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generator.rs
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generator.rs
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//! This is the implementation of the pass which transforms generators into state machines.
//!
//! MIR generation for generators creates a function which has a self argument which
//! passes by value. This argument is effectively a generator type which only contains upvars and
//! is only used for this argument inside the MIR for the generator.
//! It is passed by value to enable upvars to be moved out of it. Drop elaboration runs on that
//! MIR before this pass and creates drop flags for MIR locals.
//! It will also drop the generator argument (which only consists of upvars) if any of the upvars
//! are moved out of. This pass elaborates the drops of upvars / generator argument in the case
//! that none of the upvars were moved out of. This is because we cannot have any drops of this
//! generator in the MIR, since it is used to create the drop glue for the generator. We'd get
//! infinite recursion otherwise.
//!
//! This pass creates the implementation for the Generator::resume function and the drop shim
//! for the generator based on the MIR input. It converts the generator argument from Self to
//! &mut Self adding derefs in the MIR as needed. It computes the final layout of the generator
//! struct which looks like this:
//! First upvars are stored
//! It is followed by the generator state field.
//! Then finally the MIR locals which are live across a suspension point are stored.
//!
//! struct Generator {
//! upvars...,
//! state: u32,
//! mir_locals...,
//! }
//!
//! This pass computes the meaning of the state field and the MIR locals which are live
//! across a suspension point. There are however two hardcoded generator states:
//! 0 - Generator have not been resumed yet
//! 1 - Generator has returned / is completed
//! 2 - Generator has been poisoned
//!
//! It also rewrites `return x` and `yield y` as setting a new generator state and returning
//! GeneratorState::Complete(x) and GeneratorState::Yielded(y) respectively.
//! MIR locals which are live across a suspension point are moved to the generator struct
//! with references to them being updated with references to the generator struct.
//!
//! The pass creates two functions which have a switch on the generator state giving
//! the action to take.
//!
//! One of them is the implementation of Generator::resume.
//! For generators with state 0 (unresumed) it starts the execution of the generator.
//! For generators with state 1 (returned) and state 2 (poisoned) it panics.
//! Otherwise it continues the execution from the last suspension point.
//!
//! The other function is the drop glue for the generator.
//! For generators with state 0 (unresumed) it drops the upvars of the generator.
//! For generators with state 1 (returned) and state 2 (poisoned) it does nothing.
//! Otherwise it drops all the values in scope at the last suspension point.
use rustc::hir;
use rustc::hir::def_id::DefId;
use rustc::mir::*;
use rustc::mir::visit::{PlaceContext, Visitor, MutVisitor};
use rustc::ty::{self, TyCtxt, AdtDef, Ty};
use rustc::ty::layout::VariantIdx;
use rustc::ty::subst::SubstsRef;
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::indexed_vec::Idx;
use rustc_data_structures::bit_set::BitSet;
use std::borrow::Cow;
use std::iter::once;
use std::mem;
use crate::transform::{MirPass, MirSource};
use crate::transform::simplify;
use crate::transform::no_landing_pads::no_landing_pads;
use crate::dataflow::{do_dataflow, DebugFormatted, state_for_location};
use crate::dataflow::{MaybeStorageLive, HaveBeenBorrowedLocals};
use crate::util::dump_mir;
use crate::util::liveness;
pub struct StateTransform;
struct RenameLocalVisitor {
from: Local,
to: Local,
}
impl<'tcx> MutVisitor<'tcx> for RenameLocalVisitor {
fn visit_local(&mut self,
local: &mut Local,
_: PlaceContext<'tcx>,
_: Location) {
if *local == self.from {
*local = self.to;
}
}
}
struct DerefArgVisitor;
impl<'tcx> MutVisitor<'tcx> for DerefArgVisitor {
fn visit_local(&mut self,
local: &mut Local,
_: PlaceContext<'tcx>,
_: Location) {
assert_ne!(*local, self_arg());
}
fn visit_place(&mut self,
place: &mut Place<'tcx>,
context: PlaceContext<'tcx>,
location: Location) {
if *place == Place::Base(PlaceBase::Local(self_arg())) {
*place = Place::Projection(Box::new(Projection {
base: place.clone(),
elem: ProjectionElem::Deref,
}));
} else {
self.super_place(place, context, location);
}
}
}
struct PinArgVisitor<'tcx> {
ref_gen_ty: Ty<'tcx>,
}
impl<'tcx> MutVisitor<'tcx> for PinArgVisitor<'tcx> {
fn visit_local(&mut self,
local: &mut Local,
_: PlaceContext<'tcx>,
_: Location) {
assert_ne!(*local, self_arg());
}
fn visit_place(&mut self,
place: &mut Place<'tcx>,
context: PlaceContext<'tcx>,
location: Location) {
if *place == Place::Base(PlaceBase::Local(self_arg())) {
*place = Place::Projection(Box::new(Projection {
base: place.clone(),
elem: ProjectionElem::Field(Field::new(0), self.ref_gen_ty),
}));
} else {
self.super_place(place, context, location);
}
}
}
fn self_arg() -> Local {
Local::new(1)
}
struct SuspensionPoint {
state: u32,
resume: BasicBlock,
drop: Option<BasicBlock>,
storage_liveness: liveness::LiveVarSet,
}
struct TransformVisitor<'a, 'tcx: 'a> {
tcx: TyCtxt<'a, 'tcx, 'tcx>,
state_adt_ref: &'tcx AdtDef,
state_substs: SubstsRef<'tcx>,
// The index of the generator state in the generator struct
state_field: usize,
// Mapping from Local to (type of local, generator struct index)
// FIXME(eddyb) This should use `IndexVec<Local, Option<_>>`.
remap: FxHashMap<Local, (Ty<'tcx>, usize)>,
// A map from a suspension point in a block to the locals which have live storage at that point
// FIXME(eddyb) This should use `IndexVec<BasicBlock, Option<_>>`.
storage_liveness: FxHashMap<BasicBlock, liveness::LiveVarSet>,
// A list of suspension points, generated during the transform
suspension_points: Vec<SuspensionPoint>,
// The original RETURN_PLACE local
new_ret_local: Local,
}
impl<'a, 'tcx> TransformVisitor<'a, 'tcx> {
// Make a GeneratorState rvalue
fn make_state(&self, idx: VariantIdx, val: Operand<'tcx>) -> Rvalue<'tcx> {
let adt = AggregateKind::Adt(self.state_adt_ref, idx, self.state_substs, None, None);
Rvalue::Aggregate(box adt, vec![val])
}
// Create a Place referencing a generator struct field
fn make_field(&self, idx: usize, ty: Ty<'tcx>) -> Place<'tcx> {
let base = Place::Base(PlaceBase::Local(self_arg()));
let field = Projection {
base: base,
elem: ProjectionElem::Field(Field::new(idx), ty),
};
Place::Projection(Box::new(field))
}
// Create a statement which changes the generator state
fn set_state(&self, state_disc: u32, source_info: SourceInfo) -> Statement<'tcx> {
let state = self.make_field(self.state_field, self.tcx.types.u32);
let val = Operand::Constant(box Constant {
span: source_info.span,
ty: self.tcx.types.u32,
user_ty: None,
literal: self.tcx.mk_lazy_const(ty::LazyConst::Evaluated(ty::Const::from_bits(
self.tcx,
state_disc.into(),
ty::ParamEnv::empty().and(self.tcx.types.u32)
))),
});
Statement {
source_info,
kind: StatementKind::Assign(state, box Rvalue::Use(val)),
}
}
}
impl<'a, 'tcx> MutVisitor<'tcx> for TransformVisitor<'a, 'tcx> {
fn visit_local(&mut self,
local: &mut Local,
_: PlaceContext<'tcx>,
_: Location) {
assert_eq!(self.remap.get(local), None);
}
fn visit_place(&mut self,
place: &mut Place<'tcx>,
context: PlaceContext<'tcx>,
location: Location) {
if let Place::Base(PlaceBase::Local(l)) = *place {
// Replace an Local in the remap with a generator struct access
if let Some(&(ty, idx)) = self.remap.get(&l) {
*place = self.make_field(idx, ty);
}
} else {
self.super_place(place, context, location);
}
}
fn visit_basic_block_data(&mut self,
block: BasicBlock,
data: &mut BasicBlockData<'tcx>) {
// Remove StorageLive and StorageDead statements for remapped locals
data.retain_statements(|s| {
match s.kind {
StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => {
!self.remap.contains_key(&l)
}
_ => true
}
});
let ret_val = match data.terminator().kind {
TerminatorKind::Return => Some((VariantIdx::new(1),
None,
Operand::Move(Place::Base(PlaceBase::Local(self.new_ret_local))),
None)),
TerminatorKind::Yield { ref value, resume, drop } => Some((VariantIdx::new(0),
Some(resume),
value.clone(),
drop)),
_ => None
};
if let Some((state_idx, resume, v, drop)) = ret_val {
let source_info = data.terminator().source_info;
// We must assign the value first in case it gets declared dead below
data.statements.push(Statement {
source_info,
kind: StatementKind::Assign(Place::RETURN_PLACE,
box self.make_state(state_idx, v)),
});
let state = if let Some(resume) = resume { // Yield
let state = 3 + self.suspension_points.len() as u32;
self.suspension_points.push(SuspensionPoint {
state,
resume,
drop,
storage_liveness: self.storage_liveness.get(&block).unwrap().clone(),
});
state
} else { // Return
1 // state for returned
};
data.statements.push(self.set_state(state, source_info));
data.terminator.as_mut().unwrap().kind = TerminatorKind::Return;
}
self.super_basic_block_data(block, data);
}
}
fn make_generator_state_argument_indirect<'a, 'tcx>(
tcx: TyCtxt<'a, 'tcx, 'tcx>,
def_id: DefId,
mir: &mut Mir<'tcx>) {
let gen_ty = mir.local_decls.raw[1].ty;
let region = ty::ReFree(ty::FreeRegion {
scope: def_id,
bound_region: ty::BoundRegion::BrEnv,
});
let region = tcx.mk_region(region);
let ref_gen_ty = tcx.mk_ref(region, ty::TypeAndMut {
ty: gen_ty,
mutbl: hir::MutMutable
});
// Replace the by value generator argument
mir.local_decls.raw[1].ty = ref_gen_ty;
// Add a deref to accesses of the generator state
DerefArgVisitor.visit_mir(mir);
}
fn make_generator_state_argument_pinned<'a, 'tcx>(
tcx: TyCtxt<'a, 'tcx, 'tcx>,
mir: &mut Mir<'tcx>) {
let ref_gen_ty = mir.local_decls.raw[1].ty;
let pin_did = tcx.lang_items().pin_type().unwrap();
let pin_adt_ref = tcx.adt_def(pin_did);
let substs = tcx.intern_substs(&[ref_gen_ty.into()]);
let pin_ref_gen_ty = tcx.mk_adt(pin_adt_ref, substs);
// Replace the by ref generator argument
mir.local_decls.raw[1].ty = pin_ref_gen_ty;
// Add the Pin field access to accesses of the generator state
PinArgVisitor { ref_gen_ty }.visit_mir(mir);
}
fn replace_result_variable<'tcx>(
ret_ty: Ty<'tcx>,
mir: &mut Mir<'tcx>,
) -> Local {
let source_info = source_info(mir);
let new_ret = LocalDecl {
mutability: Mutability::Mut,
ty: ret_ty,
user_ty: UserTypeProjections::none(),
name: None,
source_info,
visibility_scope: source_info.scope,
internal: false,
is_block_tail: None,
is_user_variable: None,
};
let new_ret_local = Local::new(mir.local_decls.len());
mir.local_decls.push(new_ret);
mir.local_decls.swap(RETURN_PLACE, new_ret_local);
RenameLocalVisitor {
from: RETURN_PLACE,
to: new_ret_local,
}.visit_mir(mir);
new_ret_local
}
struct StorageIgnored(liveness::LiveVarSet);
impl<'tcx> Visitor<'tcx> for StorageIgnored {
fn visit_statement(&mut self,
_block: BasicBlock,
statement: &Statement<'tcx>,
_location: Location) {
match statement.kind {
StatementKind::StorageLive(l) |
StatementKind::StorageDead(l) => { self.0.remove(l); }
_ => (),
}
}
}
fn locals_live_across_suspend_points(
tcx: TyCtxt<'a, 'tcx, 'tcx>,
mir: &Mir<'tcx>,
source: MirSource<'tcx>,
movable: bool,
) -> (
liveness::LiveVarSet,
FxHashMap<BasicBlock, liveness::LiveVarSet>,
) {
let dead_unwinds = BitSet::new_empty(mir.basic_blocks().len());
let def_id = source.def_id();
// Calculate when MIR locals have live storage. This gives us an upper bound of their
// lifetimes.
let storage_live_analysis = MaybeStorageLive::new(mir);
let storage_live =
do_dataflow(tcx, mir, def_id, &[], &dead_unwinds, storage_live_analysis,
|bd, p| DebugFormatted::new(&bd.mir().local_decls[p]));
// Find the MIR locals which do not use StorageLive/StorageDead statements.
// The storage of these locals are always live.
let mut ignored = StorageIgnored(BitSet::new_filled(mir.local_decls.len()));
ignored.visit_mir(mir);
// Calculate the MIR locals which have been previously
// borrowed (even if they are still active).
// This is only used for immovable generators.
let borrowed_locals = if !movable {
let analysis = HaveBeenBorrowedLocals::new(mir);
let result =
do_dataflow(tcx, mir, def_id, &[], &dead_unwinds, analysis,
|bd, p| DebugFormatted::new(&bd.mir().local_decls[p]));
Some((analysis, result))
} else {
None
};
// Calculate the liveness of MIR locals ignoring borrows.
let mut set = liveness::LiveVarSet::new_empty(mir.local_decls.len());
let mut liveness = liveness::liveness_of_locals(
mir,
);
liveness::dump_mir(
tcx,
"generator_liveness",
source,
mir,
&liveness,
);
let mut storage_liveness_map = FxHashMap::default();
for (block, data) in mir.basic_blocks().iter_enumerated() {
if let TerminatorKind::Yield { .. } = data.terminator().kind {
let loc = Location {
block: block,
statement_index: data.statements.len(),
};
if let Some((ref analysis, ref result)) = borrowed_locals {
let borrowed_locals = state_for_location(loc,
analysis,
result,
mir);
// The `liveness` variable contains the liveness of MIR locals ignoring borrows.
// This is correct for movable generators since borrows cannot live across
// suspension points. However for immovable generators we need to account for
// borrows, so we conseratively assume that all borrowed locals are live until
// we find a StorageDead statement referencing the locals.
// To do this we just union our `liveness` result with `borrowed_locals`, which
// contains all the locals which has been borrowed before this suspension point.
// If a borrow is converted to a raw reference, we must also assume that it lives
// forever. Note that the final liveness is still bounded by the storage liveness
// of the local, which happens using the `intersect` operation below.
liveness.outs[block].union(&borrowed_locals);
}
let mut storage_liveness = state_for_location(loc,
&storage_live_analysis,
&storage_live,
mir);
// Store the storage liveness for later use so we can restore the state
// after a suspension point
storage_liveness_map.insert(block, storage_liveness.clone());
// Mark locals without storage statements as always having live storage
storage_liveness.union(&ignored.0);
// Locals live are live at this point only if they are used across
// suspension points (the `liveness` variable)
// and their storage is live (the `storage_liveness` variable)
storage_liveness.intersect(&liveness.outs[block]);
let live_locals = storage_liveness;
// Add the locals life at this suspension point to the set of locals which live across
// any suspension points
set.union(&live_locals);
}
}
// The generator argument is ignored
set.remove(self_arg());
(set, storage_liveness_map)
}
fn compute_layout<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
source: MirSource<'tcx>,
upvars: Vec<Ty<'tcx>>,
interior: Ty<'tcx>,
movable: bool,
mir: &mut Mir<'tcx>)
-> (FxHashMap<Local, (Ty<'tcx>, usize)>,
GeneratorLayout<'tcx>,
FxHashMap<BasicBlock, liveness::LiveVarSet>)
{
// Use a liveness analysis to compute locals which are live across a suspension point
let (live_locals, storage_liveness) = locals_live_across_suspend_points(tcx,
mir,
source,
movable);
// Erase regions from the types passed in from typeck so we can compare them with
// MIR types
let allowed_upvars = tcx.erase_regions(&upvars);
let allowed = match interior.sty {
ty::GeneratorWitness(s) => tcx.erase_late_bound_regions(&s),
_ => bug!(),
};
for (local, decl) in mir.local_decls.iter_enumerated() {
// Ignore locals which are internal or not live
if !live_locals.contains(local) || decl.internal {
continue;
}
// Sanity check that typeck knows about the type of locals which are
// live across a suspension point
if !allowed.contains(&decl.ty) && !allowed_upvars.contains(&decl.ty) {
span_bug!(mir.span,
"Broken MIR: generator contains type {} in MIR, \
but typeck only knows about {}",
decl.ty,
interior);
}
}
let upvar_len = mir.upvar_decls.len();
let dummy_local = LocalDecl::new_internal(tcx.mk_unit(), mir.span);
// Gather live locals and their indices replacing values in mir.local_decls with a dummy
// to avoid changing local indices
let live_decls = live_locals.iter().map(|local| {
let var = mem::replace(&mut mir.local_decls[local], dummy_local.clone());
(local, var)
});
// Create a map from local indices to generator struct indices.
// These are offset by (upvar_len + 1) because of fields which comes before locals.
// We also create a vector of the LocalDecls of these locals.
let (remap, vars) = live_decls.enumerate().map(|(idx, (local, var))| {
((local, (var.ty, upvar_len + 1 + idx)), var)
}).unzip();
let layout = GeneratorLayout {
fields: vars
};
(remap, layout, storage_liveness)
}
fn insert_switch<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
mir: &mut Mir<'tcx>,
cases: Vec<(u32, BasicBlock)>,
transform: &TransformVisitor<'a, 'tcx>,
default: TerminatorKind<'tcx>) {
let default_block = insert_term_block(mir, default);
let switch = TerminatorKind::SwitchInt {
discr: Operand::Copy(transform.make_field(transform.state_field, tcx.types.u32)),
switch_ty: tcx.types.u32,
values: Cow::from(cases.iter().map(|&(i, _)| i.into()).collect::<Vec<_>>()),
targets: cases.iter().map(|&(_, d)| d).chain(once(default_block)).collect(),
};
let source_info = source_info(mir);
mir.basic_blocks_mut().raw.insert(0, BasicBlockData {
statements: Vec::new(),
terminator: Some(Terminator {
source_info,
kind: switch,
}),
is_cleanup: false,
});
let blocks = mir.basic_blocks_mut().iter_mut();
for target in blocks.flat_map(|b| b.terminator_mut().successors_mut()) {
*target = BasicBlock::new(target.index() + 1);
}
}
fn elaborate_generator_drops<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
def_id: DefId,
mir: &mut Mir<'tcx>) {
use crate::util::elaborate_drops::{elaborate_drop, Unwind};
use crate::util::patch::MirPatch;
use crate::shim::DropShimElaborator;
// Note that `elaborate_drops` only drops the upvars of a generator, and
// this is ok because `open_drop` can only be reached within that own
// generator's resume function.
let param_env = tcx.param_env(def_id);
let gen = self_arg();
for block in mir.basic_blocks().indices() {
let (target, unwind, source_info) = match mir.basic_blocks()[block].terminator() {
&Terminator {
source_info,
kind: TerminatorKind::Drop {
location: Place::Base(PlaceBase::Local(local)),
target,
unwind
}
} if local == gen => (target, unwind, source_info),
_ => continue,
};
let unwind = if let Some(unwind) = unwind {
Unwind::To(unwind)
} else {
Unwind::InCleanup
};
let patch = {
let mut elaborator = DropShimElaborator {
mir: &mir,
patch: MirPatch::new(mir),
tcx,
param_env
};
elaborate_drop(
&mut elaborator,
source_info,
&Place::Base(PlaceBase::Local(gen)),
(),
target,
unwind,
block
);
elaborator.patch
};
patch.apply(mir);
}
}
fn create_generator_drop_shim<'a, 'tcx>(
tcx: TyCtxt<'a, 'tcx, 'tcx>,
transform: &TransformVisitor<'a, 'tcx>,
def_id: DefId,
source: MirSource<'tcx>,
gen_ty: Ty<'tcx>,
mir: &Mir<'tcx>,
drop_clean: BasicBlock) -> Mir<'tcx> {
let mut mir = mir.clone();
let source_info = source_info(&mir);
let mut cases = create_cases(&mut mir, transform, |point| point.drop);
cases.insert(0, (0, drop_clean));
// The returned state (1) and the poisoned state (2) falls through to
// the default case which is just to return
insert_switch(tcx, &mut mir, cases, &transform, TerminatorKind::Return);
for block in mir.basic_blocks_mut() {
let kind = &mut block.terminator_mut().kind;
if let TerminatorKind::GeneratorDrop = *kind {
*kind = TerminatorKind::Return;
}
}
// Replace the return variable
mir.local_decls[RETURN_PLACE] = LocalDecl {
mutability: Mutability::Mut,
ty: tcx.mk_unit(),
user_ty: UserTypeProjections::none(),
name: None,
source_info,
visibility_scope: source_info.scope,
internal: false,
is_block_tail: None,
is_user_variable: None,
};
make_generator_state_argument_indirect(tcx, def_id, &mut mir);
// Change the generator argument from &mut to *mut
mir.local_decls[self_arg()] = LocalDecl {
mutability: Mutability::Mut,
ty: tcx.mk_ptr(ty::TypeAndMut {
ty: gen_ty,
mutbl: hir::Mutability::MutMutable,
}),
user_ty: UserTypeProjections::none(),
name: None,
source_info,
visibility_scope: source_info.scope,
internal: false,
is_block_tail: None,
is_user_variable: None,
};
if tcx.sess.opts.debugging_opts.mir_emit_retag {
// Alias tracking must know we changed the type
mir.basic_blocks_mut()[START_BLOCK].statements.insert(0, Statement {
source_info,
kind: StatementKind::Retag(RetagKind::Raw, Place::Base(PlaceBase::Local(self_arg()))),
})
}
no_landing_pads(tcx, &mut mir);
// Make sure we remove dead blocks to remove
// unrelated code from the resume part of the function
simplify::remove_dead_blocks(&mut mir);
dump_mir(tcx, None, "generator_drop", &0, source, &mut mir, |_, _| Ok(()) );
mir
}
fn insert_term_block<'tcx>(mir: &mut Mir<'tcx>, kind: TerminatorKind<'tcx>) -> BasicBlock {
let term_block = BasicBlock::new(mir.basic_blocks().len());
let source_info = source_info(mir);
mir.basic_blocks_mut().push(BasicBlockData {
statements: Vec::new(),
terminator: Some(Terminator {
source_info,
kind,
}),
is_cleanup: false,
});
term_block
}
fn insert_panic_block<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
mir: &mut Mir<'tcx>,
message: AssertMessage<'tcx>) -> BasicBlock {
let assert_block = BasicBlock::new(mir.basic_blocks().len());
let term = TerminatorKind::Assert {
cond: Operand::Constant(box Constant {
span: mir.span,
ty: tcx.types.bool,
user_ty: None,
literal: tcx.mk_lazy_const(ty::LazyConst::Evaluated(
ty::Const::from_bool(tcx, false),
)),
}),
expected: true,
msg: message,
target: assert_block,
cleanup: None,
};
let source_info = source_info(mir);
mir.basic_blocks_mut().push(BasicBlockData {
statements: Vec::new(),
terminator: Some(Terminator {
source_info,
kind: term,
}),
is_cleanup: false,
});
assert_block
}
fn create_generator_resume_function<'a, 'tcx>(
tcx: TyCtxt<'a, 'tcx, 'tcx>,
transform: TransformVisitor<'a, 'tcx>,
def_id: DefId,
source: MirSource<'tcx>,
mir: &mut Mir<'tcx>) {
// Poison the generator when it unwinds
for block in mir.basic_blocks_mut() {
let source_info = block.terminator().source_info;
if let &TerminatorKind::Resume = &block.terminator().kind {
block.statements.push(transform.set_state(1, source_info));
}
}
let mut cases = create_cases(mir, &transform, |point| Some(point.resume));
use rustc::mir::interpret::EvalErrorKind::{
GeneratorResumedAfterPanic,
GeneratorResumedAfterReturn,
};
// Jump to the entry point on the 0 state
cases.insert(0, (0, BasicBlock::new(0)));
// Panic when resumed on the returned (1) state
cases.insert(1, (1, insert_panic_block(tcx, mir, GeneratorResumedAfterReturn)));
// Panic when resumed on the poisoned (2) state
cases.insert(2, (2, insert_panic_block(tcx, mir, GeneratorResumedAfterPanic)));
insert_switch(tcx, mir, cases, &transform, TerminatorKind::Unreachable);
make_generator_state_argument_indirect(tcx, def_id, mir);
make_generator_state_argument_pinned(tcx, mir);
no_landing_pads(tcx, mir);
// Make sure we remove dead blocks to remove
// unrelated code from the drop part of the function
simplify::remove_dead_blocks(mir);
dump_mir(tcx, None, "generator_resume", &0, source, mir, |_, _| Ok(()) );
}
fn source_info<'a, 'tcx>(mir: &Mir<'tcx>) -> SourceInfo {
SourceInfo {
span: mir.span,
scope: OUTERMOST_SOURCE_SCOPE,
}
}
fn insert_clean_drop<'a, 'tcx>(mir: &mut Mir<'tcx>) -> BasicBlock {
let return_block = insert_term_block(mir, TerminatorKind::Return);
// Create a block to destroy an unresumed generators. This can only destroy upvars.
let drop_clean = BasicBlock::new(mir.basic_blocks().len());
let term = TerminatorKind::Drop {
location: Place::Base(PlaceBase::Local(self_arg())),
target: return_block,
unwind: None,
};
let source_info = source_info(mir);
mir.basic_blocks_mut().push(BasicBlockData {
statements: Vec::new(),
terminator: Some(Terminator {
source_info,
kind: term,
}),
is_cleanup: false,
});
drop_clean
}
fn create_cases<'a, 'tcx, F>(mir: &mut Mir<'tcx>,
transform: &TransformVisitor<'a, 'tcx>,
target: F) -> Vec<(u32, BasicBlock)>
where F: Fn(&SuspensionPoint) -> Option<BasicBlock> {
let source_info = source_info(mir);
transform.suspension_points.iter().filter_map(|point| {
// Find the target for this suspension point, if applicable
target(point).map(|target| {
let block = BasicBlock::new(mir.basic_blocks().len());
let mut statements = Vec::new();
// Create StorageLive instructions for locals with live storage
for i in 0..(mir.local_decls.len()) {
let l = Local::new(i);
if point.storage_liveness.contains(l) && !transform.remap.contains_key(&l) {
statements.push(Statement {
source_info,
kind: StatementKind::StorageLive(l),
});
}
}
// Then jump to the real target
mir.basic_blocks_mut().push(BasicBlockData {
statements,
terminator: Some(Terminator {
source_info,
kind: TerminatorKind::Goto {
target,
},
}),
is_cleanup: false,
});
(point.state, block)
})
}).collect()
}
impl MirPass for StateTransform {
fn run_pass<'a, 'tcx>(&self,
tcx: TyCtxt<'a, 'tcx, 'tcx>,
source: MirSource<'tcx>,
mir: &mut Mir<'tcx>) {
let yield_ty = if let Some(yield_ty) = mir.yield_ty {
yield_ty
} else {
// This only applies to generators
return
};
assert!(mir.generator_drop.is_none());
let def_id = source.def_id();
// The first argument is the generator type passed by value
let gen_ty = mir.local_decls.raw[1].ty;
// Get the interior types and substs which typeck computed
let (upvars, interior, movable) = match gen_ty.sty {
ty::Generator(_, substs, movability) => {
(substs.upvar_tys(def_id, tcx).collect(),
substs.witness(def_id, tcx),
movability == hir::GeneratorMovability::Movable)
}
_ => bug!(),
};
// Compute GeneratorState<yield_ty, return_ty>
let state_did = tcx.lang_items().gen_state().unwrap();
let state_adt_ref = tcx.adt_def(state_did);
let state_substs = tcx.intern_substs(&[
yield_ty.into(),
mir.return_ty().into(),
]);
let ret_ty = tcx.mk_adt(state_adt_ref, state_substs);
// We rename RETURN_PLACE which has type mir.return_ty to new_ret_local
// RETURN_PLACE then is a fresh unused local with type ret_ty.
let new_ret_local = replace_result_variable(ret_ty, mir);
// Extract locals which are live across suspension point into `layout`
// `remap` gives a mapping from local indices onto generator struct indices
// `storage_liveness` tells us which locals have live storage at suspension points
let (remap, layout, storage_liveness) = compute_layout(
tcx,
source,
upvars,
interior,
movable,
mir);
let state_field = mir.upvar_decls.len();
// Run the transformation which converts Places from Local to generator struct
// accesses for locals in `remap`.
// It also rewrites `return x` and `yield y` as writing a new generator state and returning
// GeneratorState::Complete(x) and GeneratorState::Yielded(y) respectively.
let mut transform = TransformVisitor {
tcx,
state_adt_ref,
state_substs,
remap,
storage_liveness,
suspension_points: Vec::new(),
new_ret_local,
state_field,
};
transform.visit_mir(mir);
// Update our MIR struct to reflect the changed we've made
mir.yield_ty = None;
mir.arg_count = 1;
mir.spread_arg = None;
mir.generator_layout = Some(layout);
// Insert `drop(generator_struct)` which is used to drop upvars for generators in
// the unresumed (0) state.
// This is expanded to a drop ladder in `elaborate_generator_drops`.
let drop_clean = insert_clean_drop(mir);
dump_mir(tcx, None, "generator_pre-elab", &0, source, mir, |_, _| Ok(()) );
// Expand `drop(generator_struct)` to a drop ladder which destroys upvars.
// If any upvars are moved out of, drop elaboration will handle upvar destruction.
// However we need to also elaborate the code generated by `insert_clean_drop`.
elaborate_generator_drops(tcx, def_id, mir);
dump_mir(tcx, None, "generator_post-transform", &0, source, mir, |_, _| Ok(()) );
// Create a copy of our MIR and use it to create the drop shim for the generator
let drop_shim = create_generator_drop_shim(tcx,
&transform,
def_id,
source,
gen_ty,
&mir,
drop_clean);
mir.generator_drop = Some(box drop_shim);
// Create the Generator::resume function
create_generator_resume_function(tcx, transform, def_id, source, mir);
}
}