check_match.rs

use super::deconstruct_pat::{Constructor, DeconstructedPat};
use super::usefulness::{
    compute_match_usefulness, MatchArm, MatchCheckCtxt, Reachability, UsefulnessReport,
};
use super::{PatCtxt, PatternError};

use rustc_arena::TypedArena;
use rustc_ast::Mutability;
use rustc_errors::{
    error_code, pluralize, struct_span_err, Applicability, DelayDm, Diagnostic, DiagnosticBuilder,
    ErrorGuaranteed, MultiSpan,
};
use rustc_hir as hir;
use rustc_hir::def::*;
use rustc_hir::def_id::DefId;
use rustc_hir::intravisit::{self, Visitor};
use rustc_hir::{HirId, Pat};
use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt};
use rustc_session::lint::builtin::{
    BINDINGS_WITH_VARIANT_NAME, IRREFUTABLE_LET_PATTERNS, UNREACHABLE_PATTERNS,
};
use rustc_session::Session;
use rustc_span::source_map::Spanned;
use rustc_span::{BytePos, Span};

pub(crate) fn check_match(tcx: TyCtxt<'_>, def_id: DefId) {
    let body_id = match def_id.as_local() {
        None => return,
        Some(def_id) => tcx.hir().body_owned_by(def_id),
    };

    let pattern_arena = TypedArena::default();
    let mut visitor = MatchVisitor {
        tcx,
        typeck_results: tcx.typeck_body(body_id),
        param_env: tcx.param_env(def_id),
        pattern_arena: &pattern_arena,
    };
    visitor.visit_body(tcx.hir().body(body_id));
}

fn create_e0004(
    sess: &Session,
    sp: Span,
    error_message: String,
) -> DiagnosticBuilder<'_, ErrorGuaranteed> {
    struct_span_err!(sess, sp, E0004, "{}", &error_message)
}

#[derive(PartialEq)]
enum RefutableFlag {
    Irrefutable,
    Refutable,
}
use RefutableFlag::*;

struct MatchVisitor<'a, 'p, 'tcx> {
    tcx: TyCtxt<'tcx>,
    typeck_results: &'a ty::TypeckResults<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    pattern_arena: &'p TypedArena<DeconstructedPat<'p, 'tcx>>,
}

impl<'tcx> Visitor<'tcx> for MatchVisitor<'_, '_, 'tcx> {
    fn visit_expr(&mut self, ex: &'tcx hir::Expr<'tcx>) {
        intravisit::walk_expr(self, ex);
        match &ex.kind {
            hir::ExprKind::Match(scrut, arms, source) => {
                self.check_match(scrut, arms, *source, ex.span)
            }
            hir::ExprKind::Let(hir::Let { pat, init, span, .. }) => {
                self.check_let(pat, init, *span)
            }
            _ => {}
        }
    }

    fn visit_local(&mut self, loc: &'tcx hir::Local<'tcx>) {
        intravisit::walk_local(self, loc);
        let els = loc.els;
        if let Some(init) = loc.init && els.is_some() {
            self.check_let(&loc.pat, init, loc.span);
        }

        let (msg, sp) = match loc.source {
            hir::LocalSource::Normal => ("local binding", Some(loc.span)),
            hir::LocalSource::AsyncFn => ("async fn binding", None),
            hir::LocalSource::AwaitDesugar => ("`await` future binding", None),
            hir::LocalSource::AssignDesugar(_) => ("destructuring assignment binding", None),
        };
        if els.is_none() {
            self.check_irrefutable(&loc.pat, msg, sp);
        }
    }

    fn visit_param(&mut self, param: &'tcx hir::Param<'tcx>) {
        intravisit::walk_param(self, param);
        self.check_irrefutable(&param.pat, "function argument", None);
    }
}

impl PatCtxt<'_, '_> {
    fn report_inlining_errors(&self) {
        for error in &self.errors {
            match *error {
                PatternError::StaticInPattern(span) => {
                    self.span_e0158(span, "statics cannot be referenced in patterns")
                }
                PatternError::AssocConstInPattern(span) => {
                    self.span_e0158(span, "associated consts cannot be referenced in patterns")
                }
                PatternError::ConstParamInPattern(span) => {
                    self.span_e0158(span, "const parameters cannot be referenced in patterns")
                }
                PatternError::NonConstPath(span) => {
                    rustc_middle::mir::interpret::struct_error(
                        self.tcx.at(span),
                        "runtime values cannot be referenced in patterns",
                    )
                    .emit();
                }
            }
        }
    }

    fn span_e0158(&self, span: Span, text: &str) {
        struct_span_err!(self.tcx.sess, span, E0158, "{}", text).emit();
    }
}

impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
    fn check_patterns(&self, pat: &Pat<'_>, rf: RefutableFlag) {
        pat.walk_always(|pat| check_borrow_conflicts_in_at_patterns(self, pat));
        check_for_bindings_named_same_as_variants(self, pat, rf);
    }

    fn lower_pattern(
        &self,
        cx: &mut MatchCheckCtxt<'p, 'tcx>,
        pat: &'tcx hir::Pat<'tcx>,
        have_errors: &mut bool,
    ) -> &'p DeconstructedPat<'p, 'tcx> {
        let mut patcx = PatCtxt::new(self.tcx, self.param_env, self.typeck_results);
        patcx.include_lint_checks();
        let pattern = patcx.lower_pattern(pat);
        let pattern: &_ = cx.pattern_arena.alloc(DeconstructedPat::from_pat(cx, &pattern));
        if !patcx.errors.is_empty() {
            *have_errors = true;
            patcx.report_inlining_errors();
        }
        pattern
    }

    fn new_cx(&self, hir_id: HirId) -> MatchCheckCtxt<'p, 'tcx> {
        MatchCheckCtxt {
            tcx: self.tcx,
            param_env: self.param_env,
            module: self.tcx.parent_module(hir_id).to_def_id(),
            pattern_arena: &self.pattern_arena,
        }
    }

    fn check_let(&mut self, pat: &'tcx hir::Pat<'tcx>, scrutinee: &hir::Expr<'_>, span: Span) {
        self.check_patterns(pat, Refutable);
        let mut cx = self.new_cx(scrutinee.hir_id);
        let tpat = self.lower_pattern(&mut cx, pat, &mut false);
        self.check_let_reachability(&mut cx, pat.hir_id, tpat, span);
    }

    fn check_match(
        &mut self,
        scrut: &hir::Expr<'_>,
        hir_arms: &'tcx [hir::Arm<'tcx>],
        source: hir::MatchSource,
        expr_span: Span,
    ) {
        let mut cx = self.new_cx(scrut.hir_id);

        for arm in hir_arms {
            // Check the arm for some things unrelated to exhaustiveness.
            self.check_patterns(&arm.pat, Refutable);
            if let Some(hir::Guard::IfLet(ref let_expr)) = arm.guard {
                self.check_patterns(let_expr.pat, Refutable);
                let tpat = self.lower_pattern(&mut cx, let_expr.pat, &mut false);
                self.check_let_reachability(&mut cx, let_expr.pat.hir_id, tpat, tpat.span());
            }
        }

        let mut have_errors = false;

        let arms: Vec<_> = hir_arms
            .iter()
            .map(|hir::Arm { pat, guard, .. }| MatchArm {
                pat: self.lower_pattern(&mut cx, pat, &mut have_errors),
                hir_id: pat.hir_id,
                has_guard: guard.is_some(),
            })
            .collect();

        // Bail out early if lowering failed.
        if have_errors {
            return;
        }

        let scrut_ty = self.typeck_results.expr_ty_adjusted(scrut);
        let report = compute_match_usefulness(&cx, &arms, scrut.hir_id, scrut_ty);

        match source {
            // Don't report arm reachability of desugared `match $iter.into_iter() { iter => .. }`
            // when the iterator is an uninhabited type. unreachable_code will trigger instead.
            hir::MatchSource::ForLoopDesugar if arms.len() == 1 => {}
            hir::MatchSource::ForLoopDesugar | hir::MatchSource::Normal => {
                report_arm_reachability(&cx, &report)
            }
            // Unreachable patterns in try and await expressions occur when one of
            // the arms are an uninhabited type. Which is OK.
            hir::MatchSource::AwaitDesugar | hir::MatchSource::TryDesugar => {}
        }

        // Check if the match is exhaustive.
        let witnesses = report.non_exhaustiveness_witnesses;
        if !witnesses.is_empty() {
            if source == hir::MatchSource::ForLoopDesugar && hir_arms.len() == 2 {
                // the for loop pattern is not irrefutable
                let pat = hir_arms[1].pat.for_loop_some().unwrap();
                self.check_irrefutable(pat, "`for` loop binding", None);
            } else {
                non_exhaustive_match(&cx, scrut_ty, scrut.span, witnesses, hir_arms, expr_span);
            }
        }
    }

    fn check_let_reachability(
        &mut self,
        cx: &mut MatchCheckCtxt<'p, 'tcx>,
        pat_id: HirId,
        pat: &'p DeconstructedPat<'p, 'tcx>,
        span: Span,
    ) {
        if self.check_let_chain(cx, pat_id) {
            return;
        }

        if is_let_irrefutable(cx, pat_id, pat) {
            irrefutable_let_pattern(cx.tcx, pat_id, span);
        }
    }

    fn check_let_chain(&mut self, cx: &mut MatchCheckCtxt<'p, 'tcx>, pat_id: HirId) -> bool {
        let hir = self.tcx.hir();
        let parent = hir.get_parent_node(pat_id);

        // First, figure out if the given pattern is part of a let chain,
        // and if so, obtain the top node of the chain.
        let mut top = parent;
        let mut part_of_chain = false;
        loop {
            let new_top = hir.get_parent_node(top);
            if let hir::Node::Expr(
                hir::Expr {
                    kind: hir::ExprKind::Binary(Spanned { node: hir::BinOpKind::And, .. }, lhs, rhs),
                    ..
                },
                ..,
            ) = hir.get(new_top)
            {
                // If this isn't the first iteration, we need to check
                // if there is a let expr before us in the chain, so
                // that we avoid doubly checking the let chain.

                // The way a chain of &&s is encoded is ((let ... && let ...) && let ...) && let ...
                // as && is left-to-right associative. Thus, we need to check rhs.
                if part_of_chain && matches!(rhs.kind, hir::ExprKind::Let(..)) {
                    return true;
                }
                // If there is a let at the lhs, and we provide the rhs, we don't do any checking either.
                if !part_of_chain && matches!(lhs.kind, hir::ExprKind::Let(..)) && rhs.hir_id == top
                {
                    return true;
                }
            } else {
                // We've reached the top.
                break;
            }

            // Since this function is called within a let context, it is reasonable to assume that any parent
            // `&&` infers a let chain
            part_of_chain = true;
            top = new_top;
        }
        if !part_of_chain {
            return false;
        }

        // Second, obtain the refutabilities of all exprs in the chain,
        // and record chain members that aren't let exprs.
        let mut chain_refutabilities = Vec::new();
        let hir::Node::Expr(top_expr) = hir.get(top) else {
            // We ensure right above that it's an Expr
            unreachable!()
        };
        let mut cur_expr = top_expr;
        loop {
            let mut add = |expr: &hir::Expr<'tcx>| {
                let refutability = match expr.kind {
                    hir::ExprKind::Let(hir::Let { pat, init, span, .. }) => {
                        let mut ncx = self.new_cx(init.hir_id);
                        let tpat = self.lower_pattern(&mut ncx, pat, &mut false);

                        let refutable = !is_let_irrefutable(&mut ncx, pat.hir_id, tpat);
                        Some((*span, refutable))
                    }
                    _ => None,
                };
                chain_refutabilities.push(refutability);
            };
            if let hir::Expr {
                kind: hir::ExprKind::Binary(Spanned { node: hir::BinOpKind::And, .. }, lhs, rhs),
                ..
            } = cur_expr
            {
                add(rhs);
                cur_expr = lhs;
            } else {
                add(cur_expr);
                break;
            }
        }
        chain_refutabilities.reverse();

        // Third, emit the actual warnings.

        if chain_refutabilities.iter().all(|r| matches!(*r, Some((_, false)))) {
            // The entire chain is made up of irrefutable `let` statements
            let let_source = let_source_parent(self.tcx, top, None);
            irrefutable_let_patterns(
                cx.tcx,
                top,
                let_source,
                chain_refutabilities.len(),
                top_expr.span,
            );
            return true;
        }
        let lint_affix = |affix: &[Option<(Span, bool)>], kind, suggestion| {
            let span_start = affix[0].unwrap().0;
            let span_end = affix.last().unwrap().unwrap().0;
            let span = span_start.to(span_end);
            let cnt = affix.len();
            let s = pluralize!(cnt);
            cx.tcx.struct_span_lint_hir(
                IRREFUTABLE_LET_PATTERNS,
                top,
                span,
                format!("{kind} irrefutable pattern{s} in let chain"),
                |lint| {
                    lint.note(format!(
                        "{these} pattern{s} will always match",
                        these = pluralize!("this", cnt),
                    ))
                    .help(format!(
                        "consider moving {} {suggestion}",
                        if cnt > 1 { "them" } else { "it" }
                    ))
                },
            );
        };
        if let Some(until) = chain_refutabilities.iter().position(|r| !matches!(*r, Some((_, false)))) && until > 0 {
            // The chain has a non-zero prefix of irrefutable `let` statements.

            // Check if the let source is while, for there is no alternative place to put a prefix,
            // and we shouldn't lint.
            // For let guards inside a match, prefixes might use bindings of the match pattern,
            // so can't always be moved out.
            // FIXME: Add checking whether the bindings are actually used in the prefix,
            // and lint if they are not.
            let let_source = let_source_parent(self.tcx, top, None);
            if !matches!(let_source, LetSource::WhileLet | LetSource::IfLetGuard) {
                // Emit the lint
                let prefix = &chain_refutabilities[..until];
                lint_affix(prefix, "leading", "outside of the construct");
            }
        }
        if let Some(from) = chain_refutabilities.iter().rposition(|r| !matches!(*r, Some((_, false)))) && from != (chain_refutabilities.len() - 1) {
            // The chain has a non-empty suffix of irrefutable `let` statements
            let suffix = &chain_refutabilities[from + 1..];
            lint_affix(suffix, "trailing", "into the body");
        }
        true
    }

    fn check_irrefutable(&self, pat: &'tcx Pat<'tcx>, origin: &str, sp: Option<Span>) {
        let mut cx = self.new_cx(pat.hir_id);

        let pattern = self.lower_pattern(&mut cx, pat, &mut false);
        let pattern_ty = pattern.ty();
        let arms = vec![MatchArm { pat: pattern, hir_id: pat.hir_id, has_guard: false }];
        let report = compute_match_usefulness(&cx, &arms, pat.hir_id, pattern_ty);

        // Note: we ignore whether the pattern is unreachable (i.e. whether the type is empty). We
        // only care about exhaustiveness here.
        let witnesses = report.non_exhaustiveness_witnesses;
        if witnesses.is_empty() {
            // The pattern is irrefutable.
            self.check_patterns(pat, Irrefutable);
            return;
        }

        let joined_patterns = joined_uncovered_patterns(&cx, &witnesses);

        let mut bindings = vec![];

        let mut err = struct_span_err!(
            self.tcx.sess,
            pat.span,
            E0005,
            "refutable pattern in {}: {} not covered",
            origin,
            joined_patterns
        );
        let suggest_if_let = match &pat.kind {
            hir::PatKind::Path(hir::QPath::Resolved(None, path))
                if path.segments.len() == 1 && path.segments[0].args.is_none() =>
            {
                const_not_var(&mut err, cx.tcx, pat, path);
                false
            }
            _ => {
                pat.walk(&mut |pat: &hir::Pat<'_>| {
                    match pat.kind {
                        hir::PatKind::Binding(_, _, ident, _) => {
                            bindings.push(ident);
                        }
                        _ => {}
                    }
                    true
                });

                err.span_label(pat.span, pattern_not_covered_label(&witnesses, &joined_patterns));
                true
            }
        };

        if let (Some(span), true) = (sp, suggest_if_let) {
            err.note(
                "`let` bindings require an \"irrefutable pattern\", like a `struct` or \
                 an `enum` with only one variant",
            );
            if self.tcx.sess.source_map().is_span_accessible(span) {
                let semi_span = span.shrink_to_hi().with_lo(span.hi() - BytePos(1));
                let start_span = span.shrink_to_lo();
                let end_span = semi_span.shrink_to_lo();
                err.multipart_suggestion(
                    &format!(
                        "you might want to use `if let` to ignore the variant{} that {} matched",
                        pluralize!(witnesses.len()),
                        match witnesses.len() {
                            1 => "isn't",
                            _ => "aren't",
                        },
                    ),
                    vec![
                        match &bindings[..] {
                            [] => (start_span, "if ".to_string()),
                            [binding] => (start_span, format!("let {} = if ", binding)),
                            bindings => (
                                start_span,
                                format!(
                                    "let ({}) = if ",
                                    bindings
                                        .iter()
                                        .map(|ident| ident.to_string())
                                        .collect::<Vec<_>>()
                                        .join(", ")
                                ),
                            ),
                        },
                        match &bindings[..] {
                            [] => (semi_span, " { todo!() }".to_string()),
                            [binding] => {
                                (end_span, format!(" {{ {} }} else {{ todo!() }}", binding))
                            }
                            bindings => (
                                end_span,
                                format!(
                                    " {{ ({}) }} else {{ todo!() }}",
                                    bindings
                                        .iter()
                                        .map(|ident| ident.to_string())
                                        .collect::<Vec<_>>()
                                        .join(", ")
                                ),
                            ),
                        },
                    ],
                    Applicability::HasPlaceholders,
                );
                if !bindings.is_empty() {
                    err.span_suggestion_verbose(
                        semi_span.shrink_to_lo(),
                        &format!(
                            "alternatively, you might want to use \
                             let else to handle the variant{} that {} matched",
                            pluralize!(witnesses.len()),
                            match witnesses.len() {
                                1 => "isn't",
                                _ => "aren't",
                            },
                        ),
                        " else { todo!() }".to_string(),
                        Applicability::HasPlaceholders,
                    );
                }
            }
            err.note(
                "for more information, visit \
                 https://doc.rust-lang.org/book/ch18-02-refutability.html",
            );
        }

        adt_defined_here(&cx, &mut err, pattern_ty, &witnesses);
        err.note(&format!("the matched value is of type `{}`", pattern_ty));
        err.emit();
    }
}

/// A path pattern was interpreted as a constant, not a new variable.
/// This caused an irrefutable match failure in e.g. `let`.
fn const_not_var(err: &mut Diagnostic, tcx: TyCtxt<'_>, pat: &Pat<'_>, path: &hir::Path<'_>) {
    let descr = path.res.descr();
    err.span_label(
        pat.span,
        format!("interpreted as {} {} pattern, not a new variable", path.res.article(), descr,),
    );

    err.span_suggestion(
        pat.span,
        "introduce a variable instead",
        format!("{}_var", path.segments[0].ident).to_lowercase(),
        // Cannot use `MachineApplicable` as it's not really *always* correct
        // because there may be such an identifier in scope or the user maybe
        // really wanted to match against the constant. This is quite unlikely however.
        Applicability::MaybeIncorrect,
    );

    if let Some(span) = tcx.hir().res_span(path.res) {
        err.span_label(span, format!("{} defined here", descr));
    }
}

fn check_for_bindings_named_same_as_variants(
    cx: &MatchVisitor<'_, '_, '_>,
    pat: &Pat<'_>,
    rf: RefutableFlag,
) {
    pat.walk_always(|p| {
        if let hir::PatKind::Binding(_, _, ident, None) = p.kind
            && let Some(ty::BindByValue(hir::Mutability::Not)) =
                cx.typeck_results.extract_binding_mode(cx.tcx.sess, p.hir_id, p.span)
            && let pat_ty = cx.typeck_results.pat_ty(p).peel_refs()
            && let ty::Adt(edef, _) = pat_ty.kind()
            && edef.is_enum()
            && edef.variants().iter().any(|variant| {
                variant.ident(cx.tcx) == ident && variant.ctor_kind == CtorKind::Const
            })
        {
            let variant_count = edef.variants().len();
            cx.tcx.struct_span_lint_hir(
                BINDINGS_WITH_VARIANT_NAME,
                p.hir_id,
                p.span,
                DelayDm(|| format!(
                    "pattern binding `{}` is named the same as one \
                        of the variants of the type `{}`",
                    ident, cx.tcx.def_path_str(edef.did())
                )),
                |lint| {
                    let ty_path = cx.tcx.def_path_str(edef.did());
                    lint.code(error_code!(E0170));

                    // If this is an irrefutable pattern, and there's > 1 variant,
                    // then we can't actually match on this. Applying the below
                    // suggestion would produce code that breaks on `check_irrefutable`.
                    if rf == Refutable || variant_count == 1 {
                        lint.span_suggestion(
                            p.span,
                            "to match on the variant, qualify the path",
                            format!("{}::{}", ty_path, ident),
                            Applicability::MachineApplicable,
                        );
                    }

                    lint
                },
            )
        }
    });
}

/// Checks for common cases of "catchall" patterns that may not be intended as such.
fn pat_is_catchall(pat: &DeconstructedPat<'_, '_>) -> bool {
    use Constructor::*;
    match pat.ctor() {
        Wildcard => true,
        Single => pat.iter_fields().all(|pat| pat_is_catchall(pat)),
        _ => false,
    }
}

fn unreachable_pattern(tcx: TyCtxt<'_>, span: Span, id: HirId, catchall: Option<Span>) {
    tcx.struct_span_lint_hir(UNREACHABLE_PATTERNS, id, span, "unreachable pattern", |lint| {
        if let Some(catchall) = catchall {
            // We had a catchall pattern, hint at that.
            lint.span_label(span, "unreachable pattern");
            lint.span_label(catchall, "matches any value");
        }
        lint
    });
}

fn irrefutable_let_pattern(tcx: TyCtxt<'_>, id: HirId, span: Span) {
    let source = let_source(tcx, id);
    irrefutable_let_patterns(tcx, id, source, 1, span);
}

fn irrefutable_let_patterns(
    tcx: TyCtxt<'_>,
    id: HirId,
    source: LetSource,
    count: usize,
    span: Span,
) {
    let span = match source {
        LetSource::LetElse(span) => span,
        _ => span,
    };

    macro_rules! emit_diag {
        (
            $lint:expr,
            $source_name:expr,
            $note_sufix:expr,
            $help_sufix:expr
        ) => {{
            let s = pluralize!(count);
            let these = pluralize!("this", count);
            tcx.struct_span_lint_hir(
                IRREFUTABLE_LET_PATTERNS,
                id,
                span,
                format!("irrefutable {} pattern{s}", $source_name),
                |lint| {
                    lint.note(&format!(
                        "{these} pattern{s} will always match, so the {}",
                        $note_sufix
                    ))
                    .help(concat!("consider ", $help_sufix))
                },
            )
        }};
    }

    match source {
        LetSource::GenericLet => {
            emit_diag!(lint, "`let`", "`let` is useless", "removing `let`");
        }
        LetSource::IfLet => {
            emit_diag!(
                lint,
                "`if let`",
                "`if let` is useless",
                "replacing the `if let` with a `let`"
            );
        }
        LetSource::IfLetGuard => {
            emit_diag!(
                lint,
                "`if let` guard",
                "guard is useless",
                "removing the guard and adding a `let` inside the match arm"
            );
        }
        LetSource::LetElse(..) => {
            emit_diag!(
                lint,
                "`let...else`",
                "`else` clause is useless",
                "removing the `else` clause"
            );
        }
        LetSource::WhileLet => {
            emit_diag!(
                lint,
                "`while let`",
                "loop will never exit",
                "instead using a `loop { ... }` with a `let` inside it"
            );
        }
    };
}

fn is_let_irrefutable<'p, 'tcx>(
    cx: &mut MatchCheckCtxt<'p, 'tcx>,
    pat_id: HirId,
    pat: &'p DeconstructedPat<'p, 'tcx>,
) -> bool {
    let arms = [MatchArm { pat, hir_id: pat_id, has_guard: false }];
    let report = compute_match_usefulness(&cx, &arms, pat_id, pat.ty());

    // Report if the pattern is unreachable, which can only occur when the type is uninhabited.
    // This also reports unreachable sub-patterns though, so we can't just replace it with an
    // `is_uninhabited` check.
    report_arm_reachability(&cx, &report);

    // If the list of witnesses is empty, the match is exhaustive,
    // i.e. the `if let` pattern is irrefutable.
    report.non_exhaustiveness_witnesses.is_empty()
}

/// Report unreachable arms, if any.
fn report_arm_reachability<'p, 'tcx>(
    cx: &MatchCheckCtxt<'p, 'tcx>,
    report: &UsefulnessReport<'p, 'tcx>,
) {
    use Reachability::*;
    let mut catchall = None;
    for (arm, is_useful) in report.arm_usefulness.iter() {
        match is_useful {
            Unreachable => unreachable_pattern(cx.tcx, arm.pat.span(), arm.hir_id, catchall),
            Reachable(unreachables) if unreachables.is_empty() => {}
            // The arm is reachable, but contains unreachable subpatterns (from or-patterns).
            Reachable(unreachables) => {
                let mut unreachables = unreachables.clone();
                // Emit lints in the order in which they occur in the file.
                unreachables.sort_unstable();
                for span in unreachables {
                    unreachable_pattern(cx.tcx, span, arm.hir_id, None);
                }
            }
        }
        if !arm.has_guard && catchall.is_none() && pat_is_catchall(arm.pat) {
            catchall = Some(arm.pat.span());
        }
    }
}

/// Report that a match is not exhaustive.
fn non_exhaustive_match<'p, 'tcx>(
    cx: &MatchCheckCtxt<'p, 'tcx>,
    scrut_ty: Ty<'tcx>,
    sp: Span,
    witnesses: Vec<DeconstructedPat<'p, 'tcx>>,
    arms: &[hir::Arm<'tcx>],
    expr_span: Span,
) {
    let is_empty_match = arms.is_empty();
    let non_empty_enum = match scrut_ty.kind() {
        ty::Adt(def, _) => def.is_enum() && !def.variants().is_empty(),
        _ => false,
    };
    // In the case of an empty match, replace the '`_` not covered' diagnostic with something more
    // informative.
    let mut err;
    let pattern;
    let mut patterns_len = 0;
    if is_empty_match && !non_empty_enum {
        err = create_e0004(
            cx.tcx.sess,
            sp,
            format!("non-exhaustive patterns: type `{}` is non-empty", scrut_ty),
        );
        pattern = "_".to_string();
    } else {
        let joined_patterns = joined_uncovered_patterns(cx, &witnesses);
        err = create_e0004(
            cx.tcx.sess,
            sp,
            format!("non-exhaustive patterns: {} not covered", joined_patterns),
        );
        err.span_label(sp, pattern_not_covered_label(&witnesses, &joined_patterns));
        patterns_len = witnesses.len();
        pattern = if witnesses.len() < 4 {
            witnesses
                .iter()
                .map(|witness| witness.to_pat(cx).to_string())
                .collect::<Vec<String>>()
                .join(" | ")
        } else {
            "_".to_string()
        };
    };

    let is_variant_list_non_exhaustive = match scrut_ty.kind() {
        ty::Adt(def, _) if def.is_variant_list_non_exhaustive() && !def.did().is_local() => true,
        _ => false,
    };

    adt_defined_here(cx, &mut err, scrut_ty, &witnesses);
    err.note(&format!(
        "the matched value is of type `{}`{}",
        scrut_ty,
        if is_variant_list_non_exhaustive { ", which is marked as non-exhaustive" } else { "" }
    ));
    if (scrut_ty == cx.tcx.types.usize || scrut_ty == cx.tcx.types.isize)
        && !is_empty_match
        && witnesses.len() == 1
        && matches!(witnesses[0].ctor(), Constructor::NonExhaustive)
    {
        err.note(&format!(
            "`{}` does not have a fixed maximum value, so a wildcard `_` is necessary to match \
             exhaustively",
            scrut_ty,
        ));
        if cx.tcx.sess.is_nightly_build() {
            err.help(&format!(
                "add `#![feature(precise_pointer_size_matching)]` to the crate attributes to \
                 enable precise `{}` matching",
                scrut_ty,
            ));
        }
    }
    if let ty::Ref(_, sub_ty, _) = scrut_ty.kind() {
        if cx.tcx.is_ty_uninhabited_from(cx.module, *sub_ty, cx.param_env) {
            err.note("references are always considered inhabited");
        }
    }

    let mut suggestion = None;
    let sm = cx.tcx.sess.source_map();
    match arms {
        [] if sp.eq_ctxt(expr_span) => {
            // Get the span for the empty match body `{}`.
            let (indentation, more) = if let Some(snippet) = sm.indentation_before(sp) {
                (format!("\n{}", snippet), "    ")
            } else {
                (" ".to_string(), "")
            };
            suggestion = Some((
                sp.shrink_to_hi().with_hi(expr_span.hi()),
                format!(
                    " {{{indentation}{more}{pattern} => todo!(),{indentation}}}",
                    indentation = indentation,
                    more = more,
                    pattern = pattern,
                ),
            ));
        }
        [only] => {
            let (pre_indentation, is_multiline) = if let Some(snippet) = sm.indentation_before(only.span)
                && let Ok(with_trailing) = sm.span_extend_while(only.span, |c| c.is_whitespace() || c == ',')
                && sm.is_multiline(with_trailing)
            {
                (format!("\n{}", snippet), true)
            } else {
                (" ".to_string(), false)
            };
            let comma = if matches!(only.body.kind, hir::ExprKind::Block(..))
                && only.span.eq_ctxt(only.body.span)
                && is_multiline
            {
                ""
            } else {
                ","
            };
            suggestion = Some((
                only.span.shrink_to_hi(),
                format!("{}{}{} => todo!()", comma, pre_indentation, pattern),
            ));
        }
        [.., prev, last] if prev.span.eq_ctxt(last.span) => {
            let comma = if matches!(last.body.kind, hir::ExprKind::Block(..))
                && last.span.eq_ctxt(last.body.span)
            {
                ""
            } else {
                ","
            };
            let spacing = if sm.is_multiline(prev.span.between(last.span)) {
                sm.indentation_before(last.span).map(|indent| format!("\n{indent}"))
            } else {
                Some(" ".to_string())
            };
            if let Some(spacing) = spacing {
                suggestion = Some((
                    last.span.shrink_to_hi(),
                    format!("{}{}{} => todo!()", comma, spacing, pattern),
                ));
            }
        }
        _ => {}
    }

    let msg = format!(
        "ensure that all possible cases are being handled by adding a match arm with a wildcard \
         pattern{}{}",
        if patterns_len > 1 && patterns_len < 4 && suggestion.is_some() {
            ", a match arm with multiple or-patterns"
        } else {
            // we are either not suggesting anything, or suggesting `_`
            ""
        },
        match patterns_len {
            // non-exhaustive enum case
            0 if suggestion.is_some() => " as shown",
            0 => "",
            1 if suggestion.is_some() => " or an explicit pattern as shown",
            1 => " or an explicit pattern",
            _ if suggestion.is_some() => " as shown, or multiple match arms",
            _ => " or multiple match arms",
        },
    );
    if let Some((span, sugg)) = suggestion {
        err.span_suggestion_verbose(span, &msg, sugg, Applicability::HasPlaceholders);
    } else {
        err.help(&msg);
    }
    err.emit();
}

pub(crate) fn joined_uncovered_patterns<'p, 'tcx>(
    cx: &MatchCheckCtxt<'p, 'tcx>,
    witnesses: &[DeconstructedPat<'p, 'tcx>],
) -> String {
    const LIMIT: usize = 3;
    let pat_to_str = |pat: &DeconstructedPat<'p, 'tcx>| pat.to_pat(cx).to_string();
    match witnesses {
        [] => bug!(),
        [witness] => format!("`{}`", witness.to_pat(cx)),
        [head @ .., tail] if head.len() < LIMIT => {
            let head: Vec<_> = head.iter().map(pat_to_str).collect();
            format!("`{}` and `{}`", head.join("`, `"), tail.to_pat(cx))
        }
        _ => {
            let (head, tail) = witnesses.split_at(LIMIT);
            let head: Vec<_> = head.iter().map(pat_to_str).collect();
            format!("`{}` and {} more", head.join("`, `"), tail.len())
        }
    }
}

pub(crate) fn pattern_not_covered_label(
    witnesses: &[DeconstructedPat<'_, '_>],
    joined_patterns: &str,
) -> String {
    format!("pattern{} {} not covered", rustc_errors::pluralize!(witnesses.len()), joined_patterns)
}

/// Point at the definition of non-covered `enum` variants.
fn adt_defined_here<'p, 'tcx>(
    cx: &MatchCheckCtxt<'p, 'tcx>,
    err: &mut Diagnostic,
    ty: Ty<'tcx>,
    witnesses: &[DeconstructedPat<'p, 'tcx>],
) {
    let ty = ty.peel_refs();
    if let ty::Adt(def, _) = ty.kind() {
        let mut spans = vec![];
        if witnesses.len() < 5 {
            for sp in maybe_point_at_variant(cx, *def, witnesses.iter()) {
                spans.push(sp);
            }
        }
        let def_span = cx
            .tcx
            .hir()
            .get_if_local(def.did())
            .and_then(|node| node.ident())
            .map(|ident| ident.span)
            .unwrap_or_else(|| cx.tcx.def_span(def.did()));
        let mut span: MultiSpan =
            if spans.is_empty() { def_span.into() } else { spans.clone().into() };

        span.push_span_label(def_span, "");
        for pat in spans {
            span.push_span_label(pat, "not covered");
        }
        err.span_note(span, &format!("`{}` defined here", ty));
    }
}

fn maybe_point_at_variant<'a, 'p: 'a, 'tcx: 'a>(
    cx: &MatchCheckCtxt<'p, 'tcx>,
    def: AdtDef<'tcx>,
    patterns: impl Iterator<Item = &'a DeconstructedPat<'p, 'tcx>>,
) -> Vec<Span> {
    use Constructor::*;
    let mut covered = vec![];
    for pattern in patterns {
        if let Variant(variant_index) = pattern.ctor() {
            if let ty::Adt(this_def, _) = pattern.ty().kind() && this_def.did() != def.did() {
                continue;
            }
            let sp = def.variant(*variant_index).ident(cx.tcx).span;
            if covered.contains(&sp) {
                // Don't point at variants that have already been covered due to other patterns to avoid
                // visual clutter.
                continue;
            }
            covered.push(sp);
        }
        covered.extend(maybe_point_at_variant(cx, def, pattern.iter_fields()));
    }
    covered
}

/// Check if a by-value binding is by-value. That is, check if the binding's type is not `Copy`.
fn is_binding_by_move(cx: &MatchVisitor<'_, '_, '_>, hir_id: HirId) -> bool {
    !cx.typeck_results.node_type(hir_id).is_copy_modulo_regions(cx.tcx, cx.param_env)
}

/// Check that there are no borrow or move conflicts in `binding @ subpat` patterns.
///
/// For example, this would reject:
/// - `ref x @ Some(ref mut y)`,
/// - `ref mut x @ Some(ref y)`,
/// - `ref mut x @ Some(ref mut y)`,
/// - `ref mut? x @ Some(y)`, and
/// - `x @ Some(ref mut? y)`.
///
/// This analysis is *not* subsumed by NLL.
fn check_borrow_conflicts_in_at_patterns(cx: &MatchVisitor<'_, '_, '_>, pat: &Pat<'_>) {
    // Extract `sub` in `binding @ sub`.
    let (name, sub) = match &pat.kind {
        hir::PatKind::Binding(.., name, Some(sub)) => (*name, sub),
        _ => return,
    };
    let binding_span = pat.span.with_hi(name.span.hi());

    let typeck_results = cx.typeck_results;
    let sess = cx.tcx.sess;

    // Get the binding move, extract the mutability if by-ref.
    let mut_outer = match typeck_results.extract_binding_mode(sess, pat.hir_id, pat.span) {
        Some(ty::BindByValue(_)) if is_binding_by_move(cx, pat.hir_id) => {
            // We have `x @ pat` where `x` is by-move. Reject all borrows in `pat`.
            let mut conflicts_ref = Vec::new();
            sub.each_binding(|_, hir_id, span, _| {
                match typeck_results.extract_binding_mode(sess, hir_id, span) {
                    Some(ty::BindByValue(_)) | None => {}
                    Some(ty::BindByReference(_)) => conflicts_ref.push(span),
                }
            });
            if !conflicts_ref.is_empty() {
                let occurs_because = format!(
                    "move occurs because `{}` has type `{}` which does not implement the `Copy` trait",
                    name,
                    typeck_results.node_type(pat.hir_id),
                );
                sess.struct_span_err(pat.span, "borrow of moved value")
                    .span_label(binding_span, format!("value moved into `{}` here", name))
                    .span_label(binding_span, occurs_because)
                    .span_labels(conflicts_ref, "value borrowed here after move")
                    .emit();
            }
            return;
        }
        Some(ty::BindByValue(_)) | None => return,
        Some(ty::BindByReference(m)) => m,
    };

    // We now have `ref $mut_outer binding @ sub` (semantically).
    // Recurse into each binding in `sub` and find mutability or move conflicts.
    let mut conflicts_move = Vec::new();
    let mut conflicts_mut_mut = Vec::new();
    let mut conflicts_mut_ref = Vec::new();
    sub.each_binding(|_, hir_id, span, name| {
        match typeck_results.extract_binding_mode(sess, hir_id, span) {
            Some(ty::BindByReference(mut_inner)) => match (mut_outer, mut_inner) {
                (Mutability::Not, Mutability::Not) => {} // Both sides are `ref`.
                (Mutability::Mut, Mutability::Mut) => conflicts_mut_mut.push((span, name)), // 2x `ref mut`.
                _ => conflicts_mut_ref.push((span, name)), // `ref` + `ref mut` in either direction.
            },
            Some(ty::BindByValue(_)) if is_binding_by_move(cx, hir_id) => {
                conflicts_move.push((span, name)) // `ref mut?` + by-move conflict.
            }
            Some(ty::BindByValue(_)) | None => {} // `ref mut?` + by-copy is fine.
        }
    });

    // Report errors if any.
    if !conflicts_mut_mut.is_empty() {
        // Report mutability conflicts for e.g. `ref mut x @ Some(ref mut y)`.
        let mut err = sess
            .struct_span_err(pat.span, "cannot borrow value as mutable more than once at a time");
        err.span_label(binding_span, format!("first mutable borrow, by `{}`, occurs here", name));
        for (span, name) in conflicts_mut_mut {
            err.span_label(span, format!("another mutable borrow, by `{}`, occurs here", name));
        }
        for (span, name) in conflicts_mut_ref {
            err.span_label(span, format!("also borrowed as immutable, by `{}`, here", name));
        }
        for (span, name) in conflicts_move {
            err.span_label(span, format!("also moved into `{}` here", name));
        }
        err.emit();
    } else if !conflicts_mut_ref.is_empty() {
        // Report mutability conflicts for e.g. `ref x @ Some(ref mut y)` or the converse.
        let (primary, also) = match mut_outer {
            Mutability::Mut => ("mutable", "immutable"),
            Mutability::Not => ("immutable", "mutable"),
        };
        let msg =
            format!("cannot borrow value as {} because it is also borrowed as {}", also, primary);
        let mut err = sess.struct_span_err(pat.span, &msg);
        err.span_label(binding_span, format!("{} borrow, by `{}`, occurs here", primary, name));
        for (span, name) in conflicts_mut_ref {
            err.span_label(span, format!("{} borrow, by `{}`, occurs here", also, name));
        }
        for (span, name) in conflicts_move {
            err.span_label(span, format!("also moved into `{}` here", name));
        }
        err.emit();
    } else if !conflicts_move.is_empty() {
        // Report by-ref and by-move conflicts, e.g. `ref x @ y`.
        let mut err =
            sess.struct_span_err(pat.span, "cannot move out of value because it is borrowed");
        err.span_label(binding_span, format!("value borrowed, by `{}`, here", name));
        for (span, name) in conflicts_move {
            err.span_label(span, format!("value moved into `{}` here", name));
        }
        err.emit();
    }
}

#[derive(Clone, Copy, Debug)]
pub enum LetSource {
    GenericLet,
    IfLet,
    IfLetGuard,
    LetElse(Span),
    WhileLet,
}

fn let_source(tcx: TyCtxt<'_>, pat_id: HirId) -> LetSource {
    let hir = tcx.hir();

    let parent = hir.get_parent_node(pat_id);
    let_source_parent(tcx, parent, Some(pat_id))
}

fn let_source_parent(tcx: TyCtxt<'_>, parent: HirId, pat_id: Option<HirId>) -> LetSource {
    let hir = tcx.hir();

    let parent_node = hir.get(parent);

    match parent_node {
        hir::Node::Arm(hir::Arm {
            guard: Some(hir::Guard::IfLet(&hir::Let { pat: hir::Pat { hir_id, .. }, .. })),
            ..
        }) if Some(*hir_id) == pat_id => {
            return LetSource::IfLetGuard;
        }
        _ => {}
    }

    let parent_parent = hir.get_parent_node(parent);
    let parent_parent_node = hir.get(parent_parent);
    match parent_parent_node {
        hir::Node::Stmt(hir::Stmt { kind: hir::StmtKind::Local(_), span, .. }) => {
            return LetSource::LetElse(*span);
        }
        hir::Node::Arm(hir::Arm { guard: Some(hir::Guard::If(_)), .. }) => {
            return LetSource::IfLetGuard;
        }
        _ => {}
    }

    let parent_parent_parent = hir.get_parent_node(parent_parent);
    let parent_parent_parent_parent = hir.get_parent_node(parent_parent_parent);
    let parent_parent_parent_parent_node = hir.get(parent_parent_parent_parent);

    if let hir::Node::Expr(hir::Expr {
        kind: hir::ExprKind::Loop(_, _, hir::LoopSource::While, _),
        ..
    }) = parent_parent_parent_parent_node
    {
        return LetSource::WhileLet;
    }

    if let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::If(..), .. }) = parent_parent_node {
        return LetSource::IfLet;
    }

    LetSource::GenericLet
}