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use rustc_hir as hir;
use rustc_hir::lang_items::LangItem;
use rustc_middle::query::Providers;
use rustc_middle::ty::layout::{
fn_can_unwind, FnAbiError, HasParamEnv, HasTyCtxt, LayoutCx, LayoutOf, TyAndLayout,
};
use rustc_middle::ty::{self, InstanceDef, Ty, TyCtxt};
use rustc_session::config::OptLevel;
use rustc_span::def_id::DefId;
use rustc_target::abi::call::{
ArgAbi, ArgAttribute, ArgAttributes, ArgExtension, Conv, FnAbi, PassMode, Reg, RegKind,
RiscvInterruptKind,
};
use rustc_target::abi::*;
use rustc_target::spec::abi::Abi as SpecAbi;
use std::iter;
pub fn provide(providers: &mut Providers) {
*providers = Providers { fn_abi_of_fn_ptr, fn_abi_of_instance, ..*providers };
}
// NOTE(eddyb) this is private to avoid using it from outside of
// `fn_abi_of_instance` - any other uses are either too high-level
// for `Instance` (e.g. typeck would use `Ty::fn_sig` instead),
// or should go through `FnAbi` instead, to avoid losing any
// adjustments `fn_abi_of_instance` might be performing.
#[tracing::instrument(level = "debug", skip(tcx, param_env))]
fn fn_sig_for_fn_abi<'tcx>(
tcx: TyCtxt<'tcx>,
instance: ty::Instance<'tcx>,
param_env: ty::ParamEnv<'tcx>,
) -> ty::PolyFnSig<'tcx> {
if let InstanceDef::ThreadLocalShim(..) = instance.def {
return ty::Binder::dummy(tcx.mk_fn_sig(
[],
tcx.thread_local_ptr_ty(instance.def_id()),
false,
hir::Unsafety::Normal,
rustc_target::spec::abi::Abi::Unadjusted,
));
}
let ty = instance.ty(tcx, param_env);
match *ty.kind() {
ty::FnDef(..) => {
// HACK(davidtwco,eddyb): This is a workaround for polymorphization considering
// parameters unused if they show up in the signature, but not in the `mir::Body`
// (i.e. due to being inside a projection that got normalized, see
// `tests/ui/polymorphization/normalized_sig_types.rs`), and codegen not keeping
// track of a polymorphization `ParamEnv` to allow normalizing later.
//
// We normalize the `fn_sig` again after substituting at a later point.
let mut sig = match *ty.kind() {
ty::FnDef(def_id, args) => tcx
.fn_sig(def_id)
.map_bound(|fn_sig| {
tcx.normalize_erasing_regions(tcx.param_env(def_id), fn_sig)
})
.instantiate(tcx, args),
_ => unreachable!(),
};
if let ty::InstanceDef::VTableShim(..) = instance.def {
// Modify `fn(self, ...)` to `fn(self: *mut Self, ...)`.
sig = sig.map_bound(|mut sig| {
let mut inputs_and_output = sig.inputs_and_output.to_vec();
inputs_and_output[0] = Ty::new_mut_ptr(tcx, inputs_and_output[0]);
sig.inputs_and_output = tcx.mk_type_list(&inputs_and_output);
sig
});
}
sig
}
ty::Closure(def_id, args) => {
let sig = args.as_closure().sig();
let bound_vars = tcx.mk_bound_variable_kinds_from_iter(
sig.bound_vars().iter().chain(iter::once(ty::BoundVariableKind::Region(ty::BrEnv))),
);
let br = ty::BoundRegion {
var: ty::BoundVar::from_usize(bound_vars.len() - 1),
kind: ty::BoundRegionKind::BrEnv,
};
let env_region = ty::Region::new_late_bound(tcx, ty::INNERMOST, br);
let env_ty = tcx.closure_env_ty(def_id, args, env_region).unwrap();
let sig = sig.skip_binder();
ty::Binder::bind_with_vars(
tcx.mk_fn_sig(
iter::once(env_ty).chain(sig.inputs().iter().cloned()),
sig.output(),
sig.c_variadic,
sig.unsafety,
sig.abi,
),
bound_vars,
)
}
ty::Generator(did, args, _) => {
let sig = args.as_generator().poly_sig();
let bound_vars = tcx.mk_bound_variable_kinds_from_iter(
sig.bound_vars().iter().chain(iter::once(ty::BoundVariableKind::Region(ty::BrEnv))),
);
let br = ty::BoundRegion {
var: ty::BoundVar::from_usize(bound_vars.len() - 1),
kind: ty::BoundRegionKind::BrEnv,
};
let env_ty =
Ty::new_mut_ref(tcx, ty::Region::new_late_bound(tcx, ty::INNERMOST, br), ty);
let pin_did = tcx.require_lang_item(LangItem::Pin, None);
let pin_adt_ref = tcx.adt_def(pin_did);
let pin_args = tcx.mk_args(&[env_ty.into()]);
let env_ty = Ty::new_adt(tcx, pin_adt_ref, pin_args);
let sig = sig.skip_binder();
// The `FnSig` and the `ret_ty` here is for a generators main
// `Generator::resume(...) -> GeneratorState` function in case we
// have an ordinary generator, or the `Future::poll(...) -> Poll`
// function in case this is a special generator backing an async construct.
let (resume_ty, ret_ty) = if tcx.generator_is_async(did) {
// The signature should be `Future::poll(_, &mut Context<'_>) -> Poll<Output>`
let poll_did = tcx.require_lang_item(LangItem::Poll, None);
let poll_adt_ref = tcx.adt_def(poll_did);
let poll_args = tcx.mk_args(&[sig.return_ty.into()]);
let ret_ty = Ty::new_adt(tcx, poll_adt_ref, poll_args);
// We have to replace the `ResumeTy` that is used for type and borrow checking
// with `&mut Context<'_>` which is used in codegen.
#[cfg(debug_assertions)]
{
if let ty::Adt(resume_ty_adt, _) = sig.resume_ty.kind() {
let expected_adt =
tcx.adt_def(tcx.require_lang_item(LangItem::ResumeTy, None));
assert_eq!(*resume_ty_adt, expected_adt);
} else {
panic!("expected `ResumeTy`, found `{:?}`", sig.resume_ty);
};
}
let context_mut_ref = Ty::new_task_context(tcx);
(context_mut_ref, ret_ty)
} else {
// The signature should be `Generator::resume(_, Resume) -> GeneratorState<Yield, Return>`
let state_did = tcx.require_lang_item(LangItem::GeneratorState, None);
let state_adt_ref = tcx.adt_def(state_did);
let state_args = tcx.mk_args(&[sig.yield_ty.into(), sig.return_ty.into()]);
let ret_ty = Ty::new_adt(tcx, state_adt_ref, state_args);
(sig.resume_ty, ret_ty)
};
ty::Binder::bind_with_vars(
tcx.mk_fn_sig(
[env_ty, resume_ty],
ret_ty,
false,
hir::Unsafety::Normal,
rustc_target::spec::abi::Abi::Rust,
),
bound_vars,
)
}
_ => bug!("unexpected type {:?} in Instance::fn_sig", ty),
}
}
#[inline]
fn conv_from_spec_abi(tcx: TyCtxt<'_>, abi: SpecAbi) -> Conv {
use rustc_target::spec::abi::Abi::*;
match tcx.sess.target.adjust_abi(abi) {
RustIntrinsic | PlatformIntrinsic | Rust | RustCall => Conv::Rust,
// This is intentionally not using `Conv::Cold`, as that has to preserve
// even SIMD registers, which is generally not a good trade-off.
RustCold => Conv::PreserveMost,
// It's the ABI's job to select this, not ours.
System { .. } => bug!("system abi should be selected elsewhere"),
EfiApi => bug!("eficall abi should be selected elsewhere"),
Stdcall { .. } => Conv::X86Stdcall,
Fastcall { .. } => Conv::X86Fastcall,
Vectorcall { .. } => Conv::X86VectorCall,
Thiscall { .. } => Conv::X86ThisCall,
C { .. } => Conv::C,
Unadjusted => Conv::C,
Win64 { .. } => Conv::X86_64Win64,
SysV64 { .. } => Conv::X86_64SysV,
Aapcs { .. } => Conv::ArmAapcs,
CCmseNonSecureCall => Conv::CCmseNonSecureCall,
PtxKernel => Conv::PtxKernel,
Msp430Interrupt => Conv::Msp430Intr,
X86Interrupt => Conv::X86Intr,
AmdGpuKernel => Conv::AmdGpuKernel,
AvrInterrupt => Conv::AvrInterrupt,
AvrNonBlockingInterrupt => Conv::AvrNonBlockingInterrupt,
RiscvInterruptM => Conv::RiscvInterrupt { kind: RiscvInterruptKind::Machine },
RiscvInterruptS => Conv::RiscvInterrupt { kind: RiscvInterruptKind::Supervisor },
Wasm => Conv::C,
// These API constants ought to be more specific...
Cdecl { .. } => Conv::C,
}
}
fn fn_abi_of_fn_ptr<'tcx>(
tcx: TyCtxt<'tcx>,
query: ty::ParamEnvAnd<'tcx, (ty::PolyFnSig<'tcx>, &'tcx ty::List<Ty<'tcx>>)>,
) -> Result<&'tcx FnAbi<'tcx, Ty<'tcx>>, &'tcx FnAbiError<'tcx>> {
let (param_env, (sig, extra_args)) = query.into_parts();
let cx = LayoutCx { tcx, param_env };
fn_abi_new_uncached(&cx, sig, extra_args, None, None, false)
}
fn fn_abi_of_instance<'tcx>(
tcx: TyCtxt<'tcx>,
query: ty::ParamEnvAnd<'tcx, (ty::Instance<'tcx>, &'tcx ty::List<Ty<'tcx>>)>,
) -> Result<&'tcx FnAbi<'tcx, Ty<'tcx>>, &'tcx FnAbiError<'tcx>> {
let (param_env, (instance, extra_args)) = query.into_parts();
let sig = fn_sig_for_fn_abi(tcx, instance, param_env);
let caller_location =
instance.def.requires_caller_location(tcx).then(|| tcx.caller_location_ty());
fn_abi_new_uncached(
&LayoutCx { tcx, param_env },
sig,
extra_args,
caller_location,
Some(instance.def_id()),
matches!(instance.def, ty::InstanceDef::Virtual(..)),
)
}
// Handle safe Rust thin and fat pointers.
fn adjust_for_rust_scalar<'tcx>(
cx: LayoutCx<'tcx, TyCtxt<'tcx>>,
attrs: &mut ArgAttributes,
scalar: Scalar,
layout: TyAndLayout<'tcx>,
offset: Size,
is_return: bool,
drop_target_pointee: Option<Ty<'tcx>>,
) {
// Booleans are always a noundef i1 that needs to be zero-extended.
if scalar.is_bool() {
attrs.ext(ArgExtension::Zext);
attrs.set(ArgAttribute::NoUndef);
return;
}
if !scalar.is_uninit_valid() {
attrs.set(ArgAttribute::NoUndef);
}
// Only pointer types handled below.
let Scalar::Initialized { value: Pointer(_), valid_range } = scalar else { return };
// Set `nonnull` if the validity range excludes zero, or for the argument to `drop_in_place`,
// which must be nonnull per its documented safety requirements.
if !valid_range.contains(0) || drop_target_pointee.is_some() {
attrs.set(ArgAttribute::NonNull);
}
if let Some(pointee) = layout.pointee_info_at(&cx, offset) {
let kind = if let Some(kind) = pointee.safe {
Some(kind)
} else if let Some(pointee) = drop_target_pointee {
// The argument to `drop_in_place` is semantically equivalent to a mutable reference.
Some(PointerKind::MutableRef { unpin: pointee.is_unpin(cx.tcx, cx.param_env()) })
} else {
None
};
if let Some(kind) = kind {
attrs.pointee_align = Some(pointee.align);
// `Box` are not necessarily dereferenceable for the entire duration of the function as
// they can be deallocated at any time. Same for non-frozen shared references (see
// <https://github.com/rust-lang/rust/pull/98017>), and for mutable references to
// potentially self-referential types (see
// <https://github.com/rust-lang/unsafe-code-guidelines/issues/381>). If LLVM had a way
// to say "dereferenceable on entry" we could use it here.
attrs.pointee_size = match kind {
PointerKind::Box { .. }
| PointerKind::SharedRef { frozen: false }
| PointerKind::MutableRef { unpin: false } => Size::ZERO,
PointerKind::SharedRef { frozen: true }
| PointerKind::MutableRef { unpin: true } => pointee.size,
};
// The aliasing rules for `Box<T>` are still not decided, but currently we emit
// `noalias` for it. This can be turned off using an unstable flag.
// See https://github.com/rust-lang/unsafe-code-guidelines/issues/326
let noalias_for_box = cx.tcx.sess.opts.unstable_opts.box_noalias;
// LLVM prior to version 12 had known miscompiles in the presence of noalias attributes
// (see #54878), so it was conditionally disabled, but we don't support earlier
// versions at all anymore. We still support turning it off using -Zmutable-noalias.
let noalias_mut_ref = cx.tcx.sess.opts.unstable_opts.mutable_noalias;
// `&T` where `T` contains no `UnsafeCell<U>` is immutable, and can be marked as both
// `readonly` and `noalias`, as LLVM's definition of `noalias` is based solely on memory
// dependencies rather than pointer equality. However this only applies to arguments,
// not return values.
//
// `&mut T` and `Box<T>` where `T: Unpin` are unique and hence `noalias`.
let no_alias = match kind {
PointerKind::SharedRef { frozen } => frozen,
PointerKind::MutableRef { unpin } => unpin && noalias_mut_ref,
PointerKind::Box { unpin } => unpin && noalias_for_box,
};
// We can never add `noalias` in return position; that LLVM attribute has some very surprising semantics
// (see <https://github.com/rust-lang/unsafe-code-guidelines/issues/385#issuecomment-1368055745>).
if no_alias && !is_return {
attrs.set(ArgAttribute::NoAlias);
}
if matches!(kind, PointerKind::SharedRef { frozen: true }) && !is_return {
attrs.set(ArgAttribute::ReadOnly);
}
}
}
}
// FIXME(eddyb) perhaps group the signature/type-containing (or all of them?)
// arguments of this method, into a separate `struct`.
#[tracing::instrument(level = "debug", skip(cx, caller_location, fn_def_id, force_thin_self_ptr))]
fn fn_abi_new_uncached<'tcx>(
cx: &LayoutCx<'tcx, TyCtxt<'tcx>>,
sig: ty::PolyFnSig<'tcx>,
extra_args: &[Ty<'tcx>],
caller_location: Option<Ty<'tcx>>,
fn_def_id: Option<DefId>,
// FIXME(eddyb) replace this with something typed, like an `enum`.
force_thin_self_ptr: bool,
) -> Result<&'tcx FnAbi<'tcx, Ty<'tcx>>, &'tcx FnAbiError<'tcx>> {
let sig = cx.tcx.normalize_erasing_late_bound_regions(cx.param_env, sig);
let conv = conv_from_spec_abi(cx.tcx(), sig.abi);
let mut inputs = sig.inputs();
let extra_args = if sig.abi == RustCall {
assert!(!sig.c_variadic && extra_args.is_empty());
if let Some(input) = sig.inputs().last() {
if let ty::Tuple(tupled_arguments) = input.kind() {
inputs = &sig.inputs()[0..sig.inputs().len() - 1];
tupled_arguments
} else {
bug!(
"argument to function with \"rust-call\" ABI \
is not a tuple"
);
}
} else {
bug!(
"argument to function with \"rust-call\" ABI \
is not a tuple"
);
}
} else {
assert!(sig.c_variadic || extra_args.is_empty());
extra_args
};
let target = &cx.tcx.sess.target;
let target_env_gnu_like = matches!(&target.env[..], "gnu" | "musl" | "uclibc");
let win_x64_gnu = target.os == "windows" && target.arch == "x86_64" && target.env == "gnu";
let linux_s390x_gnu_like =
target.os == "linux" && target.arch == "s390x" && target_env_gnu_like;
let linux_sparc64_gnu_like =
target.os == "linux" && target.arch == "sparc64" && target_env_gnu_like;
let linux_powerpc_gnu_like =
target.os == "linux" && target.arch == "powerpc" && target_env_gnu_like;
use SpecAbi::*;
let rust_abi = matches!(sig.abi, RustIntrinsic | PlatformIntrinsic | Rust | RustCall);
let is_drop_in_place =
fn_def_id.is_some() && fn_def_id == cx.tcx.lang_items().drop_in_place_fn();
let arg_of = |ty: Ty<'tcx>, arg_idx: Option<usize>| -> Result<_, &'tcx FnAbiError<'tcx>> {
let span = tracing::debug_span!("arg_of");
let _entered = span.enter();
let is_return = arg_idx.is_none();
let is_drop_target = is_drop_in_place && arg_idx == Some(0);
let drop_target_pointee = is_drop_target.then(|| match ty.kind() {
ty::RawPtr(ty::TypeAndMut { ty, .. }) => *ty,
_ => bug!("argument to drop_in_place is not a raw ptr: {:?}", ty),
});
let layout =
cx.layout_of(ty).map_err(|err| &*cx.tcx.arena.alloc(FnAbiError::Layout(*err)))?;
let layout = if force_thin_self_ptr && arg_idx == Some(0) {
// Don't pass the vtable, it's not an argument of the virtual fn.
// Instead, pass just the data pointer, but give it the type `*const/mut dyn Trait`
// or `&/&mut dyn Trait` because this is special-cased elsewhere in codegen
make_thin_self_ptr(cx, layout)
} else {
layout
};
let mut arg = ArgAbi::new(cx, layout, |layout, scalar, offset| {
let mut attrs = ArgAttributes::new();
adjust_for_rust_scalar(
*cx,
&mut attrs,
scalar,
*layout,
offset,
is_return,
drop_target_pointee,
);
attrs
});
if arg.layout.is_zst() {
// For some forsaken reason, x86_64-pc-windows-gnu
// doesn't ignore zero-sized struct arguments.
// The same is true for {s390x,sparc64,powerpc}-unknown-linux-{gnu,musl,uclibc}.
if is_return
|| rust_abi
|| (!win_x64_gnu
&& !linux_s390x_gnu_like
&& !linux_sparc64_gnu_like
&& !linux_powerpc_gnu_like)
{
arg.mode = PassMode::Ignore;
}
}
Ok(arg)
};
let mut fn_abi = FnAbi {
ret: arg_of(sig.output(), None)?,
args: inputs
.iter()
.copied()
.chain(extra_args.iter().copied())
.chain(caller_location)
.enumerate()
.map(|(i, ty)| arg_of(ty, Some(i)))
.collect::<Result<_, _>>()?,
c_variadic: sig.c_variadic,
fixed_count: inputs.len() as u32,
conv,
can_unwind: fn_can_unwind(cx.tcx(), fn_def_id, sig.abi),
};
fn_abi_adjust_for_abi(cx, &mut fn_abi, sig.abi, fn_def_id)?;
debug!("fn_abi_new_uncached = {:?}", fn_abi);
Ok(cx.tcx.arena.alloc(fn_abi))
}
#[tracing::instrument(level = "trace", skip(cx))]
fn fn_abi_adjust_for_abi<'tcx>(
cx: &LayoutCx<'tcx, TyCtxt<'tcx>>,
fn_abi: &mut FnAbi<'tcx, Ty<'tcx>>,
abi: SpecAbi,
fn_def_id: Option<DefId>,
) -> Result<(), &'tcx FnAbiError<'tcx>> {
if abi == SpecAbi::Unadjusted {
return Ok(());
}
if abi == SpecAbi::Rust
|| abi == SpecAbi::RustCall
|| abi == SpecAbi::RustIntrinsic
|| abi == SpecAbi::PlatformIntrinsic
{
// Look up the deduced parameter attributes for this function, if we have its def ID and
// we're optimizing in non-incremental mode. We'll tag its parameters with those attributes
// as appropriate.
let deduced_param_attrs = if cx.tcx.sess.opts.optimize != OptLevel::No
&& cx.tcx.sess.opts.incremental.is_none()
{
fn_def_id.map(|fn_def_id| cx.tcx.deduced_param_attrs(fn_def_id)).unwrap_or_default()
} else {
&[]
};
let fixup = |arg: &mut ArgAbi<'tcx, Ty<'tcx>>, arg_idx: Option<usize>| {
if arg.is_ignore() {
return;
}
match arg.layout.abi {
Abi::Aggregate { .. } => {}
// This is a fun case! The gist of what this is doing is
// that we want callers and callees to always agree on the
// ABI of how they pass SIMD arguments. If we were to *not*
// make these arguments indirect then they'd be immediates
// in LLVM, which means that they'd used whatever the
// appropriate ABI is for the callee and the caller. That
// means, for example, if the caller doesn't have AVX
// enabled but the callee does, then passing an AVX argument
// across this boundary would cause corrupt data to show up.
//
// This problem is fixed by unconditionally passing SIMD
// arguments through memory between callers and callees
// which should get them all to agree on ABI regardless of
// target feature sets. Some more information about this
// issue can be found in #44367.
//
// Note that the platform intrinsic ABI is exempt here as
// that's how we connect up to LLVM and it's unstable
// anyway, we control all calls to it in libstd.
Abi::Vector { .. }
if abi != SpecAbi::PlatformIntrinsic
&& cx.tcx.sess.target.simd_types_indirect =>
{
arg.make_indirect();
return;
}
_ => return,
}
// `Aggregate` ABI must be adjusted to ensure that ABI-compatible Rust types are passed
// the same way.
let size = arg.layout.size;
if arg.layout.is_unsized() || size > Pointer(AddressSpace::DATA).size(cx) {
arg.make_indirect();
} else {
// We want to pass small aggregates as immediates, but using
// an LLVM aggregate type for this leads to bad optimizations,
// so we pick an appropriately sized integer type instead.
arg.cast_to(Reg { kind: RegKind::Integer, size });
}
// If we deduced that this parameter was read-only, add that to the attribute list now.
//
// The `readonly` parameter only applies to pointers, so we can only do this if the
// argument was passed indirectly. (If the argument is passed directly, it's an SSA
// value, so it's implicitly immutable.)
if let (Some(arg_idx), &mut PassMode::Indirect { ref mut attrs, .. }) =
(arg_idx, &mut arg.mode)
{
// The `deduced_param_attrs` list could be empty if this is a type of function
// we can't deduce any parameters for, so make sure the argument index is in
// bounds.
if let Some(deduced_param_attrs) = deduced_param_attrs.get(arg_idx) {
if deduced_param_attrs.read_only {
attrs.regular.insert(ArgAttribute::ReadOnly);
debug!("added deduced read-only attribute");
}
}
}
};
fixup(&mut fn_abi.ret, None);
for (arg_idx, arg) in fn_abi.args.iter_mut().enumerate() {
fixup(arg, Some(arg_idx));
}
} else {
fn_abi
.adjust_for_foreign_abi(cx, abi)
.map_err(|err| &*cx.tcx.arena.alloc(FnAbiError::AdjustForForeignAbi(err)))?;
}
Ok(())
}
#[tracing::instrument(level = "debug", skip(cx))]
fn make_thin_self_ptr<'tcx>(
cx: &(impl HasTyCtxt<'tcx> + HasParamEnv<'tcx>),
layout: TyAndLayout<'tcx>,
) -> TyAndLayout<'tcx> {
let tcx = cx.tcx();
let fat_pointer_ty = if layout.is_unsized() {
// unsized `self` is passed as a pointer to `self`
// FIXME (mikeyhew) change this to use &own if it is ever added to the language
Ty::new_mut_ptr(tcx, layout.ty)
} else {
match layout.abi {
Abi::ScalarPair(..) | Abi::Scalar(..) => (),
_ => bug!("receiver type has unsupported layout: {:?}", layout),
}
// In the case of Rc<Self>, we need to explicitly pass a *mut RcBox<Self>
// with a Scalar (not ScalarPair) ABI. This is a hack that is understood
// elsewhere in the compiler as a method on a `dyn Trait`.
// To get the type `*mut RcBox<Self>`, we just keep unwrapping newtypes until we
// get a built-in pointer type
let mut fat_pointer_layout = layout;
while !fat_pointer_layout.ty.is_unsafe_ptr() && !fat_pointer_layout.ty.is_ref() {
fat_pointer_layout = fat_pointer_layout
.non_1zst_field(cx)
.expect("not exactly one non-1-ZST field in a `DispatchFromDyn` type")
.1
}
fat_pointer_layout.ty
};
// we now have a type like `*mut RcBox<dyn Trait>`
// change its layout to that of `*mut ()`, a thin pointer, but keep the same type
// this is understood as a special case elsewhere in the compiler
let unit_ptr_ty = Ty::new_mut_ptr(tcx, Ty::new_unit(tcx));
TyAndLayout {
ty: fat_pointer_ty,
// NOTE(eddyb) using an empty `ParamEnv`, and `unwrap`-ing the `Result`
// should always work because the type is always `*mut ()`.
..tcx.layout_of(ty::ParamEnv::reveal_all().and(unit_ptr_ty)).unwrap()
}
}