//! Validation of patterns/matches.

mod check_match;
mod const_to_pat;
pub(crate) mod deconstruct_pat;
mod usefulness;

pub(crate) use self::check_match::check_match;
pub(crate) use self::usefulness::MatchCheckCtxt;

use crate::errors::*;
use crate::thir::util::UserAnnotatedTyHelpers;

use rustc_errors::error_code;
use rustc_hir as hir;
use rustc_hir::def::{CtorOf, DefKind, Res};
use rustc_hir::pat_util::EnumerateAndAdjustIterator;
use rustc_hir::RangeEnd;
use rustc_index::vec::Idx;
use rustc_middle::mir::interpret::{
    ConstValue, ErrorHandled, LitToConstError, LitToConstInput, Scalar,
};
use rustc_middle::mir::{self, UserTypeProjection};
use rustc_middle::mir::{BorrowKind, Field, Mutability};
use rustc_middle::thir::{Ascription, BindingMode, FieldPat, LocalVarId, Pat, PatKind, PatRange};
use rustc_middle::ty::subst::{GenericArg, SubstsRef};
use rustc_middle::ty::CanonicalUserTypeAnnotation;
use rustc_middle::ty::{self, AdtDef, ConstKind, DefIdTree, Region, Ty, TyCtxt, UserType};
use rustc_span::{Span, Symbol};

use std::cmp::Ordering;

#[derive(Clone, Debug)]
enum PatternError {
    AssocConstInPattern(Span),
    ConstParamInPattern(Span),
    StaticInPattern(Span),
    NonConstPath(Span),
}

struct PatCtxt<'a, 'tcx> {
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    typeck_results: &'a ty::TypeckResults<'tcx>,
    errors: Vec<PatternError>,
    include_lint_checks: bool,
}

pub(super) fn pat_from_hir<'a, 'tcx>(
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    typeck_results: &'a ty::TypeckResults<'tcx>,
    pat: &'tcx hir::Pat<'tcx>,
) -> Box<Pat<'tcx>> {
    let mut pcx = PatCtxt::new(tcx, param_env, typeck_results);
    let result = pcx.lower_pattern(pat);
    if !pcx.errors.is_empty() {
        let msg = format!("encountered errors lowering pattern: {:?}", pcx.errors);
        tcx.sess.delay_span_bug(pat.span, &msg);
    }
    debug!("pat_from_hir({:?}) = {:?}", pat, result);
    result
}

impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
    fn new(
        tcx: TyCtxt<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        typeck_results: &'a ty::TypeckResults<'tcx>,
    ) -> Self {
        PatCtxt { tcx, param_env, typeck_results, errors: vec![], include_lint_checks: false }
    }

    fn include_lint_checks(&mut self) -> &mut Self {
        self.include_lint_checks = true;
        self
    }

    fn lower_pattern(&mut self, pat: &'tcx hir::Pat<'tcx>) -> Box<Pat<'tcx>> {
        // When implicit dereferences have been inserted in this pattern, the unadjusted lowered
        // pattern has the type that results *after* dereferencing. For example, in this code:
        //
        // ```
        // match &&Some(0i32) {
        //     Some(n) => { ... },
        //     _ => { ... },
        // }
        // ```
        //
        // the type assigned to `Some(n)` in `unadjusted_pat` would be `Option<i32>` (this is
        // determined in rustc_hir_analysis::check::match). The adjustments would be
        //
        // `vec![&&Option<i32>, &Option<i32>]`.
        //
        // Applying the adjustments, we want to instead output `&&Some(n)` (as a THIR pattern). So
        // we wrap the unadjusted pattern in `PatKind::Deref` repeatedly, consuming the
        // adjustments in *reverse order* (last-in-first-out, so that the last `Deref` inserted
        // gets the least-dereferenced type).
        let unadjusted_pat = self.lower_pattern_unadjusted(pat);
        self.typeck_results.pat_adjustments().get(pat.hir_id).unwrap_or(&vec![]).iter().rev().fold(
            unadjusted_pat,
            |pat: Box<_>, ref_ty| {
                debug!("{:?}: wrapping pattern with type {:?}", pat, ref_ty);
                Box::new(Pat {
                    span: pat.span,
                    ty: *ref_ty,
                    kind: PatKind::Deref { subpattern: pat },
                })
            },
        )
    }

    fn lower_range_expr(
        &mut self,
        expr: &'tcx hir::Expr<'tcx>,
    ) -> (PatKind<'tcx>, Option<Ascription<'tcx>>) {
        match self.lower_lit(expr) {
            PatKind::AscribeUserType { ascription, subpattern: box Pat { kind, .. } } => {
                (kind, Some(ascription))
            }
            kind => (kind, None),
        }
    }

    fn lower_pattern_range(
        &mut self,
        ty: Ty<'tcx>,
        lo: mir::ConstantKind<'tcx>,
        hi: mir::ConstantKind<'tcx>,
        end: RangeEnd,
        span: Span,
        lo_expr: Option<&hir::Expr<'tcx>>,
        hi_expr: Option<&hir::Expr<'tcx>>,
    ) -> PatKind<'tcx> {
        assert_eq!(lo.ty(), ty);
        assert_eq!(hi.ty(), ty);
        let cmp = compare_const_vals(self.tcx, lo, hi, self.param_env);
        let max = || {
            self.tcx
                .layout_of(self.param_env.with_reveal_all_normalized(self.tcx).and(ty))
                .ok()
                .unwrap()
                .size
                .unsigned_int_max()
        };
        match (end, cmp) {
            // `x..y` where `x < y`.
            // Non-empty because the range includes at least `x`.
            (RangeEnd::Excluded, Some(Ordering::Less)) => {
                PatKind::Range(Box::new(PatRange { lo, hi, end }))
            }
            // `x..y` where `x >= y`. The range is empty => error.
            (RangeEnd::Excluded, _) => {
                let mut lower_overflow = false;
                let mut higher_overflow = false;
                if let Some(hir::Expr { kind: hir::ExprKind::Lit(lit), .. }) = lo_expr
                    && let rustc_ast::ast::LitKind::Int(val, _) = lit.node
                {
                    if lo.eval_bits(self.tcx, self.param_env, ty) != val {
                        lower_overflow = true;
                        self.tcx.sess.emit_err(LiteralOutOfRange { span: lit.span, ty, max: max() });
                    }
                }
                if let Some(hir::Expr { kind: hir::ExprKind::Lit(lit), .. }) = hi_expr
                    && let rustc_ast::ast::LitKind::Int(val, _) = lit.node
                {
                    if hi.eval_bits(self.tcx, self.param_env, ty) != val {
                        higher_overflow = true;
                        self.tcx.sess.emit_err(LiteralOutOfRange { span: lit.span, ty, max: max() });
                    }
                }
                if !lower_overflow && !higher_overflow {
                    self.tcx.sess.emit_err(LowerRangeBoundMustBeLessThanUpper { span });
                }
                PatKind::Wild
            }
            // `x..=y` where `x == y`.
            (RangeEnd::Included, Some(Ordering::Equal)) => PatKind::Constant { value: lo },
            // `x..=y` where `x < y`.
            (RangeEnd::Included, Some(Ordering::Less)) => {
                PatKind::Range(Box::new(PatRange { lo, hi, end }))
            }
            // `x..=y` where `x > y` hence the range is empty => error.
            (RangeEnd::Included, _) => {
                let mut lower_overflow = false;
                let mut higher_overflow = false;
                if let Some(hir::Expr { kind: hir::ExprKind::Lit(lit), .. }) = lo_expr
                    && let rustc_ast::ast::LitKind::Int(val, _) = lit.node
                {
                    if lo.eval_bits(self.tcx, self.param_env, ty) != val {
                        lower_overflow = true;
                        self.tcx.sess.emit_err(LiteralOutOfRange { span: lit.span, ty, max: max() });
                    }
                }
                if let Some(hir::Expr { kind: hir::ExprKind::Lit(lit), .. }) = hi_expr
                    && let rustc_ast::ast::LitKind::Int(val, _) = lit.node
                {
                    if hi.eval_bits(self.tcx, self.param_env, ty) != val {
                        higher_overflow = true;
                        self.tcx.sess.emit_err(LiteralOutOfRange { span: lit.span, ty, max: max() });
                    }
                }
                if !lower_overflow && !higher_overflow {
                    self.tcx.sess.emit_err(LowerRangeBoundMustBeLessThanOrEqualToUpper {
                        span,
                        teach: self.tcx.sess.teach(&error_code!(E0030)).then_some(()),
                    });
                }
                PatKind::Wild
            }
        }
    }

    fn normalize_range_pattern_ends(
        &self,
        ty: Ty<'tcx>,
        lo: Option<&PatKind<'tcx>>,
        hi: Option<&PatKind<'tcx>>,
    ) -> Option<(mir::ConstantKind<'tcx>, mir::ConstantKind<'tcx>)> {
        match (lo, hi) {
            (Some(PatKind::Constant { value: lo }), Some(PatKind::Constant { value: hi })) => {
                Some((*lo, *hi))
            }
            (Some(PatKind::Constant { value: lo }), None) => {
                let hi = ty.numeric_max_val(self.tcx)?;
                Some((*lo, mir::ConstantKind::from_const(hi, self.tcx)))
            }
            (None, Some(PatKind::Constant { value: hi })) => {
                let lo = ty.numeric_min_val(self.tcx)?;
                Some((mir::ConstantKind::from_const(lo, self.tcx), *hi))
            }
            _ => None,
        }
    }

    #[instrument(skip(self), level = "debug")]
    fn lower_pattern_unadjusted(&mut self, pat: &'tcx hir::Pat<'tcx>) -> Box<Pat<'tcx>> {
        let mut ty = self.typeck_results.node_type(pat.hir_id);
        let mut span = pat.span;

        let kind = match pat.kind {
            hir::PatKind::Wild => PatKind::Wild,

            hir::PatKind::Lit(value) => self.lower_lit(value),

            hir::PatKind::Range(ref lo_expr, ref hi_expr, end) => {
                let (lo_expr, hi_expr) = (lo_expr.as_deref(), hi_expr.as_deref());
                let lo_span = lo_expr.map_or(pat.span, |e| e.span);
                let lo = lo_expr.map(|e| self.lower_range_expr(e));
                let hi = hi_expr.map(|e| self.lower_range_expr(e));

                let (lp, hp) = (lo.as_ref().map(|(x, _)| x), hi.as_ref().map(|(x, _)| x));
                let mut kind = match self.normalize_range_pattern_ends(ty, lp, hp) {
                    Some((lc, hc)) => {
                        self.lower_pattern_range(ty, lc, hc, end, lo_span, lo_expr, hi_expr)
                    }
                    None => {
                        let msg = &format!(
                            "found bad range pattern `{:?}` outside of error recovery",
                            (&lo, &hi),
                        );
                        self.tcx.sess.delay_span_bug(pat.span, msg);
                        PatKind::Wild
                    }
                };

                // If we are handling a range with associated constants (e.g.
                // `Foo::<'a>::A..=Foo::B`), we need to put the ascriptions for the associated
                // constants somewhere. Have them on the range pattern.
                for end in &[lo, hi] {
                    if let Some((_, Some(ascription))) = end {
                        let subpattern = Box::new(Pat { span: pat.span, ty, kind });
                        kind =
                            PatKind::AscribeUserType { ascription: ascription.clone(), subpattern };
                    }
                }

                kind
            }

            hir::PatKind::Path(ref qpath) => {
                return self.lower_path(qpath, pat.hir_id, pat.span);
            }

            hir::PatKind::Ref(ref subpattern, _) | hir::PatKind::Box(ref subpattern) => {
                PatKind::Deref { subpattern: self.lower_pattern(subpattern) }
            }

            hir::PatKind::Slice(ref prefix, ref slice, ref suffix) => {
                self.slice_or_array_pattern(pat.span, ty, prefix, slice, suffix)
            }

            hir::PatKind::Tuple(ref pats, ddpos) => {
                let ty::Tuple(ref tys) = ty.kind() else {
                    span_bug!(pat.span, "unexpected type for tuple pattern: {:?}", ty);
                };
                let subpatterns = self.lower_tuple_subpats(pats, tys.len(), ddpos);
                PatKind::Leaf { subpatterns }
            }

            hir::PatKind::Binding(_, id, ident, ref sub) => {
                if let Some(ident_span) = ident.span.find_ancestor_inside(span) {
                    span = span.with_hi(ident_span.hi());
                }

                let bm = *self
                    .typeck_results
                    .pat_binding_modes()
                    .get(pat.hir_id)
                    .expect("missing binding mode");
                let (mutability, mode) = match bm {
                    ty::BindByValue(mutbl) => (mutbl, BindingMode::ByValue),
                    ty::BindByReference(hir::Mutability::Mut) => (
                        Mutability::Not,
                        BindingMode::ByRef(BorrowKind::Mut { allow_two_phase_borrow: false }),
                    ),
                    ty::BindByReference(hir::Mutability::Not) => {
                        (Mutability::Not, BindingMode::ByRef(BorrowKind::Shared))
                    }
                };

                // A ref x pattern is the same node used for x, and as such it has
                // x's type, which is &T, where we want T (the type being matched).
                let var_ty = ty;
                if let ty::BindByReference(_) = bm {
                    if let ty::Ref(_, rty, _) = ty.kind() {
                        ty = *rty;
                    } else {
                        bug!("`ref {}` has wrong type {}", ident, ty);
                    }
                };

                PatKind::Binding {
                    mutability,
                    mode,
                    name: ident.name,
                    var: LocalVarId(id),
                    ty: var_ty,
                    subpattern: self.lower_opt_pattern(sub),
                    is_primary: id == pat.hir_id,
                }
            }

            hir::PatKind::TupleStruct(ref qpath, ref pats, ddpos) => {
                let res = self.typeck_results.qpath_res(qpath, pat.hir_id);
                let ty::Adt(adt_def, _) = ty.kind() else {
                    span_bug!(pat.span, "tuple struct pattern not applied to an ADT {:?}", ty);
                };
                let variant_def = adt_def.variant_of_res(res);
                let subpatterns = self.lower_tuple_subpats(pats, variant_def.fields.len(), ddpos);
                self.lower_variant_or_leaf(res, pat.hir_id, pat.span, ty, subpatterns)
            }

            hir::PatKind::Struct(ref qpath, ref fields, _) => {
                let res = self.typeck_results.qpath_res(qpath, pat.hir_id);
                let subpatterns = fields
                    .iter()
                    .map(|field| FieldPat {
                        field: Field::new(self.typeck_results.field_index(field.hir_id)),
                        pattern: self.lower_pattern(&field.pat),
                    })
                    .collect();

                self.lower_variant_or_leaf(res, pat.hir_id, pat.span, ty, subpatterns)
            }

            hir::PatKind::Or(ref pats) => PatKind::Or { pats: self.lower_patterns(pats) },
        };

        Box::new(Pat { span, ty, kind })
    }

    fn lower_tuple_subpats(
        &mut self,
        pats: &'tcx [hir::Pat<'tcx>],
        expected_len: usize,
        gap_pos: hir::DotDotPos,
    ) -> Vec<FieldPat<'tcx>> {
        pats.iter()
            .enumerate_and_adjust(expected_len, gap_pos)
            .map(|(i, subpattern)| FieldPat {
                field: Field::new(i),
                pattern: self.lower_pattern(subpattern),
            })
            .collect()
    }

    fn lower_patterns(&mut self, pats: &'tcx [hir::Pat<'tcx>]) -> Box<[Box<Pat<'tcx>>]> {
        pats.iter().map(|p| self.lower_pattern(p)).collect()
    }

    fn lower_opt_pattern(
        &mut self,
        pat: &'tcx Option<&'tcx hir::Pat<'tcx>>,
    ) -> Option<Box<Pat<'tcx>>> {
        pat.map(|p| self.lower_pattern(p))
    }

    fn slice_or_array_pattern(
        &mut self,
        span: Span,
        ty: Ty<'tcx>,
        prefix: &'tcx [hir::Pat<'tcx>],
        slice: &'tcx Option<&'tcx hir::Pat<'tcx>>,
        suffix: &'tcx [hir::Pat<'tcx>],
    ) -> PatKind<'tcx> {
        let prefix = self.lower_patterns(prefix);
        let slice = self.lower_opt_pattern(slice);
        let suffix = self.lower_patterns(suffix);
        match ty.kind() {
            // Matching a slice, `[T]`.
            ty::Slice(..) => PatKind::Slice { prefix, slice, suffix },
            // Fixed-length array, `[T; len]`.
            ty::Array(_, len) => {
                let len = len.eval_target_usize(self.tcx, self.param_env);
                assert!(len >= prefix.len() as u64 + suffix.len() as u64);
                PatKind::Array { prefix, slice, suffix }
            }
            _ => span_bug!(span, "bad slice pattern type {:?}", ty),
        }
    }

    fn lower_variant_or_leaf(
        &mut self,
        res: Res,
        hir_id: hir::HirId,
        span: Span,
        ty: Ty<'tcx>,
        subpatterns: Vec<FieldPat<'tcx>>,
    ) -> PatKind<'tcx> {
        let res = match res {
            Res::Def(DefKind::Ctor(CtorOf::Variant, ..), variant_ctor_id) => {
                let variant_id = self.tcx.parent(variant_ctor_id);
                Res::Def(DefKind::Variant, variant_id)
            }
            res => res,
        };

        let mut kind = match res {
            Res::Def(DefKind::Variant, variant_id) => {
                let enum_id = self.tcx.parent(variant_id);
                let adt_def = self.tcx.adt_def(enum_id);
                if adt_def.is_enum() {
                    let substs = match ty.kind() {
                        ty::Adt(_, substs) | ty::FnDef(_, substs) => substs,
                        ty::Error(_) => {
                            // Avoid ICE (#50585)
                            return PatKind::Wild;
                        }
                        _ => bug!("inappropriate type for def: {:?}", ty),
                    };
                    PatKind::Variant {
                        adt_def,
                        substs,
                        variant_index: adt_def.variant_index_with_id(variant_id),
                        subpatterns,
                    }
                } else {
                    PatKind::Leaf { subpatterns }
                }
            }

            Res::Def(
                DefKind::Struct
                | DefKind::Ctor(CtorOf::Struct, ..)
                | DefKind::Union
                | DefKind::TyAlias
                | DefKind::AssocTy,
                _,
            )
            | Res::SelfTyParam { .. }
            | Res::SelfTyAlias { .. }
            | Res::SelfCtor(..) => PatKind::Leaf { subpatterns },
            _ => {
                let pattern_error = match res {
                    Res::Def(DefKind::ConstParam, _) => PatternError::ConstParamInPattern(span),
                    Res::Def(DefKind::Static(_), _) => PatternError::StaticInPattern(span),
                    _ => PatternError::NonConstPath(span),
                };
                self.errors.push(pattern_error);
                PatKind::Wild
            }
        };

        if let Some(user_ty) = self.user_substs_applied_to_ty_of_hir_id(hir_id) {
            debug!("lower_variant_or_leaf: kind={:?} user_ty={:?} span={:?}", kind, user_ty, span);
            let annotation = CanonicalUserTypeAnnotation {
                user_ty: Box::new(user_ty),
                span,
                inferred_ty: self.typeck_results.node_type(hir_id),
            };
            kind = PatKind::AscribeUserType {
                subpattern: Box::new(Pat { span, ty, kind }),
                ascription: Ascription { annotation, variance: ty::Variance::Covariant },
            };
        }

        kind
    }

    /// Takes a HIR Path. If the path is a constant, evaluates it and feeds
    /// it to `const_to_pat`. Any other path (like enum variants without fields)
    /// is converted to the corresponding pattern via `lower_variant_or_leaf`.
    #[instrument(skip(self), level = "debug")]
    fn lower_path(&mut self, qpath: &hir::QPath<'_>, id: hir::HirId, span: Span) -> Box<Pat<'tcx>> {
        let ty = self.typeck_results.node_type(id);
        let res = self.typeck_results.qpath_res(qpath, id);

        let pat_from_kind = |kind| Box::new(Pat { span, ty, kind });

        let (def_id, is_associated_const) = match res {
            Res::Def(DefKind::Const, def_id) => (def_id, false),
            Res::Def(DefKind::AssocConst, def_id) => (def_id, true),

            _ => return pat_from_kind(self.lower_variant_or_leaf(res, id, span, ty, vec![])),
        };

        // Use `Reveal::All` here because patterns are always monomorphic even if their function
        // isn't.
        let param_env_reveal_all = self.param_env.with_reveal_all_normalized(self.tcx);
        // N.B. There is no guarantee that substs collected in typeck results are fully normalized,
        // so they need to be normalized in order to pass to `Instance::resolve`, which will ICE
        // if given unnormalized types.
        let substs = self
            .tcx
            .normalize_erasing_regions(param_env_reveal_all, self.typeck_results.node_substs(id));
        let instance = match ty::Instance::resolve(self.tcx, param_env_reveal_all, def_id, substs) {
            Ok(Some(i)) => i,
            Ok(None) => {
                // It should be assoc consts if there's no error but we cannot resolve it.
                debug_assert!(is_associated_const);

                self.errors.push(PatternError::AssocConstInPattern(span));

                return pat_from_kind(PatKind::Wild);
            }

            Err(_) => {
                self.tcx.sess.emit_err(CouldNotEvalConstPattern { span });
                return pat_from_kind(PatKind::Wild);
            }
        };

        // `mir_const_qualif` must be called with the `DefId` of the item where the const is
        // defined, not where it is declared. The difference is significant for associated
        // constants.
        let mir_structural_match_violation = self.tcx.mir_const_qualif(instance.def_id()).custom_eq;
        debug!("mir_structural_match_violation({:?}) -> {}", qpath, mir_structural_match_violation);

        match self.tcx.const_eval_instance(param_env_reveal_all, instance, Some(span)) {
            Ok(literal) => {
                let const_ = mir::ConstantKind::Val(literal, ty);
                let pattern = self.const_to_pat(const_, id, span, mir_structural_match_violation);

                if !is_associated_const {
                    return pattern;
                }

                let user_provided_types = self.typeck_results().user_provided_types();
                if let Some(&user_ty) = user_provided_types.get(id) {
                    let annotation = CanonicalUserTypeAnnotation {
                        user_ty: Box::new(user_ty),
                        span,
                        inferred_ty: self.typeck_results().node_type(id),
                    };
                    Box::new(Pat {
                        span,
                        kind: PatKind::AscribeUserType {
                            subpattern: pattern,
                            ascription: Ascription {
                                annotation,
                                /// Note that use `Contravariant` here. See the
                                /// `variance` field documentation for details.
                                variance: ty::Variance::Contravariant,
                            },
                        },
                        ty: const_.ty(),
                    })
                } else {
                    pattern
                }
            }
            Err(ErrorHandled::TooGeneric) => {
                // While `Reported | Linted` cases will have diagnostics emitted already
                // it is not true for TooGeneric case, so we need to give user more information.
                self.tcx.sess.emit_err(ConstPatternDependsOnGenericParameter { span });
                pat_from_kind(PatKind::Wild)
            }
            Err(_) => {
                self.tcx.sess.emit_err(CouldNotEvalConstPattern { span });
                pat_from_kind(PatKind::Wild)
            }
        }
    }

    /// Converts inline const patterns.
    fn lower_inline_const(
        &mut self,
        anon_const: &'tcx hir::AnonConst,
        id: hir::HirId,
        span: Span,
    ) -> PatKind<'tcx> {
        let value = mir::ConstantKind::from_inline_const(self.tcx, anon_const.def_id);

        // Evaluate early like we do in `lower_path`.
        let value = value.eval(self.tcx, self.param_env);

        match value {
            mir::ConstantKind::Ty(c) => match c.kind() {
                ConstKind::Param(_) => {
                    self.errors.push(PatternError::ConstParamInPattern(span));
                    return PatKind::Wild;
                }
                ConstKind::Error(_) => {
                    return PatKind::Wild;
                }
                _ => bug!("Expected ConstKind::Param"),
            },
            mir::ConstantKind::Val(_, _) => self.const_to_pat(value, id, span, false).kind,
            mir::ConstantKind::Unevaluated(..) => {
                // If we land here it means the const can't be evaluated because it's `TooGeneric`.
                self.tcx.sess.emit_err(ConstPatternDependsOnGenericParameter { span });
                return PatKind::Wild;
            }
        }
    }

    /// Converts literals, paths and negation of literals to patterns.
    /// The special case for negation exists to allow things like `-128_i8`
    /// which would overflow if we tried to evaluate `128_i8` and then negate
    /// afterwards.
    fn lower_lit(&mut self, expr: &'tcx hir::Expr<'tcx>) -> PatKind<'tcx> {
        let (lit, neg) = match expr.kind {
            hir::ExprKind::Path(ref qpath) => {
                return self.lower_path(qpath, expr.hir_id, expr.span).kind;
            }
            hir::ExprKind::ConstBlock(ref anon_const) => {
                return self.lower_inline_const(anon_const, expr.hir_id, expr.span);
            }
            hir::ExprKind::Lit(ref lit) => (lit, false),
            hir::ExprKind::Unary(hir::UnOp::Neg, ref expr) => {
                let hir::ExprKind::Lit(ref lit) = expr.kind else {
                    span_bug!(expr.span, "not a literal: {:?}", expr);
                };
                (lit, true)
            }
            _ => span_bug!(expr.span, "not a literal: {:?}", expr),
        };

        let lit_input =
            LitToConstInput { lit: &lit.node, ty: self.typeck_results.expr_ty(expr), neg };
        match self.tcx.at(expr.span).lit_to_mir_constant(lit_input) {
            Ok(constant) => self.const_to_pat(constant, expr.hir_id, lit.span, false).kind,
            Err(LitToConstError::Reported(_)) => PatKind::Wild,
            Err(LitToConstError::TypeError) => bug!("lower_lit: had type error"),
        }
    }
}

impl<'tcx> UserAnnotatedTyHelpers<'tcx> for PatCtxt<'_, 'tcx> {
    fn tcx(&self) -> TyCtxt<'tcx> {
        self.tcx
    }

    fn typeck_results(&self) -> &ty::TypeckResults<'tcx> {
        self.typeck_results
    }
}

trait PatternFoldable<'tcx>: Sized {
    fn fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
        self.super_fold_with(folder)
    }

    fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self;
}

trait PatternFolder<'tcx>: Sized {
    fn fold_pattern(&mut self, pattern: &Pat<'tcx>) -> Pat<'tcx> {
        pattern.super_fold_with(self)
    }

    fn fold_pattern_kind(&mut self, kind: &PatKind<'tcx>) -> PatKind<'tcx> {
        kind.super_fold_with(self)
    }
}

impl<'tcx, T: PatternFoldable<'tcx>> PatternFoldable<'tcx> for Box<T> {
    fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
        let content: T = (**self).fold_with(folder);
        Box::new(content)
    }
}

impl<'tcx, T: PatternFoldable<'tcx>> PatternFoldable<'tcx> for Vec<T> {
    fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
        self.iter().map(|t| t.fold_with(folder)).collect()
    }
}

impl<'tcx, T: PatternFoldable<'tcx>> PatternFoldable<'tcx> for Box<[T]> {
    fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
        self.iter().map(|t| t.fold_with(folder)).collect()
    }
}

impl<'tcx, T: PatternFoldable<'tcx>> PatternFoldable<'tcx> for Option<T> {
    fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
        self.as_ref().map(|t| t.fold_with(folder))
    }
}

macro_rules! ClonePatternFoldableImpls {
    (<$lt_tcx:tt> $($ty:ty),+) => {
        $(
            impl<$lt_tcx> PatternFoldable<$lt_tcx> for $ty {
                fn super_fold_with<F: PatternFolder<$lt_tcx>>(&self, _: &mut F) -> Self {
                    Clone::clone(self)
                }
            }
        )+
    }
}

ClonePatternFoldableImpls! { <'tcx>
    Span, Field, Mutability, Symbol, LocalVarId, usize,
    Region<'tcx>, Ty<'tcx>, BindingMode, AdtDef<'tcx>,
    SubstsRef<'tcx>, &'tcx GenericArg<'tcx>, UserType<'tcx>,
    UserTypeProjection, CanonicalUserTypeAnnotation<'tcx>
}

impl<'tcx> PatternFoldable<'tcx> for FieldPat<'tcx> {
    fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
        FieldPat { field: self.field.fold_with(folder), pattern: self.pattern.fold_with(folder) }
    }
}

impl<'tcx> PatternFoldable<'tcx> for Pat<'tcx> {
    fn fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
        folder.fold_pattern(self)
    }

    fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
        Pat {
            ty: self.ty.fold_with(folder),
            span: self.span.fold_with(folder),
            kind: self.kind.fold_with(folder),
        }
    }
}

impl<'tcx> PatternFoldable<'tcx> for PatKind<'tcx> {
    fn fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
        folder.fold_pattern_kind(self)
    }

    fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
        match *self {
            PatKind::Wild => PatKind::Wild,
            PatKind::AscribeUserType {
                ref subpattern,
                ascription: Ascription { ref annotation, variance },
            } => PatKind::AscribeUserType {
                subpattern: subpattern.fold_with(folder),
                ascription: Ascription { annotation: annotation.fold_with(folder), variance },
            },
            PatKind::Binding { mutability, name, mode, var, ty, ref subpattern, is_primary } => {
                PatKind::Binding {
                    mutability: mutability.fold_with(folder),
                    name: name.fold_with(folder),
                    mode: mode.fold_with(folder),
                    var: var.fold_with(folder),
                    ty: ty.fold_with(folder),
                    subpattern: subpattern.fold_with(folder),
                    is_primary,
                }
            }
            PatKind::Variant { adt_def, substs, variant_index, ref subpatterns } => {
                PatKind::Variant {
                    adt_def: adt_def.fold_with(folder),
                    substs: substs.fold_with(folder),
                    variant_index,
                    subpatterns: subpatterns.fold_with(folder),
                }
            }
            PatKind::Leaf { ref subpatterns } => {
                PatKind::Leaf { subpatterns: subpatterns.fold_with(folder) }
            }
            PatKind::Deref { ref subpattern } => {
                PatKind::Deref { subpattern: subpattern.fold_with(folder) }
            }
            PatKind::Constant { value } => PatKind::Constant { value },
            PatKind::Range(ref range) => PatKind::Range(range.clone()),
            PatKind::Slice { ref prefix, ref slice, ref suffix } => PatKind::Slice {
                prefix: prefix.fold_with(folder),
                slice: slice.fold_with(folder),
                suffix: suffix.fold_with(folder),
            },
            PatKind::Array { ref prefix, ref slice, ref suffix } => PatKind::Array {
                prefix: prefix.fold_with(folder),
                slice: slice.fold_with(folder),
                suffix: suffix.fold_with(folder),
            },
            PatKind::Or { ref pats } => PatKind::Or { pats: pats.fold_with(folder) },
        }
    }
}

#[instrument(skip(tcx), level = "debug")]
pub(crate) fn compare_const_vals<'tcx>(
    tcx: TyCtxt<'tcx>,
    a: mir::ConstantKind<'tcx>,
    b: mir::ConstantKind<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
) -> Option<Ordering> {
    assert_eq!(a.ty(), b.ty());

    let ty = a.ty();

    // This code is hot when compiling matches with many ranges. So we
    // special-case extraction of evaluated scalars for speed, for types where
    // raw data comparisons are appropriate. E.g. `unicode-normalization` has
    // many ranges such as '\u{037A}'..='\u{037F}', and chars can be compared
    // in this way.
    match ty.kind() {
        ty::Float(_) | ty::Int(_) => {} // require special handling, see below
        _ => match (a, b) {
            (
                mir::ConstantKind::Val(ConstValue::Scalar(Scalar::Int(a)), _a_ty),
                mir::ConstantKind::Val(ConstValue::Scalar(Scalar::Int(b)), _b_ty),
            ) => return Some(a.cmp(&b)),
            _ => {}
        },
    }

    let a = a.eval_bits(tcx, param_env, ty);
    let b = b.eval_bits(tcx, param_env, ty);

    use rustc_apfloat::Float;
    match *ty.kind() {
        ty::Float(ty::FloatTy::F32) => {
            let a = rustc_apfloat::ieee::Single::from_bits(a);
            let b = rustc_apfloat::ieee::Single::from_bits(b);
            a.partial_cmp(&b)
        }
        ty::Float(ty::FloatTy::F64) => {
            let a = rustc_apfloat::ieee::Double::from_bits(a);
            let b = rustc_apfloat::ieee::Double::from_bits(b);
            a.partial_cmp(&b)
        }
        ty::Int(ity) => {
            use rustc_middle::ty::layout::IntegerExt;
            let size = rustc_target::abi::Integer::from_int_ty(&tcx, ity).size();
            let a = size.sign_extend(a);
            let b = size.sign_extend(b);
            Some((a as i128).cmp(&(b as i128)))
        }
        _ => Some(a.cmp(&b)),
    }
}