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layout.rs
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use std::fmt::Debug;
use std::iter;
use hir::def_id::DefId;
use rustc_abi::Integer::{I8, I32};
use rustc_abi::Primitive::{self, Float, Int, Pointer};
use rustc_abi::{
AbiAndPrefAlign, AddressSpace, Align, BackendRepr, FIRST_VARIANT, FieldIdx, FieldsShape,
HasDataLayout, Layout, LayoutCalculatorError, LayoutData, Niche, ReprOptions, Scalar, Size,
StructKind, TagEncoding, VariantIdx, Variants, WrappingRange,
};
use rustc_hashes::Hash64;
use rustc_index::bit_set::DenseBitSet;
use rustc_index::{IndexSlice, IndexVec};
use rustc_middle::bug;
use rustc_middle::mir::{CoroutineLayout, CoroutineSavedLocal};
use rustc_middle::query::Providers;
use rustc_middle::ty::layout::{
FloatExt, HasTyCtxt, IntegerExt, LayoutCx, LayoutError, LayoutOf, MAX_SIMD_LANES, TyAndLayout,
};
use rustc_middle::ty::print::with_no_trimmed_paths;
use rustc_middle::ty::{
self, AdtDef, CoroutineArgsExt, EarlyBinder, GenericArgsRef, PseudoCanonicalInput, Ty, TyCtxt,
TypeVisitableExt,
};
use rustc_session::{DataTypeKind, FieldInfo, FieldKind, SizeKind, VariantInfo};
use rustc_span::{Symbol, sym};
use tracing::{debug, instrument, trace};
use {rustc_abi as abi, rustc_hir as hir};
use crate::errors::{NonPrimitiveSimdType, OversizedSimdType, ZeroLengthSimdType};
mod invariant;
pub(crate) fn provide(providers: &mut Providers) {
*providers = Providers { layout_of, ..*providers };
}
#[instrument(skip(tcx, query), level = "debug")]
fn layout_of<'tcx>(
tcx: TyCtxt<'tcx>,
query: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>,
) -> Result<TyAndLayout<'tcx>, &'tcx LayoutError<'tcx>> {
let PseudoCanonicalInput { typing_env, value: ty } = query;
debug!(?ty);
// Optimization: We convert to TypingMode::PostAnalysis and convert opaque types in
// the where bounds to their hidden types. This reduces overall uncached invocations
// of `layout_of` and is thus a small performance improvement.
let typing_env = typing_env.with_post_analysis_normalized(tcx);
let unnormalized_ty = ty;
// FIXME: We might want to have two different versions of `layout_of`:
// One that can be called after typecheck has completed and can use
// `normalize_erasing_regions` here and another one that can be called
// before typecheck has completed and uses `try_normalize_erasing_regions`.
let ty = match tcx.try_normalize_erasing_regions(typing_env, ty) {
Ok(t) => t,
Err(normalization_error) => {
return Err(tcx
.arena
.alloc(LayoutError::NormalizationFailure(ty, normalization_error)));
}
};
if ty != unnormalized_ty {
// Ensure this layout is also cached for the normalized type.
return tcx.layout_of(typing_env.as_query_input(ty));
}
let cx = LayoutCx::new(tcx, typing_env);
let layout = layout_of_uncached(&cx, ty)?;
let layout = TyAndLayout { ty, layout };
// If we are running with `-Zprint-type-sizes`, maybe record layouts
// for dumping later.
if cx.tcx().sess.opts.unstable_opts.print_type_sizes {
record_layout_for_printing(&cx, layout);
}
invariant::layout_sanity_check(&cx, &layout);
Ok(layout)
}
fn error<'tcx>(cx: &LayoutCx<'tcx>, err: LayoutError<'tcx>) -> &'tcx LayoutError<'tcx> {
cx.tcx().arena.alloc(err)
}
fn map_error<'tcx>(
cx: &LayoutCx<'tcx>,
ty: Ty<'tcx>,
err: LayoutCalculatorError<TyAndLayout<'tcx>>,
) -> &'tcx LayoutError<'tcx> {
let err = match err {
LayoutCalculatorError::SizeOverflow => {
// This is sometimes not a compile error in `check` builds.
// See `tests/ui/limits/huge-enum.rs` for an example.
LayoutError::SizeOverflow(ty)
}
LayoutCalculatorError::UnexpectedUnsized(field) => {
// This is sometimes not a compile error if there are trivially false where clauses.
// See `tests/ui/layout/trivial-bounds-sized.rs` for an example.
assert!(field.layout.is_unsized(), "invalid layout error {err:#?}");
if cx.typing_env.param_env.caller_bounds().is_empty() {
cx.tcx().dcx().delayed_bug(format!(
"encountered unexpected unsized field in layout of {ty:?}: {field:#?}"
));
}
LayoutError::Unknown(ty)
}
LayoutCalculatorError::EmptyUnion => {
// This is always a compile error.
let guar =
cx.tcx().dcx().delayed_bug(format!("computed layout of empty union: {ty:?}"));
LayoutError::ReferencesError(guar)
}
LayoutCalculatorError::ReprConflict => {
// packed enums are the only known trigger of this, but others might arise
let guar = cx
.tcx()
.dcx()
.delayed_bug(format!("computed impossible repr (packed enum?): {ty:?}"));
LayoutError::ReferencesError(guar)
}
};
error(cx, err)
}
fn univariant_uninterned<'tcx>(
cx: &LayoutCx<'tcx>,
ty: Ty<'tcx>,
fields: &IndexSlice<FieldIdx, TyAndLayout<'tcx>>,
kind: StructKind,
) -> Result<LayoutData<FieldIdx, VariantIdx>, &'tcx LayoutError<'tcx>> {
let repr = ReprOptions::default();
cx.calc.univariant(fields, &repr, kind).map_err(|err| map_error(cx, ty, err))
}
fn extract_const_value<'tcx>(
cx: &LayoutCx<'tcx>,
ty: Ty<'tcx>,
ct: ty::Const<'tcx>,
) -> Result<ty::Value<'tcx>, &'tcx LayoutError<'tcx>> {
match ct.kind() {
ty::ConstKind::Value(cv) => Ok(cv),
ty::ConstKind::Param(_) | ty::ConstKind::Expr(_) => {
if !ct.has_param() {
bug!("failed to normalize const, but it is not generic: {ct:?}");
}
Err(error(cx, LayoutError::TooGeneric(ty)))
}
ty::ConstKind::Unevaluated(_) => {
let err = if ct.has_param() {
LayoutError::TooGeneric(ty)
} else {
// This case is reachable with unsatisfiable predicates and GCE (which will
// cause anon consts to inherit the unsatisfiable predicates). For example
// if we have an unsatisfiable `u8: Trait` bound, then it's not a compile
// error to mention `[u8; <u8 as Trait>::CONST]`, but we can't compute its
// layout.
LayoutError::Unknown(ty)
};
Err(error(cx, err))
}
ty::ConstKind::Infer(_)
| ty::ConstKind::Bound(..)
| ty::ConstKind::Placeholder(_)
| ty::ConstKind::Error(_) => {
// `ty::ConstKind::Error` is handled at the top of `layout_of_uncached`
// (via `ty.error_reported()`).
bug!("layout_of: unexpected const: {ct:?}");
}
}
}
fn layout_of_uncached<'tcx>(
cx: &LayoutCx<'tcx>,
ty: Ty<'tcx>,
) -> Result<Layout<'tcx>, &'tcx LayoutError<'tcx>> {
// Types that reference `ty::Error` pessimistically don't have a meaningful layout.
// The only side-effect of this is possibly worse diagnostics in case the layout
// was actually computable (like if the `ty::Error` showed up only in a `PhantomData`).
if let Err(guar) = ty.error_reported() {
return Err(error(cx, LayoutError::ReferencesError(guar)));
}
let tcx = cx.tcx();
let dl = cx.data_layout();
let scalar_unit = |value: Primitive| {
let size = value.size(dl);
assert!(size.bits() <= 128);
Scalar::Initialized { value, valid_range: WrappingRange::full(size) }
};
let scalar = |value: Primitive| tcx.mk_layout(LayoutData::scalar(cx, scalar_unit(value)));
let univariant = |fields: &IndexSlice<FieldIdx, TyAndLayout<'tcx>>, kind| {
Ok(tcx.mk_layout(univariant_uninterned(cx, ty, fields, kind)?))
};
debug_assert!(!ty.has_non_region_infer());
Ok(match *ty.kind() {
ty::Pat(ty, pat) => {
let layout = cx.layout_of(ty)?.layout;
let mut layout = LayoutData::clone(&layout.0);
match *pat {
ty::PatternKind::Range { start, end, include_end } => {
if let BackendRepr::Scalar(scalar) | BackendRepr::ScalarPair(scalar, _) =
&mut layout.backend_repr
{
if let Some(start) = start {
scalar.valid_range_mut().start = extract_const_value(cx, ty, start)?
.try_to_bits(tcx, cx.typing_env)
.ok_or_else(|| error(cx, LayoutError::Unknown(ty)))?;
}
if let Some(end) = end {
let mut end = extract_const_value(cx, ty, end)?
.try_to_bits(tcx, cx.typing_env)
.ok_or_else(|| error(cx, LayoutError::Unknown(ty)))?;
if !include_end {
end = end.wrapping_sub(1);
}
scalar.valid_range_mut().end = end;
}
let niche = Niche {
offset: Size::ZERO,
value: scalar.primitive(),
valid_range: scalar.valid_range(cx),
};
layout.largest_niche = Some(niche);
tcx.mk_layout(layout)
} else {
bug!("pattern type with range but not scalar layout: {ty:?}, {layout:?}")
}
}
}
}
// Basic scalars.
ty::Bool => tcx.mk_layout(LayoutData::scalar(
cx,
Scalar::Initialized {
value: Int(I8, false),
valid_range: WrappingRange { start: 0, end: 1 },
},
)),
ty::Char => tcx.mk_layout(LayoutData::scalar(
cx,
Scalar::Initialized {
value: Int(I32, false),
valid_range: WrappingRange { start: 0, end: 0x10FFFF },
},
)),
ty::Int(ity) => scalar(Int(abi::Integer::from_int_ty(dl, ity), true)),
ty::Uint(ity) => scalar(Int(abi::Integer::from_uint_ty(dl, ity), false)),
ty::Float(fty) => scalar(Float(abi::Float::from_float_ty(fty))),
ty::FnPtr(..) => {
let mut ptr = scalar_unit(Pointer(dl.instruction_address_space));
ptr.valid_range_mut().start = 1;
tcx.mk_layout(LayoutData::scalar(cx, ptr))
}
// The never type.
ty::Never => tcx.mk_layout(cx.calc.layout_of_never_type()),
// Potentially-wide pointers.
ty::Ref(_, pointee, _) | ty::RawPtr(pointee, _) => {
let mut data_ptr = scalar_unit(Pointer(AddressSpace::DATA));
if !ty.is_raw_ptr() {
data_ptr.valid_range_mut().start = 1;
}
if pointee.is_sized(tcx, cx.typing_env) {
return Ok(tcx.mk_layout(LayoutData::scalar(cx, data_ptr)));
}
let metadata = if let Some(metadata_def_id) = tcx.lang_items().metadata_type() {
let pointee_metadata = Ty::new_projection(tcx, metadata_def_id, [pointee]);
let metadata_ty =
match tcx.try_normalize_erasing_regions(cx.typing_env, pointee_metadata) {
Ok(metadata_ty) => metadata_ty,
Err(mut err) => {
// Usually `<Ty as Pointee>::Metadata` can't be normalized because
// its struct tail cannot be normalized either, so try to get a
// more descriptive layout error here, which will lead to less confusing
// diagnostics.
//
// We use the raw struct tail function here to get the first tail
// that is an alias, which is likely the cause of the normalization
// error.
match tcx.try_normalize_erasing_regions(
cx.typing_env,
tcx.struct_tail_raw(pointee, |ty| ty, || {}),
) {
Ok(_) => {}
Err(better_err) => {
err = better_err;
}
}
return Err(error(cx, LayoutError::NormalizationFailure(pointee, err)));
}
};
let metadata_layout = cx.layout_of(metadata_ty)?;
// If the metadata is a 1-zst, then the pointer is thin.
if metadata_layout.is_1zst() {
return Ok(tcx.mk_layout(LayoutData::scalar(cx, data_ptr)));
}
let BackendRepr::Scalar(metadata) = metadata_layout.backend_repr else {
return Err(error(cx, LayoutError::Unknown(pointee)));
};
metadata
} else {
let unsized_part = tcx.struct_tail_for_codegen(pointee, cx.typing_env);
match unsized_part.kind() {
ty::Foreign(..) => {
return Ok(tcx.mk_layout(LayoutData::scalar(cx, data_ptr)));
}
ty::Slice(_) | ty::Str => scalar_unit(Int(dl.ptr_sized_integer(), false)),
ty::Dynamic(..) => {
let mut vtable = scalar_unit(Pointer(AddressSpace::DATA));
vtable.valid_range_mut().start = 1;
vtable
}
_ => {
return Err(error(cx, LayoutError::Unknown(pointee)));
}
}
};
// Effectively a (ptr, meta) tuple.
tcx.mk_layout(cx.calc.scalar_pair(data_ptr, metadata))
}
ty::Dynamic(_, _, ty::DynStar) => {
let mut data = scalar_unit(Pointer(AddressSpace::DATA));
data.valid_range_mut().start = 0;
let mut vtable = scalar_unit(Pointer(AddressSpace::DATA));
vtable.valid_range_mut().start = 1;
tcx.mk_layout(cx.calc.scalar_pair(data, vtable))
}
// Arrays and slices.
ty::Array(element, count) => {
let count = extract_const_value(cx, ty, count)?
.try_to_target_usize(tcx)
.ok_or_else(|| error(cx, LayoutError::Unknown(ty)))?;
let element = cx.layout_of(element)?;
let size = element
.size
.checked_mul(count, dl)
.ok_or_else(|| error(cx, LayoutError::SizeOverflow(ty)))?;
let abi = BackendRepr::Memory { sized: true };
let largest_niche = if count != 0 { element.largest_niche } else { None };
let uninhabited = if count != 0 { element.uninhabited } else { false };
tcx.mk_layout(LayoutData {
variants: Variants::Single { index: FIRST_VARIANT },
fields: FieldsShape::Array { stride: element.size, count },
backend_repr: abi,
largest_niche,
uninhabited,
align: element.align,
size,
max_repr_align: None,
unadjusted_abi_align: element.align.abi,
randomization_seed: element.randomization_seed.wrapping_add(Hash64::new(count)),
})
}
ty::Slice(element) => {
let element = cx.layout_of(element)?;
tcx.mk_layout(LayoutData {
variants: Variants::Single { index: FIRST_VARIANT },
fields: FieldsShape::Array { stride: element.size, count: 0 },
backend_repr: BackendRepr::Memory { sized: false },
largest_niche: None,
uninhabited: false,
align: element.align,
size: Size::ZERO,
max_repr_align: None,
unadjusted_abi_align: element.align.abi,
// adding a randomly chosen value to distinguish slices
randomization_seed: element
.randomization_seed
.wrapping_add(Hash64::new(0x2dcba99c39784102)),
})
}
ty::Str => tcx.mk_layout(LayoutData {
variants: Variants::Single { index: FIRST_VARIANT },
fields: FieldsShape::Array { stride: Size::from_bytes(1), count: 0 },
backend_repr: BackendRepr::Memory { sized: false },
largest_niche: None,
uninhabited: false,
align: dl.i8_align,
size: Size::ZERO,
max_repr_align: None,
unadjusted_abi_align: dl.i8_align.abi,
// another random value
randomization_seed: Hash64::new(0xc1325f37d127be22),
}),
// Odd unit types.
ty::FnDef(..) => univariant(IndexSlice::empty(), StructKind::AlwaysSized)?,
ty::Dynamic(_, _, ty::Dyn) | ty::Foreign(..) => {
let mut unit =
univariant_uninterned(cx, ty, IndexSlice::empty(), StructKind::AlwaysSized)?;
match unit.backend_repr {
BackendRepr::Memory { ref mut sized } => *sized = false,
_ => bug!(),
}
tcx.mk_layout(unit)
}
ty::Coroutine(def_id, args) => coroutine_layout(cx, ty, def_id, args)?,
ty::Closure(_, args) => {
let tys = args.as_closure().upvar_tys();
univariant(
&tys.iter().map(|ty| cx.layout_of(ty)).try_collect::<IndexVec<_, _>>()?,
StructKind::AlwaysSized,
)?
}
ty::CoroutineClosure(_, args) => {
let tys = args.as_coroutine_closure().upvar_tys();
univariant(
&tys.iter().map(|ty| cx.layout_of(ty)).try_collect::<IndexVec<_, _>>()?,
StructKind::AlwaysSized,
)?
}
ty::Tuple(tys) => {
let kind =
if tys.len() == 0 { StructKind::AlwaysSized } else { StructKind::MaybeUnsized };
univariant(&tys.iter().map(|k| cx.layout_of(k)).try_collect::<IndexVec<_, _>>()?, kind)?
}
// SIMD vector types.
ty::Adt(def, args) if def.repr().simd() => {
// Supported SIMD vectors are ADTs with a single array field:
//
// * #[repr(simd)] struct S([T; 4])
//
// where T is a primitive scalar (integer/float/pointer).
let Some(ty::Array(e_ty, e_len)) = def
.is_struct()
.then(|| &def.variant(FIRST_VARIANT).fields)
.filter(|fields| fields.len() == 1)
.map(|fields| *fields[FieldIdx::ZERO].ty(tcx, args).kind())
else {
// Invalid SIMD types should have been caught by typeck by now.
let guar = tcx.dcx().delayed_bug("#[repr(simd)] was applied to an invalid ADT");
return Err(error(cx, LayoutError::ReferencesError(guar)));
};
let e_len = extract_const_value(cx, ty, e_len)?
.try_to_target_usize(tcx)
.ok_or_else(|| error(cx, LayoutError::Unknown(ty)))?;
// SIMD vectors of zero length are not supported.
// Additionally, lengths are capped at 2^16 as a fixed maximum backends must
// support.
//
// Can't be caught in typeck if the array length is generic.
if e_len == 0 {
tcx.dcx().emit_fatal(ZeroLengthSimdType { ty });
} else if e_len > MAX_SIMD_LANES {
tcx.dcx().emit_fatal(OversizedSimdType { ty, max_lanes: MAX_SIMD_LANES });
}
// Compute the ABI of the element type:
let e_ly = cx.layout_of(e_ty)?;
let BackendRepr::Scalar(e_abi) = e_ly.backend_repr else {
// This error isn't caught in typeck, e.g., if
// the element type of the vector is generic.
tcx.dcx().emit_fatal(NonPrimitiveSimdType { ty, e_ty });
};
// Compute the size and alignment of the vector:
let size = e_ly
.size
.checked_mul(e_len, dl)
.ok_or_else(|| error(cx, LayoutError::SizeOverflow(ty)))?;
let (abi, align) = if def.repr().packed() && !e_len.is_power_of_two() {
// Non-power-of-two vectors have padding up to the next power-of-two.
// If we're a packed repr, remove the padding while keeping the alignment as close
// to a vector as possible.
(
BackendRepr::Memory { sized: true },
AbiAndPrefAlign {
abi: Align::max_aligned_factor(size),
pref: dl.llvmlike_vector_align(size).pref,
},
)
} else {
(
BackendRepr::SimdVector { element: e_abi, count: e_len },
dl.llvmlike_vector_align(size),
)
};
let size = size.align_to(align.abi);
tcx.mk_layout(LayoutData {
variants: Variants::Single { index: FIRST_VARIANT },
fields: FieldsShape::Arbitrary {
offsets: [Size::ZERO].into(),
memory_index: [0].into(),
},
backend_repr: abi,
largest_niche: e_ly.largest_niche,
uninhabited: false,
size,
align,
max_repr_align: None,
unadjusted_abi_align: align.abi,
randomization_seed: e_ly.randomization_seed.wrapping_add(Hash64::new(e_len)),
})
}
// ADTs.
ty::Adt(def, args) => {
// Cache the field layouts.
let variants = def
.variants()
.iter()
.map(|v| {
v.fields
.iter()
.map(|field| cx.layout_of(field.ty(tcx, args)))
.try_collect::<IndexVec<_, _>>()
})
.try_collect::<IndexVec<VariantIdx, _>>()?;
if def.is_union() {
if def.repr().pack.is_some() && def.repr().align.is_some() {
let guar = tcx.dcx().span_delayed_bug(
tcx.def_span(def.did()),
"union cannot be packed and aligned",
);
return Err(error(cx, LayoutError::ReferencesError(guar)));
}
return Ok(tcx.mk_layout(
cx.calc
.layout_of_union(&def.repr(), &variants)
.map_err(|err| map_error(cx, ty, err))?,
));
}
let get_discriminant_type =
|min, max| abi::Integer::repr_discr(tcx, ty, &def.repr(), min, max);
let discriminants_iter = || {
def.is_enum()
.then(|| def.discriminants(tcx).map(|(v, d)| (v, d.val as i128)))
.into_iter()
.flatten()
};
let dont_niche_optimize_enum = def.repr().inhibit_enum_layout_opt()
|| def
.variants()
.iter_enumerated()
.any(|(i, v)| v.discr != ty::VariantDiscr::Relative(i.as_u32()));
let maybe_unsized = def.is_struct()
&& def.non_enum_variant().tail_opt().is_some_and(|last_field| {
let typing_env = ty::TypingEnv::post_analysis(tcx, def.did());
!tcx.type_of(last_field.did).instantiate_identity().is_sized(tcx, typing_env)
});
let layout = cx
.calc
.layout_of_struct_or_enum(
&def.repr(),
&variants,
def.is_enum(),
def.is_unsafe_cell(),
tcx.layout_scalar_valid_range(def.did()),
get_discriminant_type,
discriminants_iter(),
dont_niche_optimize_enum,
!maybe_unsized,
)
.map_err(|err| map_error(cx, ty, err))?;
if !maybe_unsized && layout.is_unsized() {
bug!("got unsized layout for type that cannot be unsized {ty:?}: {layout:#?}");
}
// If the struct tail is sized and can be unsized, check that unsizing doesn't move the fields around.
if cfg!(debug_assertions)
&& maybe_unsized
&& def.non_enum_variant().tail().ty(tcx, args).is_sized(tcx, cx.typing_env)
{
let mut variants = variants;
let tail_replacement = cx.layout_of(Ty::new_slice(tcx, tcx.types.u8)).unwrap();
*variants[FIRST_VARIANT].raw.last_mut().unwrap() = tail_replacement;
let Ok(unsized_layout) = cx.calc.layout_of_struct_or_enum(
&def.repr(),
&variants,
def.is_enum(),
def.is_unsafe_cell(),
tcx.layout_scalar_valid_range(def.did()),
get_discriminant_type,
discriminants_iter(),
dont_niche_optimize_enum,
!maybe_unsized,
) else {
bug!("failed to compute unsized layout of {ty:?}");
};
let FieldsShape::Arbitrary { offsets: sized_offsets, .. } = &layout.fields else {
bug!("unexpected FieldsShape for sized layout of {ty:?}: {:?}", layout.fields);
};
let FieldsShape::Arbitrary { offsets: unsized_offsets, .. } =
&unsized_layout.fields
else {
bug!(
"unexpected FieldsShape for unsized layout of {ty:?}: {:?}",
unsized_layout.fields
);
};
let (sized_tail, sized_fields) = sized_offsets.raw.split_last().unwrap();
let (unsized_tail, unsized_fields) = unsized_offsets.raw.split_last().unwrap();
if sized_fields != unsized_fields {
bug!("unsizing {ty:?} changed field order!\n{layout:?}\n{unsized_layout:?}");
}
if sized_tail < unsized_tail {
bug!("unsizing {ty:?} moved tail backwards!\n{layout:?}\n{unsized_layout:?}");
}
}
tcx.mk_layout(layout)
}
ty::UnsafeBinder(bound_ty) => {
let ty = tcx.instantiate_bound_regions_with_erased(bound_ty.into());
cx.layout_of(ty)?.layout
}
// Types with no meaningful known layout.
ty::Param(_) => {
return Err(error(cx, LayoutError::TooGeneric(ty)));
}
ty::Alias(..) => {
// NOTE(eddyb) `layout_of` query should've normalized these away,
// if that was possible, so there's no reason to try again here.
let err = if ty.has_param() {
LayoutError::TooGeneric(ty)
} else {
// This is only reachable with unsatisfiable predicates. For example, if we have
// `u8: Iterator`, then we can't compute the layout of `<u8 as Iterator>::Item`.
LayoutError::Unknown(ty)
};
return Err(error(cx, err));
}
ty::Placeholder(..)
| ty::Bound(..)
| ty::CoroutineWitness(..)
| ty::Infer(_)
| ty::Error(_) => {
// `ty::Error` is handled at the top of this function.
bug!("layout_of: unexpected type `{ty}`")
}
})
}
/// Overlap eligibility and variant assignment for each CoroutineSavedLocal.
#[derive(Clone, Debug, PartialEq)]
enum SavedLocalEligibility {
Unassigned,
Assigned(VariantIdx),
Ineligible(Option<FieldIdx>),
}
// When laying out coroutines, we divide our saved local fields into two
// categories: overlap-eligible and overlap-ineligible.
//
// Those fields which are ineligible for overlap go in a "prefix" at the
// beginning of the layout, and always have space reserved for them.
//
// Overlap-eligible fields are only assigned to one variant, so we lay
// those fields out for each variant and put them right after the
// prefix.
//
// Finally, in the layout details, we point to the fields from the
// variants they are assigned to. It is possible for some fields to be
// included in multiple variants. No field ever "moves around" in the
// layout; its offset is always the same.
//
// Also included in the layout are the upvars and the discriminant.
// These are included as fields on the "outer" layout; they are not part
// of any variant.
/// Compute the eligibility and assignment of each local.
fn coroutine_saved_local_eligibility(
info: &CoroutineLayout<'_>,
) -> (DenseBitSet<CoroutineSavedLocal>, IndexVec<CoroutineSavedLocal, SavedLocalEligibility>) {
use SavedLocalEligibility::*;
let mut assignments: IndexVec<CoroutineSavedLocal, SavedLocalEligibility> =
IndexVec::from_elem(Unassigned, &info.field_tys);
// The saved locals not eligible for overlap. These will get
// "promoted" to the prefix of our coroutine.
let mut ineligible_locals = DenseBitSet::new_empty(info.field_tys.len());
// Figure out which of our saved locals are fields in only
// one variant. The rest are deemed ineligible for overlap.
for (variant_index, fields) in info.variant_fields.iter_enumerated() {
for local in fields {
match assignments[*local] {
Unassigned => {
assignments[*local] = Assigned(variant_index);
}
Assigned(idx) => {
// We've already seen this local at another suspension
// point, so it is no longer a candidate.
trace!(
"removing local {:?} in >1 variant ({:?}, {:?})",
local, variant_index, idx
);
ineligible_locals.insert(*local);
assignments[*local] = Ineligible(None);
}
Ineligible(_) => {}
}
}
}
// Next, check every pair of eligible locals to see if they
// conflict.
for local_a in info.storage_conflicts.rows() {
let conflicts_a = info.storage_conflicts.count(local_a);
if ineligible_locals.contains(local_a) {
continue;
}
for local_b in info.storage_conflicts.iter(local_a) {
// local_a and local_b are storage live at the same time, therefore they
// cannot overlap in the coroutine layout. The only way to guarantee
// this is if they are in the same variant, or one is ineligible
// (which means it is stored in every variant).
if ineligible_locals.contains(local_b) || assignments[local_a] == assignments[local_b] {
continue;
}
// If they conflict, we will choose one to make ineligible.
// This is not always optimal; it's just a greedy heuristic that
// seems to produce good results most of the time.
let conflicts_b = info.storage_conflicts.count(local_b);
let (remove, other) =
if conflicts_a > conflicts_b { (local_a, local_b) } else { (local_b, local_a) };
ineligible_locals.insert(remove);
assignments[remove] = Ineligible(None);
trace!("removing local {:?} due to conflict with {:?}", remove, other);
}
}
// Count the number of variants in use. If only one of them, then it is
// impossible to overlap any locals in our layout. In this case it's
// always better to make the remaining locals ineligible, so we can
// lay them out with the other locals in the prefix and eliminate
// unnecessary padding bytes.
{
let mut used_variants = DenseBitSet::new_empty(info.variant_fields.len());
for assignment in &assignments {
if let Assigned(idx) = assignment {
used_variants.insert(*idx);
}
}
if used_variants.count() < 2 {
for assignment in assignments.iter_mut() {
*assignment = Ineligible(None);
}
ineligible_locals.insert_all();
}
}
// Write down the order of our locals that will be promoted to the prefix.
{
for (idx, local) in ineligible_locals.iter().enumerate() {
assignments[local] = Ineligible(Some(FieldIdx::from_usize(idx)));
}
}
debug!("coroutine saved local assignments: {:?}", assignments);
(ineligible_locals, assignments)
}
/// Compute the full coroutine layout.
fn coroutine_layout<'tcx>(
cx: &LayoutCx<'tcx>,
ty: Ty<'tcx>,
def_id: hir::def_id::DefId,
args: GenericArgsRef<'tcx>,
) -> Result<Layout<'tcx>, &'tcx LayoutError<'tcx>> {
use SavedLocalEligibility::*;
let tcx = cx.tcx();
let instantiate_field = |ty: Ty<'tcx>| EarlyBinder::bind(ty).instantiate(tcx, args);
let Some(info) = tcx.coroutine_layout(def_id, args.as_coroutine().kind_ty()) else {
return Err(error(cx, LayoutError::Unknown(ty)));
};
let (ineligible_locals, assignments) = coroutine_saved_local_eligibility(info);
// Build a prefix layout, including "promoting" all ineligible
// locals as part of the prefix. We compute the layout of all of
// these fields at once to get optimal packing.
let tag_index = args.as_coroutine().prefix_tys().len();
// `info.variant_fields` already accounts for the reserved variants, so no need to add them.
let max_discr = (info.variant_fields.len() - 1) as u128;
let discr_int = abi::Integer::fit_unsigned(max_discr);
let tag = Scalar::Initialized {
value: Primitive::Int(discr_int, /* signed = */ false),
valid_range: WrappingRange { start: 0, end: max_discr },
};
let tag_layout = TyAndLayout {
ty: discr_int.to_ty(tcx, /* signed = */ false),
layout: tcx.mk_layout(LayoutData::scalar(cx, tag)),
};
let promoted_layouts = ineligible_locals.iter().map(|local| {
let field_ty = instantiate_field(info.field_tys[local].ty);
let uninit_ty = Ty::new_maybe_uninit(tcx, field_ty);
cx.spanned_layout_of(uninit_ty, info.field_tys[local].source_info.span)
});
let prefix_layouts = args
.as_coroutine()
.prefix_tys()
.iter()
.map(|ty| cx.layout_of(ty))
.chain(iter::once(Ok(tag_layout)))
.chain(promoted_layouts)
.try_collect::<IndexVec<_, _>>()?;
let prefix = univariant_uninterned(cx, ty, &prefix_layouts, StructKind::AlwaysSized)?;
let (prefix_size, prefix_align) = (prefix.size, prefix.align);
// Split the prefix layout into the "outer" fields (upvars and
// discriminant) and the "promoted" fields. Promoted fields will
// get included in each variant that requested them in
// CoroutineLayout.
debug!("prefix = {:#?}", prefix);
let (outer_fields, promoted_offsets, promoted_memory_index) = match prefix.fields {
FieldsShape::Arbitrary { mut offsets, memory_index } => {
let mut inverse_memory_index = memory_index.invert_bijective_mapping();
// "a" (`0..b_start`) and "b" (`b_start..`) correspond to
// "outer" and "promoted" fields respectively.
let b_start = FieldIdx::from_usize(tag_index + 1);
let offsets_b = IndexVec::from_raw(offsets.raw.split_off(b_start.as_usize()));
let offsets_a = offsets;
// Disentangle the "a" and "b" components of `inverse_memory_index`
// by preserving the order but keeping only one disjoint "half" each.
// FIXME(eddyb) build a better abstraction for permutations, if possible.
let inverse_memory_index_b: IndexVec<u32, FieldIdx> = inverse_memory_index
.iter()
.filter_map(|&i| i.as_u32().checked_sub(b_start.as_u32()).map(FieldIdx::from_u32))
.collect();
inverse_memory_index.raw.retain(|&i| i < b_start);
let inverse_memory_index_a = inverse_memory_index;
// Since `inverse_memory_index_{a,b}` each only refer to their
// respective fields, they can be safely inverted
let memory_index_a = inverse_memory_index_a.invert_bijective_mapping();
let memory_index_b = inverse_memory_index_b.invert_bijective_mapping();
let outer_fields =
FieldsShape::Arbitrary { offsets: offsets_a, memory_index: memory_index_a };
(outer_fields, offsets_b, memory_index_b)
}
_ => bug!(),
};
let mut size = prefix.size;
let mut align = prefix.align;
let variants = info
.variant_fields
.iter_enumerated()
.map(|(index, variant_fields)| {
// Only include overlap-eligible fields when we compute our variant layout.
let variant_only_tys = variant_fields
.iter()
.filter(|local| match assignments[**local] {
Unassigned => bug!(),
Assigned(v) if v == index => true,
Assigned(_) => bug!("assignment does not match variant"),
Ineligible(_) => false,
})
.map(|local| {
let field_ty = instantiate_field(info.field_tys[*local].ty);
Ty::new_maybe_uninit(tcx, field_ty)
});
let mut variant = univariant_uninterned(
cx,
ty,
&variant_only_tys.map(|ty| cx.layout_of(ty)).try_collect::<IndexVec<_, _>>()?,
StructKind::Prefixed(prefix_size, prefix_align.abi),
)?;
variant.variants = Variants::Single { index };
let FieldsShape::Arbitrary { offsets, memory_index } = variant.fields else {
bug!();
};
// Now, stitch the promoted and variant-only fields back together in
// the order they are mentioned by our CoroutineLayout.
// Because we only use some subset (that can differ between variants)
// of the promoted fields, we can't just pick those elements of the
// `promoted_memory_index` (as we'd end up with gaps).
// So instead, we build an "inverse memory_index", as if all of the
// promoted fields were being used, but leave the elements not in the
// subset as `INVALID_FIELD_IDX`, which we can filter out later to
// obtain a valid (bijective) mapping.
const INVALID_FIELD_IDX: FieldIdx = FieldIdx::MAX;
debug_assert!(variant_fields.next_index() <= INVALID_FIELD_IDX);
let mut combined_inverse_memory_index = IndexVec::from_elem_n(
INVALID_FIELD_IDX,
promoted_memory_index.len() + memory_index.len(),
);
let mut offsets_and_memory_index = iter::zip(offsets, memory_index);
let combined_offsets = variant_fields
.iter_enumerated()
.map(|(i, local)| {
let (offset, memory_index) = match assignments[*local] {
Unassigned => bug!(),
Assigned(_) => {
let (offset, memory_index) = offsets_and_memory_index.next().unwrap();
(offset, promoted_memory_index.len() as u32 + memory_index)
}
Ineligible(field_idx) => {
let field_idx = field_idx.unwrap();
(promoted_offsets[field_idx], promoted_memory_index[field_idx])
}
};
combined_inverse_memory_index[memory_index] = i;
offset
})
.collect();
// Remove the unused slots and invert the mapping to obtain the
// combined `memory_index` (also see previous comment).
combined_inverse_memory_index.raw.retain(|&i| i != INVALID_FIELD_IDX);
let combined_memory_index = combined_inverse_memory_index.invert_bijective_mapping();
variant.fields = FieldsShape::Arbitrary {
offsets: combined_offsets,
memory_index: combined_memory_index,
};
size = size.max(variant.size);
align = align.max(variant.align);
Ok(variant)
})
.try_collect::<IndexVec<VariantIdx, _>>()?;
size = size.align_to(align.abi);
let uninhabited = prefix.uninhabited || variants.iter().all(|v| v.is_uninhabited());
let abi = BackendRepr::Memory { sized: true };
// this is similar to how ReprOptions populates its field_shuffle_seed
let def_hash = tcx.def_path_hash(def_id).0.to_smaller_hash();
let layout = tcx.mk_layout(LayoutData {
variants: Variants::Multiple {
tag,
tag_encoding: TagEncoding::Direct,
tag_field: tag_index,
variants,
},
fields: outer_fields,
backend_repr: abi,
// Suppress niches inside coroutines. If the niche is inside a field that is aliased (due to
// self-referentiality), getting the discriminant can cause aliasing violations.
// `UnsafeCell` blocks niches for the same reason, but we don't yet have `UnsafePinned` that