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use std::{fmt, iter};
use rustc_hir::lang_items::LangItem;
use rustc_index::Idx;
use rustc_middle::mir::patch::MirPatch;
use rustc_middle::mir::*;
use rustc_middle::span_bug;
use rustc_middle::traits::Reveal;
use rustc_middle::ty::util::IntTypeExt;
use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt};
use rustc_span::source_map::Spanned;
use rustc_span::DUMMY_SP;
use rustc_target::abi::{FieldIdx, VariantIdx, FIRST_VARIANT};
use tracing::{debug, instrument};
/// The value of an inserted drop flag.
#[derive(Debug, PartialEq, Eq, Copy, Clone)]
pub enum DropFlagState {
/// The tracked value is initialized and needs to be dropped when leaving its scope.
Present,
/// The tracked value is uninitialized or was moved out of and does not need to be dropped when
/// leaving its scope.
Absent,
}
impl DropFlagState {
pub fn value(self) -> bool {
match self {
DropFlagState::Present => true,
DropFlagState::Absent => false,
}
}
}
/// Describes how/if a value should be dropped.
#[derive(Debug)]
pub enum DropStyle {
/// The value is already dead at the drop location, no drop will be executed.
Dead,
/// The value is known to always be initialized at the drop location, drop will always be
/// executed.
Static,
/// Whether the value needs to be dropped depends on its drop flag.
Conditional,
/// An "open" drop is one where only the fields of a value are dropped.
///
/// For example, this happens when moving out of a struct field: The rest of the struct will be
/// dropped in such an "open" drop. It is also used to generate drop glue for the individual
/// components of a value, for example for dropping array elements.
Open,
}
/// Which drop flags to affect/check with an operation.
#[derive(Debug)]
pub enum DropFlagMode {
/// Only affect the top-level drop flag, not that of any contained fields.
Shallow,
/// Affect all nested drop flags in addition to the top-level one.
Deep,
}
/// Describes if unwinding is necessary and where to unwind to if a panic occurs.
#[derive(Copy, Clone, Debug)]
pub enum Unwind {
/// Unwind to this block.
To(BasicBlock),
/// Already in an unwind path, any panic will cause an abort.
InCleanup,
}
impl Unwind {
fn is_cleanup(self) -> bool {
match self {
Unwind::To(..) => false,
Unwind::InCleanup => true,
}
}
fn into_action(self) -> UnwindAction {
match self {
Unwind::To(bb) => UnwindAction::Cleanup(bb),
Unwind::InCleanup => UnwindAction::Terminate(UnwindTerminateReason::InCleanup),
}
}
fn map<F>(self, f: F) -> Self
where
F: FnOnce(BasicBlock) -> BasicBlock,
{
match self {
Unwind::To(bb) => Unwind::To(f(bb)),
Unwind::InCleanup => Unwind::InCleanup,
}
}
}
pub trait DropElaborator<'a, 'tcx>: fmt::Debug {
/// The type representing paths that can be moved out of.
///
/// Users can move out of individual fields of a struct, such as `a.b.c`. This type is used to
/// represent such move paths. Sometimes tracking individual move paths is not necessary, in
/// which case this may be set to (for example) `()`.
type Path: Copy + fmt::Debug;
// Accessors
fn patch(&mut self) -> &mut MirPatch<'tcx>;
fn body(&self) -> &'a Body<'tcx>;
fn tcx(&self) -> TyCtxt<'tcx>;
fn param_env(&self) -> ty::ParamEnv<'tcx>;
// Drop logic
/// Returns how `path` should be dropped, given `mode`.
fn drop_style(&self, path: Self::Path, mode: DropFlagMode) -> DropStyle;
/// Returns the drop flag of `path` as a MIR `Operand` (or `None` if `path` has no drop flag).
fn get_drop_flag(&mut self, path: Self::Path) -> Option<Operand<'tcx>>;
/// Modifies the MIR patch so that the drop flag of `path` (if any) is cleared at `location`.
///
/// If `mode` is deep, drop flags of all child paths should also be cleared by inserting
/// additional statements.
fn clear_drop_flag(&mut self, location: Location, path: Self::Path, mode: DropFlagMode);
// Subpaths
/// Returns the subpath of a field of `path` (or `None` if there is no dedicated subpath).
///
/// If this returns `None`, `field` will not get a dedicated drop flag.
fn field_subpath(&self, path: Self::Path, field: FieldIdx) -> Option<Self::Path>;
/// Returns the subpath of a dereference of `path` (or `None` if there is no dedicated subpath).
///
/// If this returns `None`, `*path` will not get a dedicated drop flag.
///
/// This is only relevant for `Box<T>`, where the contained `T` can be moved out of the box.
fn deref_subpath(&self, path: Self::Path) -> Option<Self::Path>;
/// Returns the subpath of downcasting `path` to one of its variants.
///
/// If this returns `None`, the downcast of `path` will not get a dedicated drop flag.
fn downcast_subpath(&self, path: Self::Path, variant: VariantIdx) -> Option<Self::Path>;
/// Returns the subpath of indexing a fixed-size array `path`.
///
/// If this returns `None`, elements of `path` will not get a dedicated drop flag.
///
/// This is only relevant for array patterns, which can move out of individual array elements.
fn array_subpath(&self, path: Self::Path, index: u64, size: u64) -> Option<Self::Path>;
}
#[derive(Debug)]
struct DropCtxt<'l, 'b, 'tcx, D>
where
D: DropElaborator<'b, 'tcx>,
{
elaborator: &'l mut D,
source_info: SourceInfo,
place: Place<'tcx>,
path: D::Path,
succ: BasicBlock,
unwind: Unwind,
}
/// "Elaborates" a drop of `place`/`path` and patches `bb`'s terminator to execute it.
///
/// The passed `elaborator` is used to determine what should happen at the drop terminator. It
/// decides whether the drop can be statically determined or whether it needs a dynamic drop flag,
/// and whether the drop is "open", ie. should be expanded to drop all subfields of the dropped
/// value.
///
/// When this returns, the MIR patch in the `elaborator` contains the necessary changes.
pub fn elaborate_drop<'b, 'tcx, D>(
elaborator: &mut D,
source_info: SourceInfo,
place: Place<'tcx>,
path: D::Path,
succ: BasicBlock,
unwind: Unwind,
bb: BasicBlock,
) where
D: DropElaborator<'b, 'tcx>,
'tcx: 'b,
{
DropCtxt { elaborator, source_info, place, path, succ, unwind }.elaborate_drop(bb)
}
impl<'l, 'b, 'tcx, D> DropCtxt<'l, 'b, 'tcx, D>
where
D: DropElaborator<'b, 'tcx>,
'tcx: 'b,
{
#[instrument(level = "trace", skip(self), ret)]
fn place_ty(&self, place: Place<'tcx>) -> Ty<'tcx> {
place.ty(self.elaborator.body(), self.tcx()).ty
}
fn tcx(&self) -> TyCtxt<'tcx> {
self.elaborator.tcx()
}
/// This elaborates a single drop instruction, located at `bb`, and
/// patches over it.
///
/// The elaborated drop checks the drop flags to only drop what
/// is initialized.
///
/// In addition, the relevant drop flags also need to be cleared
/// to avoid double-drops. However, in the middle of a complex
/// drop, one must avoid clearing some of the flags before they
/// are read, as that would cause a memory leak.
///
/// In particular, when dropping an ADT, multiple fields may be
/// joined together under the `rest` subpath. They are all controlled
/// by the primary drop flag, but only the last rest-field dropped
/// should clear it (and it must also not clear anything else).
//
// FIXME: I think we should just control the flags externally,
// and then we do not need this machinery.
#[instrument(level = "debug")]
pub fn elaborate_drop(&mut self, bb: BasicBlock) {
match self.elaborator.drop_style(self.path, DropFlagMode::Deep) {
DropStyle::Dead => {
self.elaborator
.patch()
.patch_terminator(bb, TerminatorKind::Goto { target: self.succ });
}
DropStyle::Static => {
self.elaborator.patch().patch_terminator(
bb,
TerminatorKind::Drop {
place: self.place,
target: self.succ,
unwind: self.unwind.into_action(),
replace: false,
},
);
}
DropStyle::Conditional => {
let drop_bb = self.complete_drop(self.succ, self.unwind);
self.elaborator
.patch()
.patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
}
DropStyle::Open => {
let drop_bb = self.open_drop();
self.elaborator
.patch()
.patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
}
}
}
/// Returns the place and move path for each field of `variant`,
/// (the move path is `None` if the field is a rest field).
fn move_paths_for_fields(
&self,
base_place: Place<'tcx>,
variant_path: D::Path,
variant: &'tcx ty::VariantDef,
args: GenericArgsRef<'tcx>,
) -> Vec<(Place<'tcx>, Option<D::Path>)> {
variant
.fields
.iter()
.enumerate()
.map(|(i, f)| {
let field = FieldIdx::new(i);
let subpath = self.elaborator.field_subpath(variant_path, field);
let tcx = self.tcx();
assert_eq!(self.elaborator.param_env().reveal(), Reveal::All);
let field_ty =
tcx.normalize_erasing_regions(self.elaborator.param_env(), f.ty(tcx, args));
(tcx.mk_place_field(base_place, field, field_ty), subpath)
})
.collect()
}
fn drop_subpath(
&mut self,
place: Place<'tcx>,
path: Option<D::Path>,
succ: BasicBlock,
unwind: Unwind,
) -> BasicBlock {
if let Some(path) = path {
debug!("drop_subpath: for std field {:?}", place);
DropCtxt {
elaborator: self.elaborator,
source_info: self.source_info,
path,
place,
succ,
unwind,
}
.elaborated_drop_block()
} else {
debug!("drop_subpath: for rest field {:?}", place);
DropCtxt {
elaborator: self.elaborator,
source_info: self.source_info,
place,
succ,
unwind,
// Using `self.path` here to condition the drop on
// our own drop flag.
path: self.path,
}
.complete_drop(succ, unwind)
}
}
/// Creates one-half of the drop ladder for a list of fields, and return
/// the list of steps in it in reverse order, with the first step
/// dropping 0 fields and so on.
///
/// `unwind_ladder` is such a list of steps in reverse order,
/// which is called if the matching step of the drop glue panics.
fn drop_halfladder(
&mut self,
unwind_ladder: &[Unwind],
mut succ: BasicBlock,
fields: &[(Place<'tcx>, Option<D::Path>)],
) -> Vec<BasicBlock> {
iter::once(succ)
.chain(fields.iter().rev().zip(unwind_ladder).map(|(&(place, path), &unwind_succ)| {
succ = self.drop_subpath(place, path, succ, unwind_succ);
succ
}))
.collect()
}
fn drop_ladder_bottom(&mut self) -> (BasicBlock, Unwind) {
// Clear the "master" drop flag at the end. This is needed
// because the "master" drop protects the ADT's discriminant,
// which is invalidated after the ADT is dropped.
(self.drop_flag_reset_block(DropFlagMode::Shallow, self.succ, self.unwind), self.unwind)
}
/// Creates a full drop ladder, consisting of 2 connected half-drop-ladders
///
/// For example, with 3 fields, the drop ladder is
///
/// .d0:
/// ELAB(drop location.0 [target=.d1, unwind=.c1])
/// .d1:
/// ELAB(drop location.1 [target=.d2, unwind=.c2])
/// .d2:
/// ELAB(drop location.2 [target=`self.succ`, unwind=`self.unwind`])
/// .c1:
/// ELAB(drop location.1 [target=.c2])
/// .c2:
/// ELAB(drop location.2 [target=`self.unwind`])
///
/// NOTE: this does not clear the master drop flag, so you need
/// to point succ/unwind on a `drop_ladder_bottom`.
fn drop_ladder(
&mut self,
fields: Vec<(Place<'tcx>, Option<D::Path>)>,
succ: BasicBlock,
unwind: Unwind,
) -> (BasicBlock, Unwind) {
debug!("drop_ladder({:?}, {:?})", self, fields);
let mut fields = fields;
fields.retain(|&(place, _)| {
self.place_ty(place).needs_drop(self.tcx(), self.elaborator.param_env())
});
debug!("drop_ladder - fields needing drop: {:?}", fields);
let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
let unwind_ladder: Vec<_> = if let Unwind::To(target) = unwind {
let halfladder = self.drop_halfladder(&unwind_ladder, target, &fields);
halfladder.into_iter().map(Unwind::To).collect()
} else {
unwind_ladder
};
let normal_ladder = self.drop_halfladder(&unwind_ladder, succ, &fields);
(*normal_ladder.last().unwrap(), *unwind_ladder.last().unwrap())
}
fn open_drop_for_tuple(&mut self, tys: &[Ty<'tcx>]) -> BasicBlock {
debug!("open_drop_for_tuple({:?}, {:?})", self, tys);
let fields = tys
.iter()
.enumerate()
.map(|(i, &ty)| {
(
self.tcx().mk_place_field(self.place, FieldIdx::new(i), ty),
self.elaborator.field_subpath(self.path, FieldIdx::new(i)),
)
})
.collect();
let (succ, unwind) = self.drop_ladder_bottom();
self.drop_ladder(fields, succ, unwind).0
}
/// Drops the T contained in a `Box<T>` if it has not been moved out of
#[instrument(level = "debug", ret)]
fn open_drop_for_box_contents(
&mut self,
adt: ty::AdtDef<'tcx>,
args: GenericArgsRef<'tcx>,
succ: BasicBlock,
unwind: Unwind,
) -> BasicBlock {
// drop glue is sent straight to codegen
// box cannot be directly dereferenced
let unique_ty = adt.non_enum_variant().fields[FieldIdx::ZERO].ty(self.tcx(), args);
let unique_variant = unique_ty.ty_adt_def().unwrap().non_enum_variant();
let nonnull_ty = unique_variant.fields[FieldIdx::ZERO].ty(self.tcx(), args);
let ptr_ty = Ty::new_imm_ptr(self.tcx(), args[0].expect_ty());
let unique_place = self.tcx().mk_place_field(self.place, FieldIdx::ZERO, unique_ty);
let nonnull_place = self.tcx().mk_place_field(unique_place, FieldIdx::ZERO, nonnull_ty);
let ptr_place = self.tcx().mk_place_field(nonnull_place, FieldIdx::ZERO, ptr_ty);
let interior = self.tcx().mk_place_deref(ptr_place);
let interior_path = self.elaborator.deref_subpath(self.path);
self.drop_subpath(interior, interior_path, succ, unwind)
}
#[instrument(level = "debug", ret)]
fn open_drop_for_adt(
&mut self,
adt: ty::AdtDef<'tcx>,
args: GenericArgsRef<'tcx>,
) -> BasicBlock {
if adt.variants().is_empty() {
return self.elaborator.patch().new_block(BasicBlockData {
statements: vec![],
terminator: Some(Terminator {
source_info: self.source_info,
kind: TerminatorKind::Unreachable,
}),
is_cleanup: self.unwind.is_cleanup(),
});
}
let skip_contents = adt.is_union() || adt.is_manually_drop();
let contents_drop = if skip_contents {
(self.succ, self.unwind)
} else {
self.open_drop_for_adt_contents(adt, args)
};
if adt.is_box() {
// we need to drop the inside of the box before running the destructor
let succ = self.destructor_call_block(contents_drop);
let unwind = contents_drop
.1
.map(|unwind| self.destructor_call_block((unwind, Unwind::InCleanup)));
self.open_drop_for_box_contents(adt, args, succ, unwind)
} else if adt.has_dtor(self.tcx()) {
self.destructor_call_block(contents_drop)
} else {
contents_drop.0
}
}
fn open_drop_for_adt_contents(
&mut self,
adt: ty::AdtDef<'tcx>,
args: GenericArgsRef<'tcx>,
) -> (BasicBlock, Unwind) {
let (succ, unwind) = self.drop_ladder_bottom();
if !adt.is_enum() {
let fields =
self.move_paths_for_fields(self.place, self.path, adt.variant(FIRST_VARIANT), args);
self.drop_ladder(fields, succ, unwind)
} else {
self.open_drop_for_multivariant(adt, args, succ, unwind)
}
}
fn open_drop_for_multivariant(
&mut self,
adt: ty::AdtDef<'tcx>,
args: GenericArgsRef<'tcx>,
succ: BasicBlock,
unwind: Unwind,
) -> (BasicBlock, Unwind) {
let mut values = Vec::with_capacity(adt.variants().len());
let mut normal_blocks = Vec::with_capacity(adt.variants().len());
let mut unwind_blocks =
if unwind.is_cleanup() { None } else { Some(Vec::with_capacity(adt.variants().len())) };
let mut have_otherwise_with_drop_glue = false;
let mut have_otherwise = false;
let tcx = self.tcx();
for (variant_index, discr) in adt.discriminants(tcx) {
let variant = &adt.variant(variant_index);
let subpath = self.elaborator.downcast_subpath(self.path, variant_index);
if let Some(variant_path) = subpath {
let base_place = tcx.mk_place_elem(
self.place,
ProjectionElem::Downcast(Some(variant.name), variant_index),
);
let fields = self.move_paths_for_fields(base_place, variant_path, variant, args);
values.push(discr.val);
if let Unwind::To(unwind) = unwind {
// We can't use the half-ladder from the original
// drop ladder, because this breaks the
// "funclet can't have 2 successor funclets"
// requirement from MSVC:
//
// switch unwind-switch
// / \ / \
// v1.0 v2.0 v2.0-unwind v1.0-unwind
// | | / |
// v1.1-unwind v2.1-unwind |
// ^ |
// \-------------------------------/
//
// Create a duplicate half-ladder to avoid that. We
// could technically only do this on MSVC, but I
// I want to minimize the divergence between MSVC
// and non-MSVC.
let unwind_blocks = unwind_blocks.as_mut().unwrap();
let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
let halfladder = self.drop_halfladder(&unwind_ladder, unwind, &fields);
unwind_blocks.push(halfladder.last().cloned().unwrap());
}
let (normal, _) = self.drop_ladder(fields, succ, unwind);
normal_blocks.push(normal);
} else {
have_otherwise = true;
let param_env = self.elaborator.param_env();
let have_field_with_drop_glue = variant
.fields
.iter()
.any(|field| field.ty(tcx, args).needs_drop(tcx, param_env));
if have_field_with_drop_glue {
have_otherwise_with_drop_glue = true;
}
}
}
if !have_otherwise {
values.pop();
} else if !have_otherwise_with_drop_glue {
normal_blocks.push(self.goto_block(succ, unwind));
if let Unwind::To(unwind) = unwind {
unwind_blocks.as_mut().unwrap().push(self.goto_block(unwind, Unwind::InCleanup));
}
} else {
normal_blocks.push(self.drop_block(succ, unwind));
if let Unwind::To(unwind) = unwind {
unwind_blocks.as_mut().unwrap().push(self.drop_block(unwind, Unwind::InCleanup));
}
}
(
self.adt_switch_block(adt, normal_blocks, &values, succ, unwind),
unwind.map(|unwind| {
self.adt_switch_block(
adt,
unwind_blocks.unwrap(),
&values,
unwind,
Unwind::InCleanup,
)
}),
)
}
fn adt_switch_block(
&mut self,
adt: ty::AdtDef<'tcx>,
blocks: Vec<BasicBlock>,
values: &[u128],
succ: BasicBlock,
unwind: Unwind,
) -> BasicBlock {
// If there are multiple variants, then if something
// is present within the enum the discriminant, tracked
// by the rest path, must be initialized.
//
// Additionally, we do not want to switch on the
// discriminant after it is free-ed, because that
// way lies only trouble.
let discr_ty = adt.repr().discr_type().to_ty(self.tcx());
let discr = Place::from(self.new_temp(discr_ty));
let discr_rv = Rvalue::Discriminant(self.place);
let switch_block = BasicBlockData {
statements: vec![self.assign(discr, discr_rv)],
terminator: Some(Terminator {
source_info: self.source_info,
kind: TerminatorKind::SwitchInt {
discr: Operand::Move(discr),
targets: SwitchTargets::new(
values.iter().copied().zip(blocks.iter().copied()),
*blocks.last().unwrap(),
),
},
}),
is_cleanup: unwind.is_cleanup(),
};
let switch_block = self.elaborator.patch().new_block(switch_block);
self.drop_flag_test_block(switch_block, succ, unwind)
}
fn destructor_call_block(&mut self, (succ, unwind): (BasicBlock, Unwind)) -> BasicBlock {
debug!("destructor_call_block({:?}, {:?})", self, succ);
let tcx = self.tcx();
let drop_trait = tcx.require_lang_item(LangItem::Drop, None);
let drop_fn = tcx.associated_item_def_ids(drop_trait)[0];
let ty = self.place_ty(self.place);
let ref_ty = Ty::new_mut_ref(tcx, tcx.lifetimes.re_erased, ty);
let ref_place = self.new_temp(ref_ty);
let unit_temp = Place::from(self.new_temp(tcx.types.unit));
let result = BasicBlockData {
statements: vec![self.assign(
Place::from(ref_place),
Rvalue::Ref(
tcx.lifetimes.re_erased,
BorrowKind::Mut { kind: MutBorrowKind::Default },
self.place,
),
)],
terminator: Some(Terminator {
kind: TerminatorKind::Call {
func: Operand::function_handle(
tcx,
drop_fn,
[ty.into()],
self.source_info.span,
),
args: [Spanned { node: Operand::Move(Place::from(ref_place)), span: DUMMY_SP }]
.into(),
destination: unit_temp,
target: Some(succ),
unwind: unwind.into_action(),
call_source: CallSource::Misc,
fn_span: self.source_info.span,
},
source_info: self.source_info,
}),
is_cleanup: unwind.is_cleanup(),
};
let destructor_block = self.elaborator.patch().new_block(result);
let block_start = Location { block: destructor_block, statement_index: 0 };
self.elaborator.clear_drop_flag(block_start, self.path, DropFlagMode::Shallow);
self.drop_flag_test_block(destructor_block, succ, unwind)
}
/// Create a loop that drops an array:
///
/// ```text
/// loop-block:
/// can_go = cur == len
/// if can_go then succ else drop-block
/// drop-block:
/// ptr = &raw mut P[cur]
/// cur = cur + 1
/// drop(ptr)
/// ```
fn drop_loop(
&mut self,
succ: BasicBlock,
cur: Local,
len: Local,
ety: Ty<'tcx>,
unwind: Unwind,
) -> BasicBlock {
let copy = |place: Place<'tcx>| Operand::Copy(place);
let move_ = |place: Place<'tcx>| Operand::Move(place);
let tcx = self.tcx();
let ptr_ty = Ty::new_mut_ptr(tcx, ety);
let ptr = Place::from(self.new_temp(ptr_ty));
let can_go = Place::from(self.new_temp(tcx.types.bool));
let one = self.constant_usize(1);
let drop_block = BasicBlockData {
statements: vec![
self.assign(
ptr,
Rvalue::RawPtr(Mutability::Mut, tcx.mk_place_index(self.place, cur)),
),
self.assign(
cur.into(),
Rvalue::BinaryOp(BinOp::Add, Box::new((move_(cur.into()), one))),
),
],
is_cleanup: unwind.is_cleanup(),
terminator: Some(Terminator {
source_info: self.source_info,
// this gets overwritten by drop elaboration.
kind: TerminatorKind::Unreachable,
}),
};
let drop_block = self.elaborator.patch().new_block(drop_block);
let loop_block = BasicBlockData {
statements: vec![self.assign(
can_go,
Rvalue::BinaryOp(BinOp::Eq, Box::new((copy(Place::from(cur)), copy(len.into())))),
)],
is_cleanup: unwind.is_cleanup(),
terminator: Some(Terminator {
source_info: self.source_info,
kind: TerminatorKind::if_(move_(can_go), succ, drop_block),
}),
};
let loop_block = self.elaborator.patch().new_block(loop_block);
self.elaborator.patch().patch_terminator(
drop_block,
TerminatorKind::Drop {
place: tcx.mk_place_deref(ptr),
target: loop_block,
unwind: unwind.into_action(),
replace: false,
},
);
loop_block
}
fn open_drop_for_array(&mut self, ety: Ty<'tcx>, opt_size: Option<u64>) -> BasicBlock {
debug!("open_drop_for_array({:?}, {:?})", ety, opt_size);
let tcx = self.tcx();
if let Some(size) = opt_size {
enum ProjectionKind<Path> {
Drop(std::ops::Range<u64>),
Keep(u64, Path),
}
// Previously, we'd make a projection for every element in the array and create a drop
// ladder if any `array_subpath` was `Some`, i.e. moving out with an array pattern.
// This caused huge memory usage when generating the drops for large arrays, so we instead
// record the *subslices* which are dropped and the *indexes* which are kept
let mut drop_ranges = vec![];
let mut dropping = true;
let mut start = 0;
for i in 0..size {
let path = self.elaborator.array_subpath(self.path, i, size);
if dropping && path.is_some() {
drop_ranges.push(ProjectionKind::Drop(start..i));
dropping = false;
} else if !dropping && path.is_none() {
dropping = true;
start = i;
}
if let Some(path) = path {
drop_ranges.push(ProjectionKind::Keep(i, path));
}
}
if !drop_ranges.is_empty() {
if dropping {
drop_ranges.push(ProjectionKind::Drop(start..size));
}
let fields = drop_ranges
.iter()
.rev()
.map(|p| {
let (project, path) = match p {
ProjectionKind::Drop(r) => (
ProjectionElem::Subslice {
from: r.start,
to: r.end,
from_end: false,
},
None,
),
&ProjectionKind::Keep(offset, path) => (
ProjectionElem::ConstantIndex {
offset,
min_length: size,
from_end: false,
},
Some(path),
),
};
(tcx.mk_place_elem(self.place, project), path)
})
.collect::<Vec<_>>();
let (succ, unwind) = self.drop_ladder_bottom();
return self.drop_ladder(fields, succ, unwind).0;
}
}
self.drop_loop_pair(ety)
}
/// Creates a pair of drop-loops of `place`, which drops its contents, even
/// in the case of 1 panic.
fn drop_loop_pair(&mut self, ety: Ty<'tcx>) -> BasicBlock {
debug!("drop_loop_pair({:?})", ety);
let tcx = self.tcx();
let len = self.new_temp(tcx.types.usize);
let cur = self.new_temp(tcx.types.usize);
let unwind =
self.unwind.map(|unwind| self.drop_loop(unwind, cur, len, ety, Unwind::InCleanup));
let loop_block = self.drop_loop(self.succ, cur, len, ety, unwind);
let zero = self.constant_usize(0);
let block = BasicBlockData {
statements: vec![
self.assign(len.into(), Rvalue::Len(self.place)),
self.assign(cur.into(), Rvalue::Use(zero)),
],
is_cleanup: unwind.is_cleanup(),
terminator: Some(Terminator {
source_info: self.source_info,
kind: TerminatorKind::Goto { target: loop_block },
}),
};
let drop_block = self.elaborator.patch().new_block(block);
// FIXME(#34708): handle partially-dropped array/slice elements.
let reset_block = self.drop_flag_reset_block(DropFlagMode::Deep, drop_block, unwind);
self.drop_flag_test_block(reset_block, self.succ, unwind)
}
/// The slow-path - create an "open", elaborated drop for a type
/// which is moved-out-of only partially, and patch `bb` to a jump
/// to it. This must not be called on ADTs with a destructor,
/// as these can't be moved-out-of, except for `Box<T>`, which is
/// special-cased.
///
/// This creates a "drop ladder" that drops the needed fields of the
/// ADT, both in the success case or if one of the destructors fail.
fn open_drop(&mut self) -> BasicBlock {
let ty = self.place_ty(self.place);
match ty.kind() {
ty::Closure(_, args) => self.open_drop_for_tuple(args.as_closure().upvar_tys()),
ty::CoroutineClosure(_, args) => {
self.open_drop_for_tuple(args.as_coroutine_closure().upvar_tys())
}
// Note that `elaborate_drops` only drops the upvars of a coroutine,
// and this is ok because `open_drop` here can only be reached
// within that own coroutine's resume function.
// This should only happen for the self argument on the resume function.
// It effectively only contains upvars until the coroutine transformation runs.
// See librustc_body/transform/coroutine.rs for more details.
ty::Coroutine(_, args) => self.open_drop_for_tuple(args.as_coroutine().upvar_tys()),
ty::Tuple(fields) => self.open_drop_for_tuple(fields),
ty::Adt(def, args) => self.open_drop_for_adt(*def, args),
ty::Dynamic(..) => self.complete_drop(self.succ, self.unwind),
ty::Array(ety, size) => {
let size = size.try_eval_target_usize(self.tcx(), self.elaborator.param_env());
self.open_drop_for_array(*ety, size)
}
ty::Slice(ety) => self.drop_loop_pair(*ety),
_ => span_bug!(self.source_info.span, "open drop from non-ADT `{:?}`", ty),
}
}
fn complete_drop(&mut self, succ: BasicBlock, unwind: Unwind) -> BasicBlock {
debug!("complete_drop(succ={:?}, unwind={:?})", succ, unwind);
let drop_block = self.drop_block(succ, unwind);
self.drop_flag_test_block(drop_block, succ, unwind)
}
/// Creates a block that resets the drop flag. If `mode` is deep, all children drop flags will
/// also be cleared.
fn drop_flag_reset_block(
&mut self,
mode: DropFlagMode,
succ: BasicBlock,
unwind: Unwind,
) -> BasicBlock {
debug!("drop_flag_reset_block({:?},{:?})", self, mode);
if unwind.is_cleanup() {
// The drop flag isn't read again on the unwind path, so don't
// bother setting it.
return succ;
}
let block = self.new_block(unwind, TerminatorKind::Goto { target: succ });
let block_start = Location { block, statement_index: 0 };
self.elaborator.clear_drop_flag(block_start, self.path, mode);
block
}
fn elaborated_drop_block(&mut self) -> BasicBlock {
debug!("elaborated_drop_block({:?})", self);
let blk = self.drop_block(self.succ, self.unwind);
self.elaborate_drop(blk);
blk
}
fn drop_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
let block = TerminatorKind::Drop {
place: self.place,
target,
unwind: unwind.into_action(),
replace: false,
};
self.new_block(unwind, block)
}
fn goto_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
let block = TerminatorKind::Goto { target };
self.new_block(unwind, block)
}
/// Returns the block to jump to in order to test the drop flag and execute the drop.
///
/// Depending on the required `DropStyle`, this might be a generated block with an `if`
/// terminator (for dynamic/open drops), or it might be `on_set` or `on_unset` itself, in case
/// the drop can be statically determined.
fn drop_flag_test_block(
&mut self,
on_set: BasicBlock,
on_unset: BasicBlock,
unwind: Unwind,
) -> BasicBlock {
let style = self.elaborator.drop_style(self.path, DropFlagMode::Shallow);
debug!(
"drop_flag_test_block({:?},{:?},{:?},{:?}) - {:?}",
self, on_set, on_unset, unwind, style
);
match style {
DropStyle::Dead => on_unset,
DropStyle::Static => on_set,
DropStyle::Conditional | DropStyle::Open => {
let flag = self.elaborator.get_drop_flag(self.path).unwrap();
let term = TerminatorKind::if_(flag, on_set, on_unset);
self.new_block(unwind, term)
}
}
}
fn new_block(&mut self, unwind: Unwind, k: TerminatorKind<'tcx>) -> BasicBlock {
self.elaborator.patch().new_block(BasicBlockData {
statements: vec![],
terminator: Some(Terminator { source_info: self.source_info, kind: k }),
is_cleanup: unwind.is_cleanup(),
})
}
fn new_temp(&mut self, ty: Ty<'tcx>) -> Local {
self.elaborator.patch().new_temp(ty, self.source_info.span)
}
fn constant_usize(&self, val: u16) -> Operand<'tcx> {
Operand::Constant(Box::new(ConstOperand {
span: self.source_info.span,
user_ty: None,
const_: Const::from_usize(self.tcx(), val.into()),
}))
}
fn assign(&self, lhs: Place<'tcx>, rhs: Rvalue<'tcx>) -> Statement<'tcx> {
Statement {
source_info: self.source_info,
kind: StatementKind::Assign(Box::new((lhs, rhs))),
}
}
}