for the type `pcx.ty`. For numbers,
        // arrays and slices we use ranges and variable-length slices when appropriate.
        //
        // If the `exhaustive_patterns` feature is enabled, we make sure to omit constructors that
        // are statically impossible. E.g., for `Option<!>`, we do not include `Some(_)` in the
        // returned list of constructors.
        // Invariant: this is empty if and only if the type is uninhabited (as determined by
        // `cx.is_uninhabited()`).
        let all_ctors = match pcx.ty.kind(&Interner) {
            TyKind::Adt(AdtId(hir_def::AdtId::EnumId(_)), _) => todo!(),
            TyKind::Adt(..) | TyKind::Tuple(..) | TyKind::Ref(..) => smallvec![Single],
            _ => todo!(),
        };
        SplitWildcard { matrix_ctors: Vec::new(), all_ctors }
    }

    /// Pass a set of constructors relative to which to split this one. Don't call twice, it won't
    /// do what you want.
    pub(super) fn split<'a>(
        &mut self,
        pcx: PatCtxt<'_>,
        ctors: impl Iterator<Item = &'a Constructor> + Clone,
    ) {
        // Since `all_ctors` never contains wildcards, this won't recurse further.
        self.all_ctors =
            self.all_ctors.iter().flat_map(|ctor| ctor.split(pcx, ctors.clone())).collect();
        self.matrix_ctors = ctors.filter(|c| !c.is_wildcard()).cloned().collect();
    }

    /// Whether there are any value constructors for this type that are not present in the matrix.
    fn any_missing(&self, pcx: PatCtxt<'_>) -> bool {
        self.iter_missing(pcx).next().is_some()
    }

    /// Iterate over the constructors for this type that are not present in the matrix.
    pub(super) fn iter_missing<'a>(
        &'a self,
        pcx: PatCtxt<'a>,
    ) -> impl Iterator<Item = &'a Constructor> {
        self.all_ctors.iter().filter(move |ctor| !ctor.is_covered_by_any(pcx, &self.matrix_ctors))
    }

    /// Return the set of constructors resulting from splitting the wildcard. As explained at the
    /// top of the file, if any constructors are missing we can ignore the present ones.
    fn into_ctors(self, pcx: PatCtxt<'_>) -> SmallVec<[Constructor; 1]> {
        if self.any_missing(pcx) {
            // Some constructors are missing, thus we can specialize with the special `Missing`
            // constructor, which stands for those constructors that are not seen in the matrix,
            // and matches the same rows as any of them (namely the wildcard rows). See the top of
            // the file for details.
            // However, when all constructors are missing we can also specialize with the full
            // `Wildcard` constructor. The difference will depend on what we want in diagnostics.

            // If some constructors are missing, we typically want to report those constructors,
            // e.g.:
            // ```
            //     enum Direction { N, S, E, W }
            //     let Direction::N = ...;
            // ```
            // we can report 3 witnesses: `S`, `E`, and `W`.
            //
            // However, if the user didn't actually specify a constructor
            // in this arm, e.g., in
            // ```
            //     let x: (Direction, Direction, bool) = ...;
            //     let (_, _, false) = x;
            // ```
            // we don't want to show all 16 possible witnesses `(<direction-1>, <direction-2>,
            // true)` - we are satisfied with `(_, _, true)`. So if all constructors are missing we
            // prefer to report just a wildcard `_`.
            //
            // The exception is: if we are at the top-level, for example in an empty match, we
            // sometimes prefer reporting the list of constructors instead of just `_`.
            let report_when_all_missing = pcx.is_top_level && !IntRange::is_integral(pcx.ty);
            let ctor = if !self.matrix_ctors.is_empty() || report_when_all_missing {
                Missing
            } else {
                Wildcard
            };
            return smallvec![ctor];
        }

        // All the constructors are present in the matrix, so we just go through them all.
        self.all_ctors
    }
}

#[test]
fn it_works2() {}

/// Some fields need to be explicitly hidden away in certain cases; see the comment above the
/// `Fields` struct. This struct represents such a potentially-hidden field.
#[derive(Debug, Copy, Clone)]
pub(super) enum FilteredField {
    Kept(PatId),
    Hidden,
}

impl FilteredField {
    fn kept(self) -> Option<PatId> {
        match self {
            FilteredField::Kept(p) => Some(p),
            FilteredField::Hidden => None,
        }
    }
}

/// A value can be decomposed into a constructor applied to some fields. This struct represents
/// those fields, generalized to allow patterns in each field. See also `Constructor`.
/// This is constructed from a constructor using [`Fields::wildcards()`].
///
/// If a private or `non_exhaustive` field is uninhabited, the code mustn't observe that it is
/// uninhabited. For that, we filter these fields out of the matrix. This is handled automatically
/// in `Fields`. This filtering is uncommon in practice, because uninhabited fields are rarely used,
/// so we avoid it when possible to preserve performance.
#[derive(Debug, Clone)]
pub(super) enum Fields {
    /// Lists of patterns that don't contain any filtered fields.
    /// `Slice` and `Vec` behave the same; the difference is only to avoid allocating and
    /// triple-dereferences when possible. Frankly this is premature optimization, I (Nadrieril)
    /// have not measured if it really made a difference.
    Vec(SmallVec<[PatId; 2]>),
}

impl Fields {
    /// Internal use. Use `Fields::wildcards()` instead.
    /// Must not be used if the pattern is a field of a struct/tuple/variant.
    fn from_single_pattern(pat: PatId) -> Self {
        Fields::Vec(smallvec![pat])
    }

    /// Convenience; internal use.
    fn wildcards_from_tys<'a>(
        cx: &MatchCheckCtx<'_>,
        tys: impl IntoIterator<Item = &'a Ty>,
    ) -> Self {
        let wilds = tys.into_iter().map(|ty| (Pat::Wild, ty));
        let pats = wilds.map(|(pat, ty)| cx.alloc_pat(pat, ty)).collect();
        Fields::Vec(pats)
    }

    pub(crate) fn wildcards(pcx: PatCtxt<'_>, constructor: &Constructor) -> Self {
        let ty = pcx.ty;
        let cx = pcx.cx;
        let wildcard_from_ty = |ty| cx.alloc_pat(Pat::Wild, ty);

        let ret = match constructor {
            Single | Variant(_) => match ty.kind(&Interner) {
                TyKind::Tuple(_, substs) => {
                    let tys = substs.iter(&Interner).map(|ty| ty.assert_ty_ref(&Interner));
                    Fields::wildcards_from_tys(cx, tys)
                }
                TyKind::Ref(.., rty) => Fields::from_single_pattern(wildcard_from_ty(rty)),
                TyKind::Adt(AdtId(adt), substs) => {
                    let adt_is_box = false; // TODO(iDawer): handle box patterns
                    if adt_is_box {
                        // Use T as the sub pattern type of Box<T>.
                        let ty = substs.at(&Interner, 0).assert_ty_ref(&Interner);
                        Fields::from_single_pattern(wildcard_from_ty(ty))
                    } else {
                        let variant_id = constructor.variant_id_for_adt(*adt, cx);
                        let variant = variant_id.variant_data(cx.db.upcast());
                        let adt_is_local =
                            variant_id.module(cx.db.upcast()).krate() == cx.module.krate();
                        // Whether we must not match the fields of this variant exhaustively.
                        let is_non_exhaustive =
                            is_field_list_non_exhaustive(variant_id, cx) && !adt_is_local;
                        let field_ty_arena = cx.db.field_types(variant_id);
                        let field_tys =
                            || field_ty_arena.iter().map(|(_, binders)| binders.skip_binders());
                        // In the following cases, we don't need to filter out any fields. This is
                        // the vast majority of real cases, since uninhabited fields are uncommon.
                        let has_no_hidden_fields = (matches!(adt, hir_def::AdtId::EnumId(_))
                            && !is_non_exhaustive)
                            || !field_tys().any(|ty| cx.is_uninhabited(ty));

                        if has_no_hidden_fields {
                            Fields::wildcards_from_tys(cx, field_tys())
                        } else {
                            //FIXME(iDawer): see MatchCheckCtx::is_uninhabited
                            unimplemented!("exhaustive_patterns feature")
                        }
                    }
                }
                _ => panic!("Unexpected type for `Single` constructor: {:?}", ty),
            },
            Slice(slice) => {
                todo!()
            }
            Str(..) | FloatRange(..) | IntRange(..) | NonExhaustive | Opaque | Missing
            | Wildcard => Fields::Vec(Default::default()),
        };
        ret
    }

    /// Apply a constructor to a list of patterns, yielding a new pattern. `self`
    /// must have as many elements as this constructor's arity.
    ///
    /// This is roughly the inverse of `specialize_constructor`.
    ///
    /// Examples:
    /// `ctor`: `Constructor::Single`
    /// `ty`: `Foo(u32, u32, u32)`
    /// `self`: `[10, 20, _]`
    /// returns `Foo(10, 20, _)`
    ///
    /// `ctor`: `Constructor::Variant(Option::Some)`
    /// `ty`: `Option<bool>`
    /// `self`: `[false]`
    /// returns `Some(false)`
    pub(super) fn apply(self, pcx: PatCtxt<'_>, ctor: &Constructor) -> Pat {
        let subpatterns_and_indices = self.patterns_and_indices();
        let mut subpatterns = subpatterns_and_indices.iter().map(|&(_, p)| p);
        // TODO witnesses are not yet used 
        const TODO: Pat = Pat::Wild;

        match ctor {
            Single | Variant(_) => match pcx.ty.kind(&Interner) {
                TyKind::Adt(..) | TyKind::Tuple(..) => {
                    // We want the real indices here.
                    // TODO indices and ellipsis interaction, tests
                    let subpatterns = subpatterns_and_indices.iter().map(|&(_, pat)| pat).collect();

                    if let Some((adt, substs)) = pcx.ty.as_adt() {
                        let item = ItemInNs::Types(adt.into());
                        let path = find_path(pcx.cx.db.upcast(), item, pcx.cx.module)
                            .map(|mpath| Path::from_known_path(mpath, Vec::new()).into());
                        match adt {
                            hir_def::AdtId::EnumId(id) => TODO,
                            hir_def::AdtId::StructId(id) => {
                                let variant_data = &pcx.cx.db.struct_data(id).variant_data;
                                let args = subpatterns_and_indices
                                    .iter()
                                    .zip(variant_data.fields().iter())
                                    .map(|(&(_, pat), (_, field_data))| RecordFieldPat {
                                        name: field_data.name.clone(),
                                        pat,
                                    })
                                    .collect();
                                Pat::Record { path, args, ellipsis: false }
                            }
                            hir_def::AdtId::UnionId(_) => Pat::Wild,
                        }
                    } else {
                        Pat::Tuple { args: subpatterns, ellipsis: None }
                    }
                }
                // Note: given the expansion of `&str` patterns done in `expand_pattern`, we should
                // be careful to reconstruct the correct constant pattern here. However a string
                // literal pattern will never be reported as a non-exhaustiveness witness, so we
                // can ignore this issue.
                TyKind::Ref(..) => {
                    Pat::Ref { pat: subpatterns.next().unwrap(), mutability: Mutability::Shared }
                }
                TyKind::Slice(..) | TyKind::Array(..) => {
                    panic!("bug: bad slice pattern {:?} {:?}", ctor, pcx.ty)
                }
                _ => Pat::Wild,
            },
            Constructor::Slice(slice) => TODO,
            Str(_) => TODO,
            FloatRange(..) => TODO,
            Constructor::IntRange(_) => TODO,
            NonExhaustive => Pat::Wild,
            Wildcard => Pat::Wild,
            Opaque => panic!("bug: we should not try to apply an opaque constructor"),
            Missing => {
                panic!("bug: trying to apply the `Missing` constructor; this should have been done in `apply_constructors`")
            }
        }
    }

    /// Returns the number of patterns. This is the same as the arity of the constructor used to
    /// construct `self`.
    pub(super) fn len(&self) -> usize {
        match self {
            Fields::Vec(pats) => pats.len(),
        }
    }

    /// Returns the list of patterns along with the corresponding field indices.
    fn patterns_and_indices(&self) -> SmallVec<[(usize, PatId); 2]> {
        match self {
            Fields::Vec(pats) => pats.iter().copied().enumerate().collect(),
        }
    }

    pub(super) fn into_patterns(self) -> SmallVec<[PatId; 2]> {
        match self {
            Fields::Vec(pats) => pats,
        }
    }

    /// Overrides some of the fields with the provided patterns. Exactly like
    /// `replace_fields_indexed`, except that it takes `FieldPat`s as input.
    fn replace_with_fieldpats(&self, new_pats: impl IntoIterator<Item = PatId>) -> Self {
        self.replace_fields_indexed(new_pats.into_iter().enumerate())
    }

    /// Overrides some of the fields with the provided patterns. This is used when a pattern
    /// defines some fields but not all, for example `Foo { field1: Some(_), .. }`: here we start
    /// with a `Fields` that is just one wildcard per field of the `Foo` struct, and override the
    /// entry corresponding to `field1` with the pattern `Some(_)`. This is also used for slice
    /// patterns for the same reason.
    fn replace_fields_indexed(&self, new_pats: impl IntoIterator<Item = (usize, PatId)>) -> Self {
        let mut fields = self.clone();

        match &mut fields {
            Fields::Vec(pats) => {
                for (i, pat) in new_pats {
                    if let Some(p) = pats.get_mut(i) {
                        *p = pat;
                    }
                }
            }
        }
        fields
    }

    /// Replaces contained fields with the given list of patterns. There must be `len()` patterns
    /// in `pats`.
    pub(super) fn replace_fields(
        &self,
        cx: &MatchCheckCtx<'_>,
        pats: impl IntoIterator<Item = Pat>,
    ) -> Self {
        let pats = {
            let mut arena = cx.pattern_arena.borrow_mut();
            pats.into_iter().map(move |pat| /* arena.alloc(pat) */ todo!()).collect()
        };

        match self {
            Fields::Vec(_) => Fields::Vec(pats),
        }
    }

    /// Replaces contained fields with the arguments of the given pattern. Only use on a pattern
    /// that is compatible with the constructor used to build `self`.
    /// This is meant to be used on the result of `Fields::wildcards()`. The idea is that
    /// `wildcards` constructs a list of fields where all entries are wildcards, and the pattern
    /// provided to this function fills some of the fields with non-wildcards.
    /// In the following example `Fields::wildcards` would return `[_, _, _, _]`. If we call
    /// `replace_with_pattern_arguments` on it with the pattern, the result will be `[Some(0), _,
    /// _, _]`.
    /// ```rust
    /// let x: [Option<u8>; 4] = foo();
    /// match x {
    ///     [Some(0), ..] => {}
    /// }
    /// ```
    /// This is guaranteed to preserve the number of patterns in `self`.
    pub(super) fn replace_with_pattern_arguments(
        &self,
        pat: PatId,
        cx: &MatchCheckCtx<'_>,
    ) -> Self {
        match &cx.pattern_arena.borrow()[pat] {
            Pat::Ref { pat: subpattern, .. } | Pat::Box { inner: subpattern } => {
                assert_eq!(self.len(), 1);
                Fields::from_single_pattern(*subpattern)
            }
            Pat::Tuple { args, ellipsis } | Pat::TupleStruct { args, ellipsis, .. } => {
                // FIXME(iDawer) handle ellipsis.
                // XXX(iDawer): in rustc, this is handled by HIR->TypedHIR lowering
                // rustc_mir_build::thir::pattern::PatCtxt::lower_tuple_subpats(..)
                self.replace_with_fieldpats(args.iter().copied())
            }
            Pat::Record { args, ellipsis, .. } => {
                // FIXME(iDawer) handle ellipsis.
                self.replace_with_fieldpats(args.iter().map(|field_pat| field_pat.pat))
            }
            Pat::Slice { .. } => {
                todo!()
            }
            Pat::Missing
            | Pat::Wild
            | Pat::Or(_)
            | Pat::Range { .. }
            | Pat::Path(_)
            | Pat::Lit(_)
            | Pat::Bind { .. }
            | Pat::ConstBlock(_) => self.clone(),
        }
    }
}

fn is_field_list_non_exhaustive(variant_id: VariantId, cx: &MatchCheckCtx<'_>) -> bool {
    let attr_def_id = match variant_id {
        VariantId::EnumVariantId(id) => id.into(),
        VariantId::StructId(id) => id.into(),
        VariantId::UnionId(id) => id.into(),
    };
    cx.db.attrs(attr_def_id).by_key("non_exhaustive").exists()
}

#[test]
fn it_works() {}