path: root/compiler/rustc_mir_build/src/thir/pattern/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, struct_span_err, Applicability, DiagnosticBuilder};
use rustc_hir as hir;
use rustc_hir::def::*;
use rustc_hir::def_id::DefId;
use rustc_hir::intravisit::{self, NestedVisitorMap, 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::{DesugaringKind, ExpnKind, Span};

crate fn check_match(tcx: TyCtxt<'_>, def_id: DefId) {
    let body_id = match def_id.as_local() {
        None => return,
        Some(id) => tcx.hir().body_owned_by(tcx.hir().local_def_id_to_hir_id(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<'_> {
    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> {
    type Map = intravisit::ErasedMap<'tcx>;

    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
        NestedVisitorMap::None
    }

    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),
            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 (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),
        };
        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);
        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,
    ) {
        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 pat, _)) = arm.guard {
                self.check_patterns(pat, Refutable);
                let tpat = self.lower_pattern(&mut cx, pat, &mut false);
                check_let_reachability(&mut cx, 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 is_empty_match = arms.is_empty();
        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, is_empty_match);
            }
        }
    }

    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 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
            }
            _ => {
                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 let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
                err.span_suggestion(
                    span,
                    "you might want to use `if let` to ignore the variant that isn't matched",
                    format!("if {} {{ /* */ }}", &snippet[..snippet.len() - 1]),
                    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 DiagnosticBuilder<'_>,
    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 {
            if 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();
                if let ty::Adt(edef, _) = pat_ty.kind() {
                    if 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,
                            |lint| {
                                let ty_path = cx.tcx.def_path_str(edef.did);
                                let mut err = lint.build(&format!(
                                    "pattern binding `{}` is named the same as one \
                                                    of the variants of the type `{}`",
                                    ident, ty_path
                                ));
                                err.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 {
                                    err.span_suggestion(
                                        p.span,
                                        "to match on the variant, qualify the path",
                                        format!("{}::{}", ty_path, ident),
                                        Applicability::MachineApplicable,
                                    );
                                }
                                err.emit();
                            },
                        )
                    }
                }
            }
        }
    });
}

/// 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, |lint| {
        let mut err = lint.build("unreachable pattern");
        if let Some(catchall) = catchall {
            // We had a catchall pattern, hint at that.
            err.span_label(span, "unreachable pattern");
            err.span_label(catchall, "matches any value");
        }
        err.emit();
    });
}

fn irrefutable_let_pattern(tcx: TyCtxt<'_>, id: HirId, span: Span) {
    macro_rules! emit_diag {
        (
            $lint:expr,
            $source_name:expr,
            $note_sufix:expr,
            $help_sufix:expr
        ) => {{
            let mut diag = $lint.build(concat!("irrefutable ", $source_name, " pattern"));
            diag.note(concat!("this pattern will always match, so the ", $note_sufix));
            diag.help(concat!("consider ", $help_sufix));
            diag.emit()
        }};
    }

    let source = let_source(tcx, id);
    let span = match source {
        LetSource::LetElse(span) => span,
        _ => span,
    };
    tcx.struct_span_lint_hir(IRREFUTABLE_LET_PATTERNS, id, span, |lint| 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 check_let_reachability<'p, 'tcx>(
    cx: &mut MatchCheckCtxt<'p, 'tcx>,
    pat_id: HirId,
    pat: &'p DeconstructedPat<'p, 'tcx>,
    span: Span,
) {
    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 report.non_exhaustiveness_witnesses.is_empty() {
        // The match is exhaustive, i.e. the `if let` pattern is irrefutable.
        irrefutable_let_pattern(cx.tcx, pat_id, span);
    }
}

/// 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>>,
    is_empty_match: bool,
) {
    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;
    if is_empty_match && !non_empty_enum {
        err = create_e0004(
            cx.tcx.sess,
            sp,
            format!("non-exhaustive patterns: type `{}` is non-empty", scrut_ty),
        );
    } 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));
    };

    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.help(
        "ensure that all possible cases are being handled, \
              possibly by adding wildcards or more match arms",
    );
    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");
        }
    }
    err.emit();
}

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())
        }
    }
}

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 DiagnosticBuilder<'_>,
    ty: Ty<'tcx>,
    witnesses: &[DeconstructedPat<'p, 'tcx>],
) {
    let ty = ty.peel_refs();
    if let ty::Adt(def, _) = ty.kind() {
        if let Some(sp) = cx.tcx.hir().span_if_local(def.did) {
            err.span_label(sp, format!("`{}` defined here", ty));
        }

        if witnesses.len() < 4 {
            for sp in maybe_point_at_variant(cx, def, witnesses.iter()) {
                err.span_label(sp, "not covered");
            }
        }
    }
}

fn maybe_point_at_variant<'a, 'p: 'a, 'tcx: 'a>(
    cx: &MatchCheckCtxt<'p, 'tcx>,
    def: &AdtDef,
    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() {
                if this_def.did != def.did {
                    continue;
                }
            }
            let sp = def.variants[*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, span: Span) -> bool {
    !cx.typeck_results.node_type(hir_id).is_copy_modulo_regions(cx.tcx.at(span), 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, pat.span) => {
            // 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, span) => {
                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);
    match hir.get(parent) {
        hir::Node::Arm(hir::Arm {
            guard: Some(hir::Guard::IfLet(&hir::Pat { hir_id, .. }, _)),
            ..
        }) if hir_id == pat_id => {
            return LetSource::IfLetGuard;
        }
        hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Let(..), span, .. }) => {
            let expn_data = span.ctxt().outer_expn_data();
            if let ExpnKind::Desugaring(DesugaringKind::LetElse) = expn_data.kind {
                return LetSource::LetElse(expn_data.call_site);
            }
        }
        _ => {}
    }
    let parent_parent = hir.get_parent_node(parent);
    let parent_parent_node = hir.get(parent_parent);

    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
    {
        LetSource::WhileLet
    } else if let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::If { .. }, .. }) =
        parent_parent_node
    {
        LetSource::IfLet
    } else {
        LetSource::GenericLet
    }
}