mutability_errors.rs

use hir::ExprKind;
use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
use rustc_hir as hir;
use rustc_hir::intravisit::Visitor;
use rustc_hir::Node;
use rustc_middle::mir::{Mutability, Place, PlaceRef, ProjectionElem};
use rustc_middle::ty::{self, InstanceDef, Ty, TyCtxt};
use rustc_middle::{
    hir::place::PlaceBase,
    mir::{self, BindingForm, Local, LocalDecl, LocalInfo, LocalKind, Location},
};
use rustc_span::symbol::{kw, Symbol};
use rustc_span::{sym, BytePos, DesugaringKind, Span};
use rustc_target::abi::FieldIdx;

use crate::diagnostics::BorrowedContentSource;
use crate::util::FindAssignments;
use crate::MirBorrowckCtxt;

#[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_coroutine(
                    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 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_ref());
                    debug_assert!(is_closure_or_coroutine(
                        the_place_err.ty(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::Subtype(_)
                        | ProjectionElem::ConstantIndex { .. }
                        | ProjectionElem::OpaqueCast { .. }
                        | 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 mut suggest = true;
        let mut mut_error = None;
        let mut count = 1;

        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();
                match the_place_err {
                    PlaceRef { local, projection: [] }
                        if self.body.local_decls[local].can_be_made_mutable() =>
                    {
                        let span = self.body.local_decls[local].source_info.span;
                        mut_error = Some(span);
                        if let Some((buffer, c)) = self.get_buffered_mut_error(span) {
                            // We've encountered a second (or more) attempt to mutably borrow an
                            // immutable binding, so the likely problem is with the binding
                            // declaration, not the use. We collect these in a single diagnostic
                            // and make the binding the primary span of the error.
                            err = buffer;
                            count = c + 1;
                            if count == 2 {
                                err.replace_span_with(span, false);
                                err.span_label(span, "not mutable");
                            }
                            suggest = false;
                        } else {
                            err = self.cannot_borrow_path_as_mutable_because(
                                borrow_span,
                                &item_msg,
                                &reason,
                            );
                        }
                    }
                    _ => {
                        err = self.cannot_borrow_path_as_mutable_because(
                            borrow_span,
                            &item_msg,
                            &reason,
                        );
                    }
                }
                if suggest {
                    borrow_spans.var_subdiag(
                        None,
                        &mut err,
                        Some(mir::BorrowKind::Mut { kind: mir::MutBorrowKind::Default }),
                        |_kind, var_span| {
                            let place = self.describe_any_place(access_place.as_ref());
                            crate::session_diagnostics::CaptureVarCause::MutableBorrowUsePlaceClosure {
                                place,
                                var_span,
                            }
                        },
                    );
                }
                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:
                    [
                        proj_base @ ..,
                        ProjectionElem::Deref,
                        ProjectionElem::Field(field, _),
                        ProjectionElem::Deref,
                    ],
            } => {
                err.span_label(span, format!("cannot {act}"));

                let place = Place::ty_from(local, proj_base, self.body, self.infcx.tcx);
                if let Some(span) = get_mut_span_in_struct_field(self.infcx.tcx, place.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)
                    .is_some_and(|l| mut_borrow_of_mutable_ref(l, self.local_names[local])) =>
            {
                let decl = &self.body.local_decls[local];
                err.span_label(span, format!("cannot {act}"));
                if let Some(mir::Statement {
                    source_info,
                    kind:
                        mir::StatementKind::Assign(box (
                            _,
                            mir::Rvalue::Ref(
                                _,
                                mir::BorrowKind::Mut { kind: mir::MutBorrowKind::Default },
                                _,
                            ),
                        )),
                    ..
                }) = &self.body[location.block].statements.get(location.statement_index)
                {
                    match *decl.local_info() {
                        LocalInfo::User(BindingForm::Var(mir::VarBindingForm {
                            binding_mode: ty::BindingMode::BindByValue(Mutability::Not),
                            opt_ty_info: Some(sp),
                            opt_match_place: _,
                            pat_span: _,
                        })) => {
                            if suggest {
                                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.is_not() {
                    if matches!(
                        decl.local_info(),
                        LocalInfo::User(BindingForm::ImplicitSelf(hir::ImplicitSelfKind::MutRef))
                    ) {
                        err.note(
                            "as `Self` may be unsized, this call attempts to take `&mut &mut self`",
                        );
                        err.note("however, `&mut self` expands to `self: &mut Self`, therefore `self` cannot be borrowed mutably");
                    } else {
                        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);

                if count < 10 {
                    err.span_label(span, format!("cannot {act}"));
                }
                if suggest {
                    self.construct_mut_suggestion_for_local_binding_patterns(&mut err, local);
                    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.expect_local(), 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_coroutine(
                    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}"));

                let upvar_hir_id = captured_place.get_root_variable();

                if let Some(Node::Pat(pat)) = self.infcx.tcx.hir().find(upvar_hir_id)
                    && let hir::PatKind::Binding(hir::BindingAnnotation::NONE, _, upvar_ident, _) =
                        pat.kind
                {
                    if upvar_ident.name == kw::SelfLower {
                        for (_, node) in self.infcx.tcx.hir().parent_iter(upvar_hir_id) {
                            if let Some(fn_decl) = node.fn_decl() {
                                if !matches!(
                                    fn_decl.implicit_self,
                                    hir::ImplicitSelfKind::ImmRef | hir::ImplicitSelfKind::MutRef
                                ) {
                                    err.span_suggestion(
                                        upvar_ident.span,
                                        "consider changing this to be mutable",
                                        format!("mut {}", upvar_ident.name),
                                        Applicability::MachineApplicable,
                                    );
                                    break;
                                }
                            }
                        }
                    } else {
                        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.expect_local(), 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 self
                    .infcx
                    .tcx
                    .sess
                    .source_map()
                    .span_to_snippet(span)
                    .is_ok_and(|snippet| snippet.starts_with("&mut ")) =>
            {
                err.span_label(span, format!("cannot {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}"));
                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_ref() { ("&", "reference") } else { ("*const", "pointer") };

                match self.local_names[local] {
                    Some(name) if !local_decl.from_compiler_desugaring() => {
                        err.span_label(
                            span,
                            format!(
                                "`{name}` is a `{pointer_sigil}` {pointer_desc}, \
                                 so the data it refers to cannot be {acted_on}",
                            ),
                        );

                        self.suggest_using_iter_mut(&mut err);
                        self.suggest_make_local_mut(&mut err, local, name);
                    }
                    _ => {
                        err.span_label(
                            span,
                            format!("cannot {act} through `{pointer_sigil}` {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}"));

                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}`",
                        ));
                        self.suggest_map_index_mut_alternatives(ty, &mut err, span);
                    }
                    _ => (),
                }
            }

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

        if let Some(span) = mut_error {
            self.buffer_mut_error(span, err, count);
        } else {
            self.buffer_error(err);
        }
    }

    fn suggest_map_index_mut_alternatives(&self, ty: Ty<'tcx>, err: &mut Diagnostic, span: Span) {
        let Some(adt) = ty.ty_adt_def() else { return };
        let did = adt.did();
        if self.infcx.tcx.is_diagnostic_item(sym::HashMap, did)
            || self.infcx.tcx.is_diagnostic_item(sym::BTreeMap, did)
        {
            struct V<'a, 'tcx> {
                assign_span: Span,
                err: &'a mut Diagnostic,
                ty: Ty<'tcx>,
                suggested: bool,
            }
            impl<'a, 'tcx> Visitor<'tcx> for V<'a, 'tcx> {
                fn visit_stmt(&mut self, stmt: &'tcx hir::Stmt<'tcx>) {
                    hir::intravisit::walk_stmt(self, stmt);
                    let expr = match stmt.kind {
                        hir::StmtKind::Semi(expr) | hir::StmtKind::Expr(expr) => expr,
                        hir::StmtKind::Local(hir::Local { init: Some(expr), .. }) => expr,
                        _ => {
                            return;
                        }
                    };
                    if let hir::ExprKind::Assign(place, rv, _sp) = expr.kind
                        && let hir::ExprKind::Index(val, index, _) = place.kind
                        && (expr.span == self.assign_span || place.span == self.assign_span)
                    {
                        // val[index] = rv;
                        // ---------- place
                        self.err.multipart_suggestions(
                            format!(
                                "to modify a `{}`, use `.get_mut()`, `.insert()` or the entry API",
                                self.ty,
                            ),
                            vec![
                                vec![
                                    // val.insert(index, rv);
                                    (
                                        val.span.shrink_to_hi().with_hi(index.span.lo()),
                                        ".insert(".to_string(),
                                    ),
                                    (
                                        index.span.shrink_to_hi().with_hi(rv.span.lo()),
                                        ", ".to_string(),
                                    ),
                                    (rv.span.shrink_to_hi(), ")".to_string()),
                                ],
                                vec![
                                    // val.get_mut(index).map(|v| { *v = rv; });
                                    (
                                        val.span.shrink_to_hi().with_hi(index.span.lo()),
                                        ".get_mut(".to_string(),
                                    ),
                                    (
                                        index.span.shrink_to_hi().with_hi(place.span.hi()),
                                        ").map(|val| { *val".to_string(),
                                    ),
                                    (rv.span.shrink_to_hi(), "; })".to_string()),
                                ],
                                vec![
                                    // let x = val.entry(index).or_insert(rv);
                                    (val.span.shrink_to_lo(), "let val = ".to_string()),
                                    (
                                        val.span.shrink_to_hi().with_hi(index.span.lo()),
                                        ".entry(".to_string(),
                                    ),
                                    (
                                        index.span.shrink_to_hi().with_hi(rv.span.lo()),
                                        ").or_insert(".to_string(),
                                    ),
                                    (rv.span.shrink_to_hi(), ")".to_string()),
                                ],
                            ],
                            Applicability::MachineApplicable,
                        );
                        self.suggested = true;
                    } else if let hir::ExprKind::MethodCall(_path, receiver, _, sp) = expr.kind
                        && let hir::ExprKind::Index(val, index, _) = receiver.kind
                        && expr.span == self.assign_span
                    {
                        // val[index].path(args..);
                        self.err.multipart_suggestion(
                            format!("to modify a `{}` use `.get_mut()`", self.ty),
                            vec![
                                (
                                    val.span.shrink_to_hi().with_hi(index.span.lo()),
                                    ".get_mut(".to_string(),
                                ),
                                (
                                    index.span.shrink_to_hi().with_hi(receiver.span.hi()),
                                    ").map(|val| val".to_string(),
                                ),
                                (sp.shrink_to_hi(), ")".to_string()),
                            ],
                            Applicability::MachineApplicable,
                        );
                        self.suggested = true;
                    }
                }
            }
            let hir_map = self.infcx.tcx.hir();
            let def_id = self.body.source.def_id();
            let Some(local_def_id) = def_id.as_local() else { return };
            let Some(body_id) = hir_map.maybe_body_owned_by(local_def_id) else { return };
            let body = self.infcx.tcx.hir().body(body_id);

            let mut v = V { assign_span: span, err, ty, suggested: false };
            v.visit_body(body);
            if !v.suggested {
                err.help(format!(
                    "to modify a `{ty}`, use `.get_mut()`, `.insert()` or the entry API",
                ));
            }
        }
    }

    /// 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 = self.infcx.tcx.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,
            }),
        )
    }

    fn construct_mut_suggestion_for_local_binding_patterns(
        &self,
        err: &mut DiagnosticBuilder<'_, ErrorGuaranteed>,
        local: Local,
    ) {
        let local_decl = &self.body.local_decls[local];
        debug!("local_decl: {:?}", local_decl);
        let pat_span = match *local_decl.local_info() {
            LocalInfo::User(BindingForm::Var(mir::VarBindingForm {
                binding_mode: ty::BindingMode::BindByValue(Mutability::Not),
                opt_ty_info: _,
                opt_match_place: _,
                pat_span,
            })) => pat_span,
            _ => local_decl.source_info.span,
        };

        struct BindingFinder {
            span: Span,
            hir_id: Option<hir::HirId>,
        }

        impl<'tcx> Visitor<'tcx> for BindingFinder {
            fn visit_stmt(&mut self, s: &'tcx hir::Stmt<'tcx>) {
                if let hir::StmtKind::Local(local) = s.kind {
                    if local.pat.span == self.span {
                        self.hir_id = Some(local.hir_id);
                    }
                }
                hir::intravisit::walk_stmt(self, s);
            }
        }

        let hir_map = self.infcx.tcx.hir();
        let def_id = self.body.source.def_id();
        let hir_id = if let Some(local_def_id) = def_id.as_local()
            && let Some(body_id) = hir_map.maybe_body_owned_by(local_def_id)
        {
            let body = hir_map.body(body_id);
            let mut v = BindingFinder { span: pat_span, hir_id: None };
            v.visit_body(body);
            v.hir_id
        } else {
            None
        };

        // With ref-binding patterns, the mutability suggestion has to apply to
        // the binding, not the reference (which would be a type error):
        //
        // `let &b = a;` -> `let &(mut b) = a;`
        if let Some(hir_id) = hir_id
            && let Some(hir::Node::Local(hir::Local {
                pat: hir::Pat { kind: hir::PatKind::Ref(_, _), .. },
                ..
            })) = hir_map.find(hir_id)
            && let Ok(name) =
                self.infcx.tcx.sess.source_map().span_to_snippet(local_decl.source_info.span)
        {
            err.span_suggestion(
                pat_span,
                "consider changing this to be mutable",
                format!("&(mut {name})"),
                Applicability::MachineApplicable,
            );
            return;
        }

        err.span_suggestion_verbose(
            local_decl.source_info.span.shrink_to_lo(),
            "consider changing this to be mutable",
            "mut ",
            Applicability::MachineApplicable,
        );
    }

    // point to span of upvar making closure call require mutable borrow
    fn show_mutating_upvar(
        &self,
        tcx: TyCtxt<'_>,
        closure_local_def_id: hir::def_id::LocalDefId,
        the_place_err: PlaceRef<'tcx>,
        err: &mut Diagnostic,
    ) {
        let tables = tcx.typeck(closure_local_def_id);
        if let Some((span, closure_kind_origin)) = tcx.closure_kind_origin(closure_local_def_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[&closure_local_def_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, span: Span) {
        use hir::{
            BorrowKind, Expr,
            ExprKind::{AddrOf, Block, Call, MethodCall},
        };

        let hir_map = self.infcx.tcx.hir();
        struct Finder<'tcx> {
            span: Span,
            expr: Option<&'tcx Expr<'tcx>>,
        }

        impl<'tcx> Visitor<'tcx> for Finder<'tcx> {
            fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
                if e.span == self.span && self.expr.is_none() {
                    self.expr = Some(e);
                }
                hir::intravisit::walk_expr(self, e);
            }
        }
        if let Some(body_id) = hir_map.maybe_body_owned_by(self.mir_def_id())
            && let Block(block, _) = hir_map.body(body_id).value.kind
        {
            // `span` corresponds to the expression being iterated, find the `for`-loop desugared
            // expression with that span in order to identify potential fixes when encountering a
            // read-only iterator that should be mutable.
            let mut v = Finder { span, expr: None };
            v.visit_block(block);
            if let Some(expr) = v.expr
                && let Call(_, [expr]) = expr.kind
            {
                match expr.kind {
                    MethodCall(path_segment, _, _, span) => {
                        // We have `for _ in iter.read_only_iter()`, try to
                        // suggest `for _ in iter.mutable_iter()` instead.
                        let opt_suggestions = self
                            .infcx
                            .tcx
                            .typeck(path_segment.hir_id.owner.def_id)
                            .type_dependent_def_id(expr.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,
                            );
                        }
                    }
                    AddrOf(BorrowKind::Ref, Mutability::Not, expr) => {
                        // We have `for _ in &i`, suggest `for _ in &mut i`.
                        err.span_suggestion_verbose(
                            expr.span.shrink_to_lo(),
                            "use a mutable iterator instead",
                            "mut ".to_string(),
                            Applicability::MachineApplicable,
                        );
                    }
                    _ => {}
                }
            }
        }
    }

    /// 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 closure_span = self.infcx.tcx.def_span(self.mir_def_id());
        let fn_call_id = hir.parent_id(closure_id);
        let node = hir.get(fn_call_id);
        let def_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.hir_id == closure_id)
                .map(|(pos, _)| pos)
                .next();
            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`");
                    err.span_label(closure_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).def_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(closure_span, "in this closure");
                }
                _ => {}
            }
        }
    }

    fn suggest_using_iter_mut(&self, err: &mut DiagnosticBuilder<'_, ErrorGuaranteed>) {
        let source = self.body.source;
        let hir = self.infcx.tcx.hir();
        if let InstanceDef::Item(def_id) = source.instance
            && let Some(Node::Expr(hir::Expr { hir_id, kind, .. })) = hir.get_if_local(def_id)
            && let ExprKind::Closure(closure) = kind
            && closure.movability == None
            && let Some(Node::Expr(expr)) = hir.find_parent(*hir_id)
        {
            let mut cur_expr = expr;
            while let ExprKind::MethodCall(path_segment, recv, _, _) = cur_expr.kind {
                if path_segment.ident.name == sym::iter {
                    // check `_ty` has `iter_mut` method
                    let res = self
                        .infcx
                        .tcx
                        .typeck(path_segment.hir_id.owner.def_id)
                        .type_dependent_def_id(cur_expr.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.filter_by_name_unhygienic(sym::iter_mut).peekable()
                        });

                    if let Some(mut res) = res
                        && res.peek().is_some()
                    {
                        err.span_suggestion_verbose(
                            path_segment.ident.span,
                            "you may want to use `iter_mut` here",
                            "iter_mut",
                            Applicability::MaybeIncorrect,
                        );
                    }
                    break;
                } else {
                    cur_expr = recv;
                }
            }
        }
    }

    fn suggest_make_local_mut(
        &self,
        err: &mut DiagnosticBuilder<'_, ErrorGuaranteed>,
        local: Local,
        name: Symbol,
    ) {
        let local_decl = &self.body.local_decls[local];

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

        let (is_trait_sig, local_trait) = self.is_error_in_trait(local);
        if is_trait_sig && local_trait.is_none() {
            return;
        }

        let decl_span = match local_trait {
            Some(span) => span,
            None => local_decl.source_info.span,
        };

        let label = match *local_decl.local_info() {
            LocalInfo::User(mir::BindingForm::ImplicitSelf(_)) => {
                let suggestion = suggest_ampmut_self(self.infcx.tcx, decl_span);
                Some((true, decl_span, suggestion))
            }

            LocalInfo::User(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) => {
                        let span = opt_assignment_rhs_span.unwrap();
                        self.suggest_similar_mut_method_for_for_loop(err, span);
                        err.span_label(
                            span,
                            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.ty,
                                decl_span,
                                opt_assignment_rhs_span,
                                opt_ty_info,
                            )
                        } else {
                            match local_decl.local_info() {
                                LocalInfo::User(mir::BindingForm::Var(mir::VarBindingForm {
                                    opt_ty_info: None,
                                    ..
                                })) => {
                                    let sugg = suggest_ampmut_self(self.infcx.tcx, decl_span);
                                    (true, decl_span, sugg)
                                }
                                // explicit self (eg `self: &'a Self`)
                                _ => suggest_ampmut(
                                    self.infcx.tcx,
                                    local_decl.ty,
                                    decl_span,
                                    opt_assignment_rhs_span,
                                    opt_ty_info,
                                ),
                            }
                        };
                        Some(label)
                    }
                }
            }

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

            _ => unreachable!(),
        };

        match label {
            Some((true, err_help_span, suggested_code)) => {
                err.span_suggestion_verbose(
                    err_help_span,
                    format!("consider changing this to be a mutable {pointer_desc}"),
                    suggested_code,
                    Applicability::MachineApplicable,
                );
            }
            Some((false, err_label_span, message)) => {
                struct BindingFinder {
                    span: Span,
                    hir_id: Option<hir::HirId>,
                }

                impl<'tcx> Visitor<'tcx> for BindingFinder {
                    fn visit_stmt(&mut self, s: &'tcx hir::Stmt<'tcx>) {
                        if let hir::StmtKind::Local(local) = s.kind {
                            if local.pat.span == self.span {
                                self.hir_id = Some(local.hir_id);
                            }
                        }
                        hir::intravisit::walk_stmt(self, s);
                    }
                }
                let hir_map = self.infcx.tcx.hir();
                let def_id = self.body.source.def_id();
                let hir_id = if let Some(local_def_id) = def_id.as_local()
                    && let Some(body_id) = hir_map.maybe_body_owned_by(local_def_id)
                {
                    let body = hir_map.body(body_id);
                    let mut v = BindingFinder { span: err_label_span, hir_id: None };
                    v.visit_body(body);
                    v.hir_id
                } else {
                    None
                };

                if let Some(hir_id) = hir_id
                    && let Some(hir::Node::Local(local)) = hir_map.find(hir_id)
                {
                    let (changing, span, sugg) = match local.ty {
                        Some(ty) => ("changing", ty.span, message),
                        None => {
                            ("specifying", local.pat.span.shrink_to_hi(), format!(": {message}"))
                        }
                    };
                    err.span_suggestion_verbose(
                        span,
                        format!("consider {changing} this binding's type"),
                        sugg,
                        Applicability::HasPlaceholders,
                    );
                } else {
                    err.span_label(
                        err_label_span,
                        format!("consider changing this binding's type to be: `{message}`"),
                    );
                }
            }
            None => {}
        }
    }
}

pub 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() {
        // Check if mutably borrowing a mutable reference.
        LocalInfo::User(mir::BindingForm::Var(mir::VarBindingForm {
            binding_mode: ty::BindingMode::BindByValue(Mutability::Not),
            ..
        })) => matches!(local_decl.ty.kind(), ty::Ref(_, _, hir::Mutability::Mut)),
        LocalInfo::User(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 into the next case with them.
            kind == hir::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>, span: Span) -> String {
    match tcx.sess.source_map().span_to_snippet(span) {
        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>,
    decl_ty: Ty<'tcx>,
    decl_span: Span,
    opt_assignment_rhs_span: Option<Span>,
    opt_ty_info: Option<Span>,
) -> (bool, Span, String) {
    // if there is a RHS and it starts with a `&` from it, then check if it is
    // mutable, and if not, put suggest putting `mut ` to make it mutable.
    // we don't have to worry about lifetime annotations here because they are
    // not valid when taking a reference. For example, the following is not valid Rust:
    //
    // let x: &i32 = &'a 5;
    //                ^^ lifetime annotation not allowed
    //
    if let Some(assignment_rhs_span) = opt_assignment_rhs_span
        && let Ok(src) = tcx.sess.source_map().span_to_snippet(assignment_rhs_span)
        && let Some(stripped) = src.strip_prefix('&')
    {
        let is_mut = if let Some(rest) = stripped.trim_start().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 the reference is already mutable then there is nothing we can do
        // here.
        if !is_mut {
            let span = assignment_rhs_span;
            // shrink the span to just after the `&` in `&variable`
            let span = span.with_lo(span.lo() + BytePos(1)).shrink_to_lo();

            // FIXME(Ezrashaw): returning is bad because we still might want to
            // update the annotated type, see #106857.
            return (true, span, "mut ".to_owned());
        }
    }

    let (binding_exists, span) = match opt_ty_info {
        // if this is a variable binding with an explicit type,
        // then we will suggest changing it to be mutable.
        // this is `Applicability::MachineApplicable`.
        Some(ty_span) => (true, ty_span),

        // otherwise, we'll suggest *adding* an annotated type, we'll suggest
        // the RHS's type for that.
        // this is `Applicability::HasPlaceholders`.
        None => (false, decl_span),
    };

    // if the binding already exists and is a reference with an explicit
    // lifetime, then we can suggest adding ` mut`. this is special-cased from
    // the path without an explicit lifetime.
    if let Ok(src) = tcx.sess.source_map().span_to_snippet(span)
        && src.starts_with("&'")
        // note that `&     'a T` is invalid so this is correct.
        && let Some(ws_pos) = src.find(char::is_whitespace)
    {
        let span = span.with_lo(span.lo() + BytePos(ws_pos as u32)).shrink_to_lo();
        (true, span, " mut".to_owned())
    // if there is already a binding, we modify it to be `mut`
    } else if binding_exists {
        // shrink the span to just after the `&` in `&variable`
        let span = span.with_lo(span.lo() + BytePos(1)).shrink_to_lo();
        (true, span, "mut ".to_owned())
    } else {
        // otherwise, suggest that the user annotates the binding; we provide the
        // type of the local.
        let ty_mut = decl_ty.builtin_deref(true).unwrap();
        assert_eq!(ty_mut.mutbl, hir::Mutability::Not);

        (false, span, format!("{}mut {}", if decl_ty.is_ref() { "&" } else { "*" }, ty_mut.ty))
    }
}

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

/// 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: FieldIdx,
) -> 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::Ref(lt, hir::MutTy { mutbl: hir::Mutability::Not, ty }) = field.ty.kind
    {
        return Some(lt.ident.span.between(ty.span));
    }

    None
}

/// If possible, suggest replacing `ref` with `ref mut`.
fn suggest_ref_mut(tcx: TyCtxt<'_>, span: Span) -> Option<Span> {
    let pattern_str = tcx.sess.source_map().span_to_snippet(span).ok()?;
    if pattern_str.starts_with("ref")
        && pattern_str["ref".len()..].starts_with(rustc_lexer::is_whitespace)
    {
        let span = span.with_lo(span.lo() + BytePos(4)).shrink_to_lo();
        Some(span)
    } else {
        None
    }
}