rustc_mir_build/builder/expr/

as_place.rs

//! See docs in build/expr/mod.rs

use std::{assert_matches, iter};

use rustc_abi::{FIRST_VARIANT, FieldIdx, VariantIdx};
use rustc_hir::def_id::LocalDefId;
use rustc_middle::hir::place::{Projection as HirProjection, ProjectionKind as HirProjectionKind};
use rustc_middle::mir::AssertKind::BoundsCheck;
use rustc_middle::mir::*;
use rustc_middle::thir::*;
use rustc_middle::ty::{self, AdtDef, CanonicalUserTypeAnnotation, Ty, Variance};
use rustc_middle::{bug, span_bug};
use rustc_span::Span;
use tracing::{debug, instrument, trace};

use crate::builder::ForGuard::{OutsideGuard, RefWithinGuard};
use crate::builder::expr::category::Category;
use crate::builder::scope::LintLevel;
use crate::builder::{BlockAnd, BlockAndExtension, Builder, Capture, CaptureMap};

/// The "outermost" place that holds this value.
#[derive(Copy, Clone, Debug, PartialEq)]
pub(crate) enum PlaceBase {
    /// Denotes the start of a `Place`.
    Local(Local),

    /// When building place for an expression within a closure, the place might start off a
    /// captured path. When `capture_disjoint_fields` is enabled, we might not know the capture
    /// index (within the desugared closure) of the captured path until most of the projections
    /// are applied. We use `PlaceBase::Upvar` to keep track of the root variable off of which the
    /// captured path starts, the closure the capture belongs to and the trait the closure
    /// implements.
    ///
    /// Once we have figured out the capture index, we can convert the place builder to start from
    /// `PlaceBase::Local`.
    ///
    /// Consider the following example
    /// ```rust
    /// let t = (((10, 10), 10), 10);
    ///
    /// let c = || {
    ///     println!("{}", t.0.0.0);
    /// };
    /// ```
    /// Here the THIR expression for `t.0.0.0` will be something like
    ///
    /// ```ignore (illustrative)
    /// * Field(0)
    ///     * Field(0)
    ///         * Field(0)
    ///             * UpvarRef(t)
    /// ```
    ///
    /// When `capture_disjoint_fields` is enabled, `t.0.0.0` is captured and we won't be able to
    /// figure out that it is captured until all the `Field` projections are applied.
    Upvar {
        /// HirId of the upvar
        var_hir_id: LocalVarId,
        /// DefId of the closure
        closure_def_id: LocalDefId,
    },
}

/// `PlaceBuilder` is used to create places during MIR construction. It allows you to "build up" a
/// place by pushing more and more projections onto the end, and then convert the final set into a
/// place using the `to_place` method.
///
/// This is used internally when building a place for an expression like `a.b.c`. The fields `b`
/// and `c` can be progressively pushed onto the place builder that is created when converting `a`.
#[derive(Clone, Debug, PartialEq)]
pub(in crate::builder) struct PlaceBuilder<'tcx> {
    base: PlaceBase,
    projection: Vec<PlaceElem<'tcx>>,
}

/// Given a list of MIR projections, convert them to list of HIR ProjectionKind.
/// The projections are truncated to represent a path that might be captured by a
/// closure/coroutine. This implies the vector returned from this function doesn't contain
/// ProjectionElems `Downcast`, `ConstantIndex`, `Index`, or `Subslice` because those will never be
/// part of a path that is captured by a closure. We stop applying projections once we see the first
/// projection that isn't captured by a closure.
fn convert_to_hir_projections_and_truncate_for_capture(
    mir_projections: &[PlaceElem<'_>],
) -> Vec<HirProjectionKind> {
    let mut hir_projections = Vec::new();
    let mut variant = None;

    for mir_projection in mir_projections {
        let hir_projection = match mir_projection {
            ProjectionElem::Deref => HirProjectionKind::Deref,
            ProjectionElem::Field(field, _) => {
                let variant = variant.unwrap_or(FIRST_VARIANT);
                HirProjectionKind::Field(*field, variant)
            }
            ProjectionElem::Downcast(.., idx) => {
                // We don't expect to see multi-variant enums here, as earlier
                // phases will have truncated them already. However, there can
                // still be downcasts, thanks to single-variant enums.
                // We keep track of VariantIdx so we can use this information
                // if the next ProjectionElem is a Field.
                variant = Some(*idx);
                continue;
            }
            ProjectionElem::UnwrapUnsafeBinder(_) => HirProjectionKind::UnwrapUnsafeBinder,
            // These do not affect anything, they just make sure we know the right type.
            ProjectionElem::OpaqueCast(_) => continue,
            ProjectionElem::Index(..)
            | ProjectionElem::ConstantIndex { .. }
            | ProjectionElem::Subslice { .. } => {
                // We don't capture array-access projections.
                // We can stop here as arrays are captured completely.
                break;
            }
        };
        variant = None;
        hir_projections.push(hir_projection);
    }

    hir_projections
}

/// Return true if the `proj_possible_ancestor` represents an ancestor path
/// to `proj_capture` or `proj_possible_ancestor` is same as `proj_capture`,
/// assuming they both start off of the same root variable.
///
/// **Note:** It's the caller's responsibility to ensure that both lists of projections
///           start off of the same root variable.
///
/// Eg: 1. `foo.x` which is represented using `projections=[Field(x)]` is an ancestor of
///        `foo.x.y` which is represented using `projections=[Field(x), Field(y)]`.
///        Note both `foo.x` and `foo.x.y` start off of the same root variable `foo`.
///     2. Since we only look at the projections here function will return `bar.x` as a valid
///        ancestor of `foo.x.y`. It's the caller's responsibility to ensure that both projections
///        list are being applied to the same root variable.
fn is_ancestor_or_same_capture(
    proj_possible_ancestor: &[HirProjectionKind],
    proj_capture: &[HirProjectionKind],
) -> bool {
    // We want to make sure `is_ancestor_or_same_capture("x.0.0", "x.0")` to return false.
    // Therefore we can't just check if all projections are same in the zipped iterator below.
    if proj_possible_ancestor.len() > proj_capture.len() {
        return false;
    }

    iter::zip(proj_possible_ancestor, proj_capture).all(|(a, b)| a == b)
}

/// Given a closure, returns the index of a capture within the desugared closure struct and the
/// `ty::CapturedPlace` which is the ancestor of the Place represented using the `var_hir_id`
/// and `projection`.
///
/// Note there will be at most one ancestor for any given Place.
///
/// Returns None, when the ancestor is not found.
fn find_capture_matching_projections<'a, 'tcx>(
    upvars: &'a CaptureMap<'tcx>,
    var_hir_id: LocalVarId,
    projections: &[PlaceElem<'tcx>],
) -> Option<(usize, &'a Capture<'tcx>)> {
    let hir_projections = convert_to_hir_projections_and_truncate_for_capture(projections);

    upvars.get_by_key_enumerated(var_hir_id.0.local_id).find(|(_, capture)| {
        let possible_ancestor_proj_kinds: Vec<_> =
            capture.captured_place.place.projections.iter().map(|proj| proj.kind).collect();
        is_ancestor_or_same_capture(&possible_ancestor_proj_kinds, &hir_projections)
    })
}

/// Takes an upvar place and tries to resolve it into a `PlaceBuilder`
/// with `PlaceBase::Local`
#[instrument(level = "trace", skip(cx), ret)]
fn to_upvars_resolved_place_builder<'tcx>(
    cx: &Builder<'_, 'tcx>,
    var_hir_id: LocalVarId,
    closure_def_id: LocalDefId,
    projection: &[PlaceElem<'tcx>],
) -> Option<PlaceBuilder<'tcx>> {
    let Some((capture_index, capture)) =
        find_capture_matching_projections(&cx.upvars, var_hir_id, projection)
    else {
        let closure_span = cx.tcx.def_span(closure_def_id);
        if !enable_precise_capture(closure_span) {
            bug!(
                "No associated capture found for {:?}[{:#?}] even though \
                    capture_disjoint_fields isn't enabled",
                var_hir_id,
                projection
            )
        } else {
            debug!("No associated capture found for {:?}[{:#?}]", var_hir_id, projection,);
        }
        return None;
    };

    // Access the capture by accessing the field within the Closure struct.
    let capture_info = &cx.upvars[capture_index];

    let mut upvar_resolved_place_builder = PlaceBuilder::from(capture_info.use_place);

    // We used some of the projections to build the capture itself,
    // now we apply the remaining to the upvar resolved place.
    trace!(?capture.captured_place, ?projection);
    let remaining_projections = strip_prefix(
        capture.captured_place.place.base_ty,
        projection,
        &capture.captured_place.place.projections,
    );
    upvar_resolved_place_builder.projection.extend(remaining_projections);

    Some(upvar_resolved_place_builder)
}

/// Returns projections remaining after stripping an initial prefix of HIR
/// projections.
///
/// Supports only HIR projection kinds that represent a path that might be
/// captured by a closure or a coroutine, i.e., an `Index` or a `Subslice`
/// projection kinds are unsupported.
fn strip_prefix<'tcx>(
    mut base_ty: Ty<'tcx>,
    projections: &[PlaceElem<'tcx>],
    prefix_projections: &[HirProjection<'tcx>],
) -> impl Iterator<Item = PlaceElem<'tcx>> {
    let mut iter = projections
        .iter()
        .copied()
        // Filter out opaque casts, they are unnecessary in the prefix.
        .filter(|elem| !matches!(elem, ProjectionElem::OpaqueCast(..)));
    for projection in prefix_projections {
        match projection.kind {
            HirProjectionKind::Deref => {
                assert_matches!(iter.next(), Some(ProjectionElem::Deref));
            }
            HirProjectionKind::Field(..) => {
                if base_ty.is_enum() {
                    assert_matches!(iter.next(), Some(ProjectionElem::Downcast(..)));
                }
                assert_matches!(iter.next(), Some(ProjectionElem::Field(..)));
            }
            HirProjectionKind::OpaqueCast => {
                assert_matches!(iter.next(), Some(ProjectionElem::OpaqueCast(..)));
            }
            HirProjectionKind::UnwrapUnsafeBinder => {
                assert_matches!(iter.next(), Some(ProjectionElem::UnwrapUnsafeBinder(..)));
            }
            HirProjectionKind::Index | HirProjectionKind::Subslice => {
                bug!("unexpected projection kind: {:?}", projection);
            }
        }
        base_ty = projection.ty;
    }
    iter
}

impl<'tcx> PlaceBuilder<'tcx> {
    pub(in crate::builder) fn to_place(&self, cx: &Builder<'_, 'tcx>) -> Place<'tcx> {
        self.try_to_place(cx).unwrap_or_else(|| match self.base {
            PlaceBase::Local(local) => span_bug!(
                cx.local_decls[local].source_info.span,
                "could not resolve local: {local:#?} + {:?}",
                self.projection
            ),
            PlaceBase::Upvar { var_hir_id, closure_def_id: _ } => span_bug!(
                cx.tcx.hir_span(var_hir_id.0),
                "could not resolve upvar: {var_hir_id:?} + {:?}",
                self.projection
            ),
        })
    }

    /// Creates a `Place` or returns `None` if an upvar cannot be resolved
    pub(in crate::builder) fn try_to_place(&self, cx: &Builder<'_, 'tcx>) -> Option<Place<'tcx>> {
        let resolved = self.resolve_upvar(cx);
        let builder = resolved.as_ref().unwrap_or(self);
        let PlaceBase::Local(local) = builder.base else { return None };
        let projection = cx.tcx.mk_place_elems(&builder.projection);
        Some(Place { local, projection })
    }

    /// Attempts to resolve the `PlaceBuilder`.
    /// Returns `None` if this is not an upvar.
    ///
    /// Upvars resolve may fail for a `PlaceBuilder` when attempting to
    /// resolve a disjoint field whose root variable is not captured
    /// (destructured assignments) or when attempting to resolve a root
    /// variable (discriminant matching with only wildcard arm) that is
    /// not captured. This can happen because the final mir that will be
    /// generated doesn't require a read for this place. Failures will only
    /// happen inside closures.
    pub(in crate::builder) fn resolve_upvar(
        &self,
        cx: &Builder<'_, 'tcx>,
    ) -> Option<PlaceBuilder<'tcx>> {
        let PlaceBase::Upvar { var_hir_id, closure_def_id } = self.base else {
            return None;
        };
        to_upvars_resolved_place_builder(cx, var_hir_id, closure_def_id, &self.projection)
    }

    pub(crate) fn base(&self) -> PlaceBase {
        self.base
    }

    pub(crate) fn projection(&self) -> &[PlaceElem<'tcx>] {
        &self.projection
    }

    pub(crate) fn field(self, f: FieldIdx, ty: Ty<'tcx>) -> Self {
        self.project(PlaceElem::Field(f, ty))
    }

    pub(crate) fn deref(self) -> Self {
        self.project(PlaceElem::Deref)
    }

    pub(crate) fn downcast(self, adt_def: AdtDef<'tcx>, variant_index: VariantIdx) -> Self {
        self.project(PlaceElem::Downcast(Some(adt_def.variant(variant_index).name), variant_index))
    }

    fn index(self, index: Local) -> Self {
        self.project(PlaceElem::Index(index))
    }

    pub(crate) fn project(mut self, elem: PlaceElem<'tcx>) -> Self {
        self.projection.push(elem);
        self
    }

    /// Same as `.clone().project(..)` but more efficient
    pub(crate) fn clone_project(&self, elem: PlaceElem<'tcx>) -> Self {
        Self {
            base: self.base,
            projection: Vec::from_iter(self.projection.iter().copied().chain([elem])),
        }
    }
}

impl<'tcx> From<Local> for PlaceBuilder<'tcx> {
    fn from(local: Local) -> Self {
        Self { base: PlaceBase::Local(local), projection: Vec::new() }
    }
}

impl<'tcx> From<PlaceBase> for PlaceBuilder<'tcx> {
    fn from(base: PlaceBase) -> Self {
        Self { base, projection: Vec::new() }
    }
}

impl<'tcx> From<Place<'tcx>> for PlaceBuilder<'tcx> {
    fn from(p: Place<'tcx>) -> Self {
        Self { base: PlaceBase::Local(p.local), projection: p.projection.to_vec() }
    }
}

impl<'a, 'tcx> Builder<'a, 'tcx> {
    /// Compile `expr`, yielding a place that we can move from etc.
    ///
    /// WARNING: Any user code might:
    /// * Invalidate any slice bounds checks performed.
    /// * Change the address that this `Place` refers to.
    /// * Modify the memory that this place refers to.
    /// * Invalidate the memory that this place refers to, this will be caught
    ///   by borrow checking.
    ///
    /// Extra care is needed if any user code is allowed to run between calling
    /// this method and using it, as is the case for `match` and index
    /// expressions.
    pub(crate) fn as_place(
        &mut self,
        mut block: BasicBlock,
        expr_id: ExprId,
    ) -> BlockAnd<Place<'tcx>> {
        let place_builder = unpack!(block = self.as_place_builder(block, expr_id));
        block.and(place_builder.to_place(self))
    }

    /// This is used when constructing a compound `Place`, so that we can avoid creating
    /// intermediate `Place` values until we know the full set of projections.
    pub(crate) fn as_place_builder(
        &mut self,
        block: BasicBlock,
        expr_id: ExprId,
    ) -> BlockAnd<PlaceBuilder<'tcx>> {
        self.expr_as_place(block, expr_id, Mutability::Mut, None)
    }

    /// Compile `expr`, yielding a place that we can move from etc.
    /// Mutability note: The caller of this method promises only to read from the resulting
    /// place. The place itself may or may not be mutable:
    /// * If this expr is a place expr like a.b, then we will return that place.
    /// * Otherwise, a temporary is created: in that event, it will be an immutable temporary.
    pub(crate) fn as_read_only_place(
        &mut self,
        mut block: BasicBlock,
        expr_id: ExprId,
    ) -> BlockAnd<Place<'tcx>> {
        let place_builder = unpack!(block = self.as_read_only_place_builder(block, expr_id));
        block.and(place_builder.to_place(self))
    }

    /// This is used when constructing a compound `Place`, so that we can avoid creating
    /// intermediate `Place` values until we know the full set of projections.
    /// Mutability note: The caller of this method promises only to read from the resulting
    /// place. The place itself may or may not be mutable:
    /// * If this expr is a place expr like a.b, then we will return that place.
    /// * Otherwise, a temporary is created: in that event, it will be an immutable temporary.
    fn as_read_only_place_builder(
        &mut self,
        block: BasicBlock,
        expr_id: ExprId,
    ) -> BlockAnd<PlaceBuilder<'tcx>> {
        self.expr_as_place(block, expr_id, Mutability::Not, None)
    }

    fn expr_as_place(
        &mut self,
        mut block: BasicBlock,
        expr_id: ExprId,
        mutability: Mutability,
        fake_borrow_temps: Option<&mut Vec<Local>>,
    ) -> BlockAnd<PlaceBuilder<'tcx>> {
        let expr = &self.thir[expr_id];
        debug!("expr_as_place(block={:?}, expr={:?}, mutability={:?})", block, expr, mutability);

        let this = self; // See "LET_THIS_SELF".
        let expr_span = expr.span;
        let source_info = this.source_info(expr_span);
        match expr.kind {
            ExprKind::Scope { region_scope, hir_id, value } => {
                this.in_scope((region_scope, source_info), LintLevel::Explicit(hir_id), |this| {
                    this.expr_as_place(block, value, mutability, fake_borrow_temps)
                })
            }
            ExprKind::Field { lhs, variant_index, name } => {
                let lhs_expr = &this.thir[lhs];
                let mut place_builder =
                    unpack!(block = this.expr_as_place(block, lhs, mutability, fake_borrow_temps,));
                if let ty::Adt(adt_def, _) = lhs_expr.ty.kind() {
                    if adt_def.is_enum() {
                        place_builder = place_builder.downcast(*adt_def, variant_index);
                    }
                }
                block.and(place_builder.field(name, expr.ty))
            }
            ExprKind::Deref { arg } => {
                let place_builder =
                    unpack!(block = this.expr_as_place(block, arg, mutability, fake_borrow_temps,));
                block.and(place_builder.deref())
            }
            ExprKind::Index { lhs, index } => this.lower_index_expression(
                block,
                lhs,
                index,
                mutability,
                fake_borrow_temps,
                expr_span,
                source_info,
            ),
            ExprKind::UpvarRef { closure_def_id, var_hir_id } => {
                this.lower_captured_upvar(block, closure_def_id.expect_local(), var_hir_id)
            }

            ExprKind::VarRef { id } => {
                let place_builder = if this.is_bound_var_in_guard(id) {
                    let index = this.var_local_id(id, RefWithinGuard);
                    PlaceBuilder::from(index).deref()
                } else {
                    let index = this.var_local_id(id, OutsideGuard);
                    PlaceBuilder::from(index)
                };
                block.and(place_builder)
            }

            ExprKind::PlaceTypeAscription { source, ref user_ty, user_ty_span } => {
                let place_builder = unpack!(
                    block = this.expr_as_place(block, source, mutability, fake_borrow_temps,)
                );
                if let Some(user_ty) = user_ty {
                    let ty_source_info = this.source_info(user_ty_span);
                    let annotation_index =
                        this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                            span: user_ty_span,
                            user_ty: user_ty.clone(),
                            inferred_ty: expr.ty,
                        });

                    let place = place_builder.to_place(this);
                    this.cfg.push(
                        block,
                        Statement::new(
                            ty_source_info,
                            StatementKind::AscribeUserType(
                                Box::new((
                                    place,
                                    UserTypeProjection { base: annotation_index, projs: vec![] },
                                )),
                                Variance::Invariant,
                            ),
                        ),
                    );
                }
                block.and(place_builder)
            }
            ExprKind::ValueTypeAscription { source, ref user_ty, user_ty_span } => {
                let temp_lifetime =
                    this.region_scope_tree.temporary_scope(this.thir[source].temp_scope_id);
                let temp = unpack!(block = this.as_temp(block, temp_lifetime, source, mutability));
                if let Some(user_ty) = user_ty {
                    let ty_source_info = this.source_info(user_ty_span);
                    let annotation_index =
                        this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                            span: user_ty_span,
                            user_ty: user_ty.clone(),
                            inferred_ty: expr.ty,
                        });
                    this.cfg.push(
                        block,
                        Statement::new(
                            ty_source_info,
                            StatementKind::AscribeUserType(
                                Box::new((
                                    Place::from(temp),
                                    UserTypeProjection { base: annotation_index, projs: vec![] },
                                )),
                                Variance::Invariant,
                            ),
                        ),
                    );
                }
                block.and(PlaceBuilder::from(temp))
            }

            ExprKind::PlaceUnwrapUnsafeBinder { source } => {
                let place_builder = unpack!(
                    block = this.expr_as_place(block, source, mutability, fake_borrow_temps,)
                );
                block.and(place_builder.project(PlaceElem::UnwrapUnsafeBinder(expr.ty)))
            }
            ExprKind::ValueUnwrapUnsafeBinder { source } => {
                let temp_lifetime =
                    this.region_scope_tree.temporary_scope(this.thir[source].temp_scope_id);
                let temp = unpack!(block = this.as_temp(block, temp_lifetime, source, mutability));
                block.and(PlaceBuilder::from(temp).project(PlaceElem::UnwrapUnsafeBinder(expr.ty)))
            }

            ExprKind::Array { .. }
            | ExprKind::Tuple { .. }
            | ExprKind::Adt { .. }
            | ExprKind::Closure { .. }
            | ExprKind::Unary { .. }
            | ExprKind::Binary { .. }
            | ExprKind::LogicalOp { .. }
            | ExprKind::Cast { .. }
            | ExprKind::Use { .. }
            | ExprKind::NeverToAny { .. }
            | ExprKind::PointerCoercion { .. }
            | ExprKind::Repeat { .. }
            | ExprKind::Borrow { .. }
            | ExprKind::RawBorrow { .. }
            | ExprKind::Match { .. }
            | ExprKind::If { .. }
            | ExprKind::Loop { .. }
            | ExprKind::LoopMatch { .. }
            | ExprKind::Block { .. }
            | ExprKind::Let { .. }
            | ExprKind::Assign { .. }
            | ExprKind::AssignOp { .. }
            | ExprKind::Break { .. }
            | ExprKind::Continue { .. }
            | ExprKind::ConstContinue { .. }
            | ExprKind::Return { .. }
            | ExprKind::Become { .. }
            | ExprKind::Literal { .. }
            | ExprKind::NamedConst { .. }
            | ExprKind::NonHirLiteral { .. }
            | ExprKind::ZstLiteral { .. }
            | ExprKind::ConstParam { .. }
            | ExprKind::ConstBlock { .. }
            | ExprKind::StaticRef { .. }
            | ExprKind::InlineAsm { .. }
            | ExprKind::Yield { .. }
            | ExprKind::ThreadLocalRef(_)
            | ExprKind::Call { .. }
            | ExprKind::ByUse { .. }
            | ExprKind::WrapUnsafeBinder { .. } => {
                // these are not places, so we need to make a temporary.
                debug_assert!(!matches!(Category::of(&expr.kind), Some(Category::Place)));
                let temp_lifetime = this.region_scope_tree.temporary_scope(expr.temp_scope_id);
                let temp = unpack!(block = this.as_temp(block, temp_lifetime, expr_id, mutability));
                block.and(PlaceBuilder::from(temp))
            }
            ExprKind::Reborrow { .. } => {
                // FIXME(reborrow): it should currently be impossible to end up evaluating a
                // Reborrow expression as a place. That might not in the future, but what this then
                // evaluates to requires further thought.
                unreachable!();
            }
        }
    }

    /// Lower a captured upvar. Note we might not know the actual capture index,
    /// so we create a place starting from `PlaceBase::Upvar`, which will be resolved
    /// once all projections that allow us to identify a capture have been applied.
    fn lower_captured_upvar(
        &mut self,
        block: BasicBlock,
        closure_def_id: LocalDefId,
        var_hir_id: LocalVarId,
    ) -> BlockAnd<PlaceBuilder<'tcx>> {
        block.and(PlaceBuilder::from(PlaceBase::Upvar { var_hir_id, closure_def_id }))
    }

    /// Lower an index expression
    ///
    /// This has two complications;
    ///
    /// * We need to do a bounds check.
    /// * We need to ensure that the bounds check can't be invalidated using an
    ///   expression like `x[1][{x = y; 2}]`. We use fake borrows here to ensure
    ///   that this is the case.
    fn lower_index_expression(
        &mut self,
        mut block: BasicBlock,
        base: ExprId,
        index: ExprId,
        mutability: Mutability,
        fake_borrow_temps: Option<&mut Vec<Local>>,
        expr_span: Span,
        source_info: SourceInfo,
    ) -> BlockAnd<PlaceBuilder<'tcx>> {
        let base_fake_borrow_temps = &mut Vec::new();
        let is_outermost_index = fake_borrow_temps.is_none();
        let fake_borrow_temps = fake_borrow_temps.unwrap_or(base_fake_borrow_temps);

        let base_place =
            unpack!(block = self.expr_as_place(block, base, mutability, Some(fake_borrow_temps),));

        // Making this a *fresh* temporary means we do not have to worry about
        // the index changing later: Nothing will ever change this temporary.
        // The "retagging" transformation (for Stacked Borrows) relies on this.
        // Using the enclosing temporary scope for the index ensures it will live past where this
        // place is used. This lifetime may be larger than strictly necessary but it means we don't
        // need to pass a scope for operands to `as_place`.
        let index_lifetime = self.region_scope_tree.temporary_scope(self.thir[index].temp_scope_id);
        let idx = unpack!(block = self.as_temp(block, index_lifetime, index, Mutability::Not));

        block = self.bounds_check(block, &base_place, idx, expr_span, source_info);

        if is_outermost_index {
            self.read_fake_borrows(block, fake_borrow_temps, source_info)
        } else {
            self.add_fake_borrows_of_base(
                base_place.to_place(self),
                block,
                fake_borrow_temps,
                expr_span,
                source_info,
            );
        }

        block.and(base_place.index(idx))
    }

    /// Given a place that's either an array or a slice, returns an operand
    /// with the length of the array/slice.
    ///
    /// For arrays it'll be `Operand::Constant` with the actual length;
    /// For slices it'll be `Operand::Move` of a local using `PtrMetadata`.
    pub(in crate::builder) fn len_of_slice_or_array(
        &mut self,
        block: BasicBlock,
        place: Place<'tcx>,
        span: Span,
        source_info: SourceInfo,
    ) -> Operand<'tcx> {
        let place_ty = place.ty(&self.local_decls, self.tcx).ty;
        match place_ty.kind() {
            ty::Array(_elem_ty, len_const) => {
                // We know how long an array is, so just use that as a constant
                // directly -- no locals needed. We do need one statement so
                // that borrow- and initialization-checking consider it used,
                // though. FIXME: Do we really *need* to count this as a use?
                // Could partial array tracking work off something else instead?
                self.cfg.push_fake_read(block, source_info, FakeReadCause::ForIndex, place);
                let const_ = Const::Ty(self.tcx.types.usize, *len_const);
                Operand::Constant(Box::new(ConstOperand { span, user_ty: None, const_ }))
            }
            ty::Slice(_elem_ty) => {
                let ptr_or_ref = if let [PlaceElem::Deref] = place.projection[..]
                    && let local_ty = self.local_decls[place.local].ty
                    && local_ty.is_trivially_pure_clone_copy()
                {
                    // It's extremely common that we have something that can be
                    // directly passed to `PtrMetadata`, so avoid an unnecessary
                    // temporary and statement in those cases. Note that we can
                    // only do that for `Copy` types -- not `&mut [_]` -- because
                    // the MIR we're building here needs to pass NLL later.
                    Operand::Copy(Place::from(place.local))
                } else {
                    let ptr_ty = Ty::new_imm_ptr(self.tcx, place_ty);
                    let slice_ptr = self.temp(ptr_ty, span);
                    self.cfg.push_assign(
                        block,
                        source_info,
                        slice_ptr,
                        Rvalue::RawPtr(RawPtrKind::FakeForPtrMetadata, place),
                    );
                    Operand::Move(slice_ptr)
                };

                let len = self.temp(self.tcx.types.usize, span);
                self.cfg.push_assign(
                    block,
                    source_info,
                    len,
                    Rvalue::UnaryOp(UnOp::PtrMetadata, ptr_or_ref),
                );

                Operand::Move(len)
            }
            _ => {
                span_bug!(span, "len called on place of type {place_ty:?}")
            }
        }
    }

    fn bounds_check(
        &mut self,
        block: BasicBlock,
        slice: &PlaceBuilder<'tcx>,
        index: Local,
        expr_span: Span,
        source_info: SourceInfo,
    ) -> BasicBlock {
        let slice = slice.to_place(self);

        // len = len(slice)
        let len = self.len_of_slice_or_array(block, slice, expr_span, source_info);

        // lt = idx < len
        let bool_ty = self.tcx.types.bool;
        let lt = self.temp(bool_ty, expr_span);
        self.cfg.push_assign(
            block,
            source_info,
            lt,
            Rvalue::BinaryOp(
                BinOp::Lt,
                Box::new((Operand::Copy(Place::from(index)), len.to_copy())),
            ),
        );
        let msg = BoundsCheck { len, index: Operand::Copy(Place::from(index)) };

        // assert!(lt, "...")
        self.assert(block, Operand::Move(lt), true, msg, expr_span)
    }

    fn add_fake_borrows_of_base(
        &mut self,
        base_place: Place<'tcx>,
        block: BasicBlock,
        fake_borrow_temps: &mut Vec<Local>,
        expr_span: Span,
        source_info: SourceInfo,
    ) {
        let tcx = self.tcx;

        let place_ty = base_place.ty(&self.local_decls, tcx);
        if let ty::Slice(_) = place_ty.ty.kind() {
            // We need to create fake borrows to ensure that the bounds
            // check that we just did stays valid. Since we can't assign to
            // unsized values, we only need to ensure that none of the
            // pointers in the base place are modified.
            for (base_place, elem) in base_place.iter_projections().rev() {
                match elem {
                    ProjectionElem::Deref => {
                        let fake_borrow_deref_ty = base_place.ty(&self.local_decls, tcx).ty;
                        let fake_borrow_ty =
                            Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, fake_borrow_deref_ty);
                        let fake_borrow_temp =
                            self.local_decls.push(LocalDecl::new(fake_borrow_ty, expr_span));
                        let projection = tcx.mk_place_elems(base_place.projection);
                        self.cfg.push_assign(
                            block,
                            source_info,
                            fake_borrow_temp.into(),
                            Rvalue::Ref(
                                tcx.lifetimes.re_erased,
                                BorrowKind::Fake(FakeBorrowKind::Shallow),
                                Place { local: base_place.local, projection },
                            ),
                        );
                        fake_borrow_temps.push(fake_borrow_temp);
                    }
                    ProjectionElem::Index(_) => {
                        let index_ty = base_place.ty(&self.local_decls, tcx);
                        match index_ty.ty.kind() {
                            // The previous index expression has already
                            // done any index expressions needed here.
                            ty::Slice(_) => break,
                            ty::Array(..) => (),
                            _ => bug!("unexpected index base"),
                        }
                    }
                    ProjectionElem::Field(..)
                    | ProjectionElem::Downcast(..)
                    | ProjectionElem::OpaqueCast(..)
                    | ProjectionElem::ConstantIndex { .. }
                    | ProjectionElem::Subslice { .. }
                    | ProjectionElem::UnwrapUnsafeBinder(_) => (),
                }
            }
        }
    }

    fn read_fake_borrows(
        &mut self,
        bb: BasicBlock,
        fake_borrow_temps: &mut Vec<Local>,
        source_info: SourceInfo,
    ) {
        // All indexes have been evaluated now, read all of the
        // fake borrows so that they are live across those index
        // expressions.
        for temp in fake_borrow_temps {
            self.cfg.push_fake_read(bb, source_info, FakeReadCause::ForIndex, Place::from(*temp));
        }
    }
}

/// Precise capture is enabled if user is using Rust Edition 2021 or higher.
fn enable_precise_capture(closure_span: Span) -> bool {
    closure_span.at_least_rust_2021()
}