mutability_errors.rs

use rustc_hir as hir;
use rustc_hir::Node;
use rustc_middle::hir::map::Map;
use rustc_middle::mir::{Mutability, Place, PlaceRef, ProjectionElem};
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_middle::{
    hir::place::PlaceBase,
    mir::{
        self, BindingForm, ClearCrossCrate, ImplicitSelfKind, Local, LocalDecl, LocalInfo,
        LocalKind, Location,
    },
};
use rustc_span::source_map::DesugaringKind;
use rustc_span::symbol::{kw, Symbol};
use rustc_span::{BytePos, Span};

use crate::diagnostics::BorrowedContentSource;
use crate::MirBorrowckCtxt;
use rustc_const_eval::util::collect_writes::FindAssignments;
use rustc_errors::{Applicability, Diagnostic};

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub(crate) enum AccessKind {
    MutableBorrow,
    Mutate,
}

impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
    pub(crate) fn report_mutability_error(
        &mut self,
        access_place: Place<'tcx>,
        span: Span,
        the_place_err: PlaceRef<'tcx>,
        error_access: AccessKind,
        location: Location,
    ) {
        debug!(
            "report_mutability_error(\
                access_place={:?}, span={:?}, the_place_err={:?}, error_access={:?}, location={:?},\
            )",
            access_place, span, the_place_err, error_access, location,
        );

        let mut err;
        let item_msg;
        let reason;
        let mut opt_source = None;
        let access_place_desc = self.describe_any_place(access_place.as_ref());
        debug!("report_mutability_error: access_place_desc={:?}", access_place_desc);

        match the_place_err {
            PlaceRef { local, projection: [] } => {
                item_msg = access_place_desc;
                if access_place.as_local().is_some() {
                    reason = ", as it is not declared as mutable".to_string();
                } else {
                    let name = self.local_names[local].expect("immutable unnamed local");
                    reason = format!(", as `{name}` is not declared as mutable");
                }
            }

            PlaceRef {
                local,
                projection: [proj_base @ .., ProjectionElem::Field(upvar_index, _)],
            } => {
                debug_assert!(is_closure_or_generator(
                    Place::ty_from(local, proj_base, self.body, self.infcx.tcx).ty
                ));

                let imm_borrow_derefed = self.upvars[upvar_index.index()]
                    .place
                    .place
                    .deref_tys()
                    .any(|ty| matches!(ty.kind(), ty::Ref(.., hir::Mutability::Not)));

                // If the place is immutable then:
                //
                // - Either we deref an immutable ref to get to our final place.
                //    - We don't capture derefs of raw ptrs
                // - Or the final place is immut because the root variable of the capture
                //   isn't marked mut and we should suggest that to the user.
                if imm_borrow_derefed {
                    // If we deref an immutable ref then the suggestion here doesn't help.
                    return;
                } else {
                    item_msg = access_place_desc;
                    if self.is_upvar_field_projection(access_place.as_ref()).is_some() {
                        reason = ", as it is not declared as mutable".to_string();
                    } else {
                        let name = self.upvars[upvar_index.index()].place.to_string(self.infcx.tcx);
                        reason = format!(", as `{name}` is not declared as mutable");
                    }
                }
            }

            PlaceRef { local, projection: [ProjectionElem::Deref] }
                if self.body.local_decls[local].is_ref_for_guard() =>
            {
                item_msg = access_place_desc;
                reason = ", as it is immutable for the pattern guard".to_string();
            }
            PlaceRef { local, projection: [ProjectionElem::Deref] }
                if self.body.local_decls[local].is_ref_to_static() =>
            {
                if access_place.projection.len() == 1 {
                    item_msg = format!("immutable static item {access_place_desc}");
                    reason = String::new();
                } else {
                    item_msg = access_place_desc;
                    let local_info = &self.body.local_decls[local].local_info;
                    if let Some(box LocalInfo::StaticRef { def_id, .. }) = *local_info {
                        let static_name = &self.infcx.tcx.item_name(def_id);
                        reason = format!(", as `{static_name}` is an immutable static item");
                    } else {
                        bug!("is_ref_to_static return true, but not ref to static?");
                    }
                }
            }
            PlaceRef { local: _, projection: [proj_base @ .., ProjectionElem::Deref] } => {
                if the_place_err.local == ty::CAPTURE_STRUCT_LOCAL
                    && proj_base.is_empty()
                    && !self.upvars.is_empty()
                {
                    item_msg = access_place_desc;
                    debug_assert!(
                        self.body.local_decls[ty::CAPTURE_STRUCT_LOCAL].ty.is_region_ptr()
                    );
                    debug_assert!(is_closure_or_generator(
                        Place::ty_from(
                            the_place_err.local,
                            the_place_err.projection,
                            self.body,
                            self.infcx.tcx
                        )
                        .ty
                    ));

                    reason = if self.is_upvar_field_projection(access_place.as_ref()).is_some() {
                        ", as it is a captured variable in a `Fn` closure".to_string()
                    } else {
                        ", as `Fn` closures cannot mutate their captured variables".to_string()
                    }
                } else {
                    let source = self.borrowed_content_source(PlaceRef {
                        local: the_place_err.local,
                        projection: proj_base,
                    });
                    let pointer_type = source.describe_for_immutable_place(self.infcx.tcx);
                    opt_source = Some(source);
                    if let Some(desc) = self.describe_place(access_place.as_ref()) {
                        item_msg = format!("`{desc}`");
                        reason = match error_access {
                            AccessKind::Mutate => format!(", which is behind {pointer_type}"),
                            AccessKind::MutableBorrow => {
                                format!(", as it is behind {pointer_type}")
                            }
                        }
                    } else {
                        item_msg = format!("data in {pointer_type}");
                        reason = String::new();
                    }
                }
            }

            PlaceRef {
                local: _,
                projection:
                    [
                        ..,
                        ProjectionElem::Index(_)
                        | ProjectionElem::ConstantIndex { .. }
                        | ProjectionElem::Subslice { .. }
                        | ProjectionElem::Downcast(..),
                    ],
            } => bug!("Unexpected immutable place."),
        }

        debug!("report_mutability_error: item_msg={:?}, reason={:?}", item_msg, reason);

        // `act` and `acted_on` are strings that let us abstract over
        // the verbs used in some diagnostic messages.
        let act;
        let acted_on;

        let span = match error_access {
            AccessKind::Mutate => {
                err = self.cannot_assign(span, &(item_msg + &reason));
                act = "assign";
                acted_on = "written";
                span
            }
            AccessKind::MutableBorrow => {
                act = "borrow as mutable";
                acted_on = "borrowed as mutable";

                let borrow_spans = self.borrow_spans(span, location);
                let borrow_span = borrow_spans.args_or_use();
                err = self.cannot_borrow_path_as_mutable_because(borrow_span, &item_msg, &reason);
                borrow_spans.var_span_label(
                    &mut err,
                    format!(
                        "mutable borrow occurs due to use of {} in closure",
                        self.describe_any_place(access_place.as_ref()),
                    ),
                    "mutable",
                );
                borrow_span
            }
        };

        debug!("report_mutability_error: act={:?}, acted_on={:?}", act, acted_on);

        match the_place_err {
            // Suggest making an existing shared borrow in a struct definition a mutable borrow.
            //
            // This is applicable when we have a deref of a field access to a deref of a local -
            // something like `*((*_1).0`. The local that we get will be a reference to the
            // struct we've got a field access of (it must be a reference since there's a deref
            // after the field access).
            PlaceRef {
                local,
                projection:
                    &[
                        ref proj_base @ ..,
                        ProjectionElem::Deref,
                        ProjectionElem::Field(field, _),
                        ProjectionElem::Deref,
                    ],
            } => {
                err.span_label(span, format!("cannot {ACT}", ACT = act));

                if let Some(span) = get_mut_span_in_struct_field(
                    self.infcx.tcx,
                    Place::ty_from(local, proj_base, self.body, self.infcx.tcx).ty,
                    field,
                ) {
                    err.span_suggestion_verbose(
                        span,
                        "consider changing this to be mutable",
                        " mut ",
                        Applicability::MaybeIncorrect,
                    );
                }
            }

            // Suggest removing a `&mut` from the use of a mutable reference.
            PlaceRef { local, projection: [] }
                if self
                    .body
                    .local_decls
                    .get(local)
                    .map(|l| mut_borrow_of_mutable_ref(l, self.local_names[local]))
                    .unwrap_or(false) =>
            {
                let decl = &self.body.local_decls[local];
                err.span_label(span, format!("cannot {ACT}", ACT = act));
                if let Some(mir::Statement {
                    source_info,
                    kind:
                        mir::StatementKind::Assign(box (
                            _,
                            mir::Rvalue::Ref(
                                _,
                                mir::BorrowKind::Mut { allow_two_phase_borrow: false },
                                _,
                            ),
                        )),
                    ..
                }) = &self.body[location.block].statements.get(location.statement_index)
                {
                    match decl.local_info {
                        Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
                            mir::VarBindingForm {
                                binding_mode: ty::BindingMode::BindByValue(Mutability::Not),
                                opt_ty_info: Some(sp),
                                opt_match_place: _,
                                pat_span: _,
                            },
                        )))) => {
                            err.span_note(sp, "the binding is already a mutable borrow");
                        }
                        _ => {
                            err.span_note(
                                decl.source_info.span,
                                "the binding is already a mutable borrow",
                            );
                        }
                    }
                    if let Ok(snippet) =
                        self.infcx.tcx.sess.source_map().span_to_snippet(source_info.span)
                    {
                        if snippet.starts_with("&mut ") {
                            // We don't have access to the HIR to get accurate spans, but we can
                            // give a best effort structured suggestion.
                            err.span_suggestion_verbose(
                                source_info.span.with_hi(source_info.span.lo() + BytePos(5)),
                                "try removing `&mut` here",
                                "",
                                Applicability::MachineApplicable,
                            );
                        } else {
                            // This can occur with things like `(&mut self).foo()`.
                            err.span_help(source_info.span, "try removing `&mut` here");
                        }
                    } else {
                        err.span_help(source_info.span, "try removing `&mut` here");
                    }
                } else if decl.mutability == Mutability::Not
                    && !matches!(
                        decl.local_info,
                        Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::ImplicitSelf(
                            ImplicitSelfKind::MutRef
                        ))))
                    )
                {
                    err.span_suggestion_verbose(
                        decl.source_info.span.shrink_to_lo(),
                        "consider making the binding mutable",
                        "mut ",
                        Applicability::MachineApplicable,
                    );
                }
            }

            // We want to suggest users use `let mut` for local (user
            // variable) mutations...
            PlaceRef { local, projection: [] }
                if self.body.local_decls[local].can_be_made_mutable() =>
            {
                // ... but it doesn't make sense to suggest it on
                // variables that are `ref x`, `ref mut x`, `&self`,
                // or `&mut self` (such variables are simply not
                // mutable).
                let local_decl = &self.body.local_decls[local];
                assert_eq!(local_decl.mutability, Mutability::Not);

                err.span_label(span, format!("cannot {ACT}", ACT = act));
                err.span_suggestion(
                    local_decl.source_info.span,
                    "consider changing this to be mutable",
                    format!("mut {}", self.local_names[local].unwrap()),
                    Applicability::MachineApplicable,
                );
                let tcx = self.infcx.tcx;
                if let ty::Closure(id, _) = *the_place_err.ty(self.body, tcx).ty.kind() {
                    self.show_mutating_upvar(tcx, id, the_place_err, &mut err);
                }
            }

            // Also suggest adding mut for upvars
            PlaceRef {
                local,
                projection: [proj_base @ .., ProjectionElem::Field(upvar_index, _)],
            } => {
                debug_assert!(is_closure_or_generator(
                    Place::ty_from(local, proj_base, self.body, self.infcx.tcx).ty
                ));

                let captured_place = &self.upvars[upvar_index.index()].place;

                err.span_label(span, format!("cannot {ACT}", ACT = act));

                let upvar_hir_id = captured_place.get_root_variable();

                if let Some(Node::Binding(pat)) = self.infcx.tcx.hir().find(upvar_hir_id)
                    && let hir::PatKind::Binding(
                        hir::BindingAnnotation::Unannotated,
                        _,
                        upvar_ident,
                        _,
                    ) = pat.kind
                {
                    err.span_suggestion(
                        upvar_ident.span,
                        "consider changing this to be mutable",
                        format!("mut {}", upvar_ident.name),
                        Applicability::MachineApplicable,
                    );
                }

                let tcx = self.infcx.tcx;
                if let ty::Ref(_, ty, Mutability::Mut) = the_place_err.ty(self.body, tcx).ty.kind()
                    && let ty::Closure(id, _) = *ty.kind()
                {
                    self.show_mutating_upvar(tcx, id, the_place_err, &mut err);
                }
            }

            // complete hack to approximate old AST-borrowck
            // diagnostic: if the span starts with a mutable borrow of
            // a local variable, then just suggest the user remove it.
            PlaceRef { local: _, projection: [] }
                if {
                    if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
                        snippet.starts_with("&mut ")
                    } else {
                        false
                    }
                } =>
            {
                err.span_label(span, format!("cannot {ACT}", ACT = act));
                err.span_suggestion(
                    span,
                    "try removing `&mut` here",
                    "",
                    Applicability::MaybeIncorrect,
                );
            }

            PlaceRef { local, projection: [ProjectionElem::Deref] }
                if self.body.local_decls[local].is_ref_for_guard() =>
            {
                err.span_label(span, format!("cannot {ACT}", ACT = act));
                err.note(
                    "variables bound in patterns are immutable until the end of the pattern guard",
                );
            }

            // We want to point out when a `&` can be readily replaced
            // with an `&mut`.
            //
            // FIXME: can this case be generalized to work for an
            // arbitrary base for the projection?
            PlaceRef { local, projection: [ProjectionElem::Deref] }
                if self.body.local_decls[local].is_user_variable() =>
            {
                let local_decl = &self.body.local_decls[local];

                let (pointer_sigil, pointer_desc) = if local_decl.ty.is_region_ptr() {
                    ("&", "reference")
                } else {
                    ("*const", "pointer")
                };

                match self.local_names[local] {
                    Some(name) if !local_decl.from_compiler_desugaring() => {
                        let label = match local_decl.local_info.as_deref().unwrap() {
                            LocalInfo::User(ClearCrossCrate::Set(
                                mir::BindingForm::ImplicitSelf(_),
                            )) => {
                                let (span, suggestion) =
                                    suggest_ampmut_self(self.infcx.tcx, local_decl);
                                Some((true, span, suggestion))
                            }

                            LocalInfo::User(ClearCrossCrate::Set(mir::BindingForm::Var(
                                mir::VarBindingForm {
                                    binding_mode: ty::BindingMode::BindByValue(_),
                                    opt_ty_info,
                                    ..
                                },
                            ))) => {
                                // check if the RHS is from desugaring
                                let opt_assignment_rhs_span =
                                    self.body.find_assignments(local).first().map(|&location| {
                                        if let Some(mir::Statement {
                                            source_info: _,
                                            kind:
                                                mir::StatementKind::Assign(box (
                                                    _,
                                                    mir::Rvalue::Use(mir::Operand::Copy(place)),
                                                )),
                                        }) = self.body[location.block]
                                            .statements
                                            .get(location.statement_index)
                                        {
                                            self.body.local_decls[place.local].source_info.span
                                        } else {
                                            self.body.source_info(location).span
                                        }
                                    });
                                match opt_assignment_rhs_span.and_then(|s| s.desugaring_kind()) {
                                    // on for loops, RHS points to the iterator part
                                    Some(DesugaringKind::ForLoop) => {
                                        self.suggest_similar_mut_method_for_for_loop(&mut err);
                                        err.span_label(opt_assignment_rhs_span.unwrap(), format!(
                                            "this iterator yields `{pointer_sigil}` {pointer_desc}s",
                                        ));
                                        None
                                    }
                                    // don't create labels for compiler-generated spans
                                    Some(_) => None,
                                    None => {
                                        let label = if name != kw::SelfLower {
                                            suggest_ampmut(
                                                self.infcx.tcx,
                                                local_decl,
                                                opt_assignment_rhs_span,
                                                *opt_ty_info,
                                            )
                                        } else {
                                            match local_decl.local_info.as_deref() {
                                                Some(LocalInfo::User(ClearCrossCrate::Set(
                                                    mir::BindingForm::Var(mir::VarBindingForm {
                                                        opt_ty_info: None,
                                                        ..
                                                    }),
                                                ))) => {
                                                    let (span, sugg) = suggest_ampmut_self(
                                                        self.infcx.tcx,
                                                        local_decl,
                                                    );
                                                    (true, span, sugg)
                                                }
                                                // explicit self (eg `self: &'a Self`)
                                                _ => suggest_ampmut(
                                                    self.infcx.tcx,
                                                    local_decl,
                                                    opt_assignment_rhs_span,
                                                    *opt_ty_info,
                                                ),
                                            }
                                        };
                                        Some(label)
                                    }
                                }
                            }

                            LocalInfo::User(ClearCrossCrate::Set(mir::BindingForm::Var(
                                mir::VarBindingForm {
                                    binding_mode: ty::BindingMode::BindByReference(_),
                                    ..
                                },
                            ))) => {
                                let pattern_span = local_decl.source_info.span;
                                suggest_ref_mut(self.infcx.tcx, pattern_span)
                                    .map(|replacement| (true, pattern_span, replacement))
                            }

                            LocalInfo::User(ClearCrossCrate::Clear) => {
                                bug!("saw cleared local state")
                            }

                            _ => unreachable!(),
                        };

                        match label {
                            Some((true, err_help_span, suggested_code)) => {
                                let (is_trait_sig, local_trait) = self.is_error_in_trait(local);
                                if !is_trait_sig {
                                    err.span_suggestion(
                                        err_help_span,
                                        &format!(
                                            "consider changing this to be a mutable {pointer_desc}"
                                        ),
                                        suggested_code,
                                        Applicability::MachineApplicable,
                                    );
                                } else if let Some(x) = local_trait {
                                    err.span_suggestion(
                                        x,
                                        &format!(
                                            "consider changing that to be a mutable {pointer_desc}"
                                        ),
                                        suggested_code,
                                        Applicability::MachineApplicable,
                                    );
                                }
                            }
                            Some((false, err_label_span, message)) => {
                                err.span_label(
                                    err_label_span,
                                    &format!(
                                        "consider changing this binding's type to be: `{message}`"
                                    ),
                                );
                            }
                            None => {}
                        }
                        err.span_label(
                            span,
                            format!(
                                "`{NAME}` is a `{SIGIL}` {DESC}, \
                                so the data it refers to cannot be {ACTED_ON}",
                                NAME = name,
                                SIGIL = pointer_sigil,
                                DESC = pointer_desc,
                                ACTED_ON = acted_on
                            ),
                        );
                    }
                    _ => {
                        err.span_label(
                            span,
                            format!(
                                "cannot {ACT} through `{SIGIL}` {DESC}",
                                ACT = act,
                                SIGIL = pointer_sigil,
                                DESC = pointer_desc
                            ),
                        );
                    }
                }
            }

            PlaceRef { local, projection: [ProjectionElem::Deref] }
                if local == ty::CAPTURE_STRUCT_LOCAL && !self.upvars.is_empty() =>
            {
                self.expected_fn_found_fn_mut_call(&mut err, span, act);
            }

            PlaceRef { local: _, projection: [.., ProjectionElem::Deref] } => {
                err.span_label(span, format!("cannot {ACT}", ACT = act));

                match opt_source {
                    Some(BorrowedContentSource::OverloadedDeref(ty)) => {
                        err.help(&format!(
                            "trait `DerefMut` is required to modify through a dereference, \
                                but it is not implemented for `{ty}`",
                        ));
                    }
                    Some(BorrowedContentSource::OverloadedIndex(ty)) => {
                        err.help(&format!(
                            "trait `IndexMut` is required to modify indexed content, \
                                but it is not implemented for `{ty}`",
                        ));
                    }
                    _ => (),
                }
            }

            _ => {
                err.span_label(span, format!("cannot {ACT}", ACT = act));
            }
        }

        self.buffer_error(err);
    }

    /// User cannot make signature of a trait mutable without changing the
    /// trait. So we find if this error belongs to a trait and if so we move
    /// suggestion to the trait or disable it if it is out of scope of this crate
    fn is_error_in_trait(&self, local: Local) -> (bool, Option<Span>) {
        if self.body.local_kind(local) != LocalKind::Arg {
            return (false, None);
        }
        let hir_map = self.infcx.tcx.hir();
        let my_def = self.body.source.def_id();
        let my_hir = hir_map.local_def_id_to_hir_id(my_def.as_local().unwrap());
        let Some(td) =
            self.infcx.tcx.impl_of_method(my_def).and_then(|x| self.infcx.tcx.trait_id_of_impl(x))
        else {
            return (false, None);
        };
        (
            true,
            td.as_local().and_then(|tld| match hir_map.find_by_def_id(tld) {
                Some(Node::Item(hir::Item {
                    kind: hir::ItemKind::Trait(_, _, _, _, items),
                    ..
                })) => {
                    let mut f_in_trait_opt = None;
                    for hir::TraitItemRef { id: fi, kind: k, .. } in *items {
                        let hi = fi.hir_id();
                        if !matches!(k, hir::AssocItemKind::Fn { .. }) {
                            continue;
                        }
                        if hir_map.name(hi) != hir_map.name(my_hir) {
                            continue;
                        }
                        f_in_trait_opt = Some(hi);
                        break;
                    }
                    f_in_trait_opt.and_then(|f_in_trait| match hir_map.find(f_in_trait) {
                        Some(Node::TraitItem(hir::TraitItem {
                            kind:
                                hir::TraitItemKind::Fn(
                                    hir::FnSig { decl: hir::FnDecl { inputs, .. }, .. },
                                    _,
                                ),
                            ..
                        })) => {
                            let hir::Ty { span, .. } = inputs[local.index() - 1];
                            Some(span)
                        }
                        _ => None,
                    })
                }
                _ => None,
            }),
        )
    }

    // point to span of upvar making closure call require mutable borrow
    fn show_mutating_upvar(
        &self,
        tcx: TyCtxt<'_>,
        id: hir::def_id::DefId,
        the_place_err: PlaceRef<'tcx>,
        err: &mut Diagnostic,
    ) {
        let closure_local_def_id = id.expect_local();
        let tables = tcx.typeck(closure_local_def_id);
        let closure_hir_id = tcx.hir().local_def_id_to_hir_id(closure_local_def_id);
        if let Some((span, closure_kind_origin)) =
            &tables.closure_kind_origins().get(closure_hir_id)
        {
            let reason = if let PlaceBase::Upvar(upvar_id) = closure_kind_origin.base {
                let upvar = ty::place_to_string_for_capture(tcx, closure_kind_origin);
                let root_hir_id = upvar_id.var_path.hir_id;
                // we have an origin for this closure kind starting at this root variable so it's safe to unwrap here
                let captured_places = tables.closure_min_captures[&id].get(&root_hir_id).unwrap();

                let origin_projection = closure_kind_origin
                    .projections
                    .iter()
                    .map(|proj| proj.kind)
                    .collect::<Vec<_>>();
                let mut capture_reason = String::new();
                for captured_place in captured_places {
                    let captured_place_kinds = captured_place
                        .place
                        .projections
                        .iter()
                        .map(|proj| proj.kind)
                        .collect::<Vec<_>>();
                    if rustc_middle::ty::is_ancestor_or_same_capture(
                        &captured_place_kinds,
                        &origin_projection,
                    ) {
                        match captured_place.info.capture_kind {
                            ty::UpvarCapture::ByRef(
                                ty::BorrowKind::MutBorrow | ty::BorrowKind::UniqueImmBorrow,
                            ) => {
                                capture_reason = format!("mutable borrow of `{upvar}`");
                            }
                            ty::UpvarCapture::ByValue => {
                                capture_reason = format!("possible mutation of `{upvar}`");
                            }
                            _ => bug!("upvar `{upvar}` borrowed, but not mutably"),
                        }
                        break;
                    }
                }
                if capture_reason.is_empty() {
                    bug!("upvar `{upvar}` borrowed, but cannot find reason");
                }
                capture_reason
            } else {
                bug!("not an upvar")
            };
            err.span_label(
                *span,
                format!(
                    "calling `{}` requires mutable binding due to {}",
                    self.describe_place(the_place_err).unwrap(),
                    reason
                ),
            );
        }
    }

    // Attempt to search similar mutable associated items for suggestion.
    // In the future, attempt in all path but initially for RHS of for_loop
    fn suggest_similar_mut_method_for_for_loop(&self, err: &mut Diagnostic) {
        use hir::{
            BodyId, Expr,
            ExprKind::{Block, Call, DropTemps, Match, MethodCall},
            HirId, ImplItem, ImplItemKind, Item, ItemKind,
        };

        fn maybe_body_id_of_fn(hir_map: Map<'_>, id: HirId) -> Option<BodyId> {
            match hir_map.find(id) {
                Some(Node::Item(Item { kind: ItemKind::Fn(_, _, body_id), .. }))
                | Some(Node::ImplItem(ImplItem { kind: ImplItemKind::Fn(_, body_id), .. })) => {
                    Some(*body_id)
                }
                _ => None,
            }
        }
        let hir_map = self.infcx.tcx.hir();
        let mir_body_hir_id = self.mir_hir_id();
        if let Some(fn_body_id) = maybe_body_id_of_fn(hir_map, mir_body_hir_id) {
            if let Block(
                hir::Block {
                    expr:
                        Some(Expr {
                            kind:
                                DropTemps(Expr {
                                    kind:
                                        Match(
                                            Expr {
                                                kind:
                                                    Call(
                                                        _,
                                                        [
                                                            Expr {
                                                                kind:
                                                                    MethodCall(
                                                                        path_segment,
                                                                        _args,
                                                                        span,
                                                                    ),
                                                                hir_id,
                                                                ..
                                                            },
                                                            ..,
                                                        ],
                                                    ),
                                                ..
                                            },
                                            ..,
                                        ),
                                    ..
                                }),
                            ..
                        }),
                    ..
                },
                _,
            ) = hir_map.body(fn_body_id).value.kind
            {
                let opt_suggestions = path_segment
                    .hir_id
                    .map(|path_hir_id| self.infcx.tcx.typeck(path_hir_id.owner))
                    .and_then(|typeck| typeck.type_dependent_def_id(*hir_id))
                    .and_then(|def_id| self.infcx.tcx.impl_of_method(def_id))
                    .map(|def_id| self.infcx.tcx.associated_items(def_id))
                    .map(|assoc_items| {
                        assoc_items
                            .in_definition_order()
                            .map(|assoc_item_def| assoc_item_def.ident(self.infcx.tcx))
                            .filter(|&ident| {
                                let original_method_ident = path_segment.ident;
                                original_method_ident != ident
                                    && ident
                                        .as_str()
                                        .starts_with(&original_method_ident.name.to_string())
                            })
                            .map(|ident| format!("{ident}()"))
                            .peekable()
                    });

                if let Some(mut suggestions) = opt_suggestions
                    && suggestions.peek().is_some()
                {
                    err.span_suggestions(
                        *span,
                        "use mutable method",
                        suggestions,
                        Applicability::MaybeIncorrect,
                    );
                }
            }
        };
    }

    /// Targeted error when encountering an `FnMut` closure where an `Fn` closure was expected.
    fn expected_fn_found_fn_mut_call(&self, err: &mut Diagnostic, sp: Span, act: &str) {
        err.span_label(sp, format!("cannot {act}"));

        let hir = self.infcx.tcx.hir();
        let closure_id = self.mir_hir_id();
        let fn_call_id = hir.get_parent_node(closure_id);
        let node = hir.get(fn_call_id);
        let item_id = hir.enclosing_body_owner(fn_call_id);
        let mut look_at_return = true;
        // If we can detect the expression to be an `fn` call where the closure was an argument,
        // we point at the `fn` definition argument...
        if let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Call(func, args), .. }) = node {
            let arg_pos = args
                .iter()
                .enumerate()
                .filter(|(_, arg)| arg.span == self.body.span)
                .map(|(pos, _)| pos)
                .next();
            let def_id = hir.local_def_id(item_id);
            let tables = self.infcx.tcx.typeck(def_id);
            if let Some(ty::FnDef(def_id, _)) =
                tables.node_type_opt(func.hir_id).as_ref().map(|ty| ty.kind())
            {
                let arg = match hir.get_if_local(*def_id) {
                    Some(
                        hir::Node::Item(hir::Item {
                            ident, kind: hir::ItemKind::Fn(sig, ..), ..
                        })
                        | hir::Node::TraitItem(hir::TraitItem {
                            ident,
                            kind: hir::TraitItemKind::Fn(sig, _),
                            ..
                        })
                        | hir::Node::ImplItem(hir::ImplItem {
                            ident,
                            kind: hir::ImplItemKind::Fn(sig, _),
                            ..
                        }),
                    ) => Some(
                        arg_pos
                            .and_then(|pos| {
                                sig.decl.inputs.get(
                                    pos + if sig.decl.implicit_self.has_implicit_self() {
                                        1
                                    } else {
                                        0
                                    },
                                )
                            })
                            .map(|arg| arg.span)
                            .unwrap_or(ident.span),
                    ),
                    _ => None,
                };
                if let Some(span) = arg {
                    err.span_label(span, "change this to accept `FnMut` instead of `Fn`");
                    err.span_label(func.span, "expects `Fn` instead of `FnMut`");
                    if self.infcx.tcx.sess.source_map().is_multiline(self.body.span) {
                        err.span_label(self.body.span, "in this closure");
                    }
                    look_at_return = false;
                }
            }
        }

        if look_at_return && hir.get_return_block(closure_id).is_some() {
            // ...otherwise we are probably in the tail expression of the function, point at the
            // return type.
            match hir.get_by_def_id(hir.get_parent_item(fn_call_id)) {
                hir::Node::Item(hir::Item { ident, kind: hir::ItemKind::Fn(sig, ..), .. })
                | hir::Node::TraitItem(hir::TraitItem {
                    ident,
                    kind: hir::TraitItemKind::Fn(sig, _),
                    ..
                })
                | hir::Node::ImplItem(hir::ImplItem {
                    ident,
                    kind: hir::ImplItemKind::Fn(sig, _),
                    ..
                }) => {
                    err.span_label(ident.span, "");
                    err.span_label(
                        sig.decl.output.span(),
                        "change this to return `FnMut` instead of `Fn`",
                    );
                    err.span_label(self.body.span, "in this closure");
                }
                _ => {}
            }
        }
    }
}

fn mut_borrow_of_mutable_ref(local_decl: &LocalDecl<'_>, local_name: Option<Symbol>) -> bool {
    debug!("local_info: {:?}, ty.kind(): {:?}", local_decl.local_info, local_decl.ty.kind());

    match local_decl.local_info.as_deref() {
        // Check if mutably borrowing a mutable reference.
        Some(LocalInfo::User(ClearCrossCrate::Set(mir::BindingForm::Var(
            mir::VarBindingForm {
                binding_mode: ty::BindingMode::BindByValue(Mutability::Not), ..
            },
        )))) => matches!(local_decl.ty.kind(), ty::Ref(_, _, hir::Mutability::Mut)),
        Some(LocalInfo::User(ClearCrossCrate::Set(mir::BindingForm::ImplicitSelf(kind)))) => {
            // Check if the user variable is a `&mut self` and we can therefore
            // suggest removing the `&mut`.
            //
            // Deliberately fall into this case for all implicit self types,
            // so that we don't fall in to the next case with them.
            *kind == mir::ImplicitSelfKind::MutRef
        }
        _ if Some(kw::SelfLower) == local_name => {
            // Otherwise, check if the name is the `self` keyword - in which case
            // we have an explicit self. Do the same thing in this case and check
            // for a `self: &mut Self` to suggest removing the `&mut`.
            matches!(local_decl.ty.kind(), ty::Ref(_, _, hir::Mutability::Mut))
        }
        _ => false,
    }
}

fn suggest_ampmut_self<'tcx>(
    tcx: TyCtxt<'tcx>,
    local_decl: &mir::LocalDecl<'tcx>,
) -> (Span, String) {
    let sp = local_decl.source_info.span;
    (
        sp,
        match tcx.sess.source_map().span_to_snippet(sp) {
            Ok(snippet) => {
                let lt_pos = snippet.find('\'');
                if let Some(lt_pos) = lt_pos {
                    format!("&{}mut self", &snippet[lt_pos..snippet.len() - 4])
                } else {
                    "&mut self".to_string()
                }
            }
            _ => "&mut self".to_string(),
        },
    )
}

// When we want to suggest a user change a local variable to be a `&mut`, there
// are three potential "obvious" things to highlight:
//
// let ident [: Type] [= RightHandSideExpression];
//     ^^^^^    ^^^^     ^^^^^^^^^^^^^^^^^^^^^^^
//     (1.)     (2.)              (3.)
//
// We can always fallback on highlighting the first. But chances are good that
// the user experience will be better if we highlight one of the others if possible;
// for example, if the RHS is present and the Type is not, then the type is going to
// be inferred *from* the RHS, which means we should highlight that (and suggest
// that they borrow the RHS mutably).
//
// This implementation attempts to emulate AST-borrowck prioritization
// by trying (3.), then (2.) and finally falling back on (1.).
fn suggest_ampmut<'tcx>(
    tcx: TyCtxt<'tcx>,
    local_decl: &mir::LocalDecl<'tcx>,
    opt_assignment_rhs_span: Option<Span>,
    opt_ty_info: Option<Span>,
) -> (bool, Span, String) {
    if let Some(assignment_rhs_span) = opt_assignment_rhs_span
        && let Ok(src) = tcx.sess.source_map().span_to_snippet(assignment_rhs_span)
    {
        let is_mutbl = |ty: &str| -> bool {
            if let Some(rest) = ty.strip_prefix("mut") {
                match rest.chars().next() {
                    // e.g. `&mut x`
                    Some(c) if c.is_whitespace() => true,
                    // e.g. `&mut(x)`
                    Some('(') => true,
                    // e.g. `&mut{x}`
                    Some('{') => true,
                    // e.g. `&mutablevar`
                    _ => false,
                }
            } else {
                false
            }
        };
        if let (true, Some(ws_pos)) = (src.starts_with("&'"), src.find(char::is_whitespace)) {
            let lt_name = &src[1..ws_pos];
            let ty = src[ws_pos..].trim_start();
            if !is_mutbl(ty) {
                return (true, assignment_rhs_span, format!("&{lt_name} mut {ty}"));
            }
        } else if let Some(stripped) = src.strip_prefix('&') {
            let stripped = stripped.trim_start();
            if !is_mutbl(stripped) {
                return (true, assignment_rhs_span, format!("&mut {stripped}"));
            }
        }
    }

    let (suggestability, highlight_span) = match opt_ty_info {
        // if this is a variable binding with an explicit type,
        // try to highlight that for the suggestion.
        Some(ty_span) => (true, ty_span),

        // otherwise, just highlight the span associated with
        // the (MIR) LocalDecl.
        None => (false, local_decl.source_info.span),
    };

    if let Ok(src) = tcx.sess.source_map().span_to_snippet(highlight_span)
        && let (true, Some(ws_pos)) = (src.starts_with("&'"), src.find(char::is_whitespace))
    {
        let lt_name = &src[1..ws_pos];
        let ty = &src[ws_pos..];
        return (true, highlight_span, format!("&{} mut{}", lt_name, ty));
    }

    let ty_mut = local_decl.ty.builtin_deref(true).unwrap();
    assert_eq!(ty_mut.mutbl, hir::Mutability::Not);
    (
        suggestability,
        highlight_span,
        if local_decl.ty.is_region_ptr() {
            format!("&mut {}", ty_mut.ty)
        } else {
            format!("*mut {}", ty_mut.ty)
        },
    )
}

fn is_closure_or_generator(ty: Ty<'_>) -> bool {
    ty.is_closure() || ty.is_generator()
}

/// Given a field that needs to be mutable, returns a span where the " mut " could go.
/// This function expects the local to be a reference to a struct in order to produce a span.
///
/// ```text
/// LL |     s: &'a   String
///    |           ^^^ returns a span taking up the space here
/// ```
fn get_mut_span_in_struct_field<'tcx>(
    tcx: TyCtxt<'tcx>,
    ty: Ty<'tcx>,
    field: mir::Field,
) -> Option<Span> {
    // Expect our local to be a reference to a struct of some kind.
    if let ty::Ref(_, ty, _) = ty.kind()
        && let ty::Adt(def, _) = ty.kind()
        && let field = def.all_fields().nth(field.index())?
        // Use the HIR types to construct the diagnostic message.
        && let node = tcx.hir().find_by_def_id(field.did.as_local()?)?
        // Now we're dealing with the actual struct that we're going to suggest a change to,
        // we can expect a field that is an immutable reference to a type.
        && let hir::Node::Field(field) = node
        && let hir::TyKind::Rptr(lt, hir::MutTy { mutbl: hir::Mutability::Not, ty }) = field.ty.kind
    {
        return Some(lt.span.between(ty.span));
    }

    None
}

/// If possible, suggest replacing `ref` with `ref mut`.
fn suggest_ref_mut(tcx: TyCtxt<'_>, binding_span: Span) -> Option<String> {
    let hi_src = tcx.sess.source_map().span_to_snippet(binding_span).ok()?;
    if hi_src.starts_with("ref") && hi_src["ref".len()..].starts_with(rustc_lexer::is_whitespace) {
        let replacement = format!("ref mut{}", &hi_src["ref".len()..]);
        Some(replacement)
    } else {
        None
    }
}