rust/src/abi/mod.rs

//! Handling of everything related to the calling convention. Also fills `fx.local_map`.

#[cfg(debug_assertions)]
mod comments;
mod pass_mode;
mod returning;

use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
use rustc_target::spec::abi::Abi;

use cranelift_codegen::ir::{AbiParam, ArgumentPurpose};

use self::pass_mode::*;
use crate::prelude::*;

pub(crate) use self::returning::{can_return_to_ssa_var, codegen_return};

// Copied from https://github.com/rust-lang/rust/blob/f52c72948aa1dd718cc1f168d21c91c584c0a662/src/librustc_middle/ty/layout.rs#L2301
#[rustfmt::skip]
pub(crate) fn fn_sig_for_fn_abi<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) -> ty::PolyFnSig<'tcx> {
    use rustc_middle::ty::subst::Subst;

    // FIXME(davidtwco,eddyb): A `ParamEnv` should be passed through to this function.
    let ty = instance.ty(tcx, ty::ParamEnv::reveal_all());
    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
            // `src/test/ui/polymorphization/normalized_sig_types.rs`), and codegen not keeping
            // track of a polymorphization `ParamEnv` to allow normalizing later.
            let mut sig = match *ty.kind() {
                ty::FnDef(def_id, substs) => tcx
                    .normalize_erasing_regions(tcx.param_env(def_id), tcx.fn_sig(def_id))
                    .subst(tcx, substs),
                _ => 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] = tcx.mk_mut_ptr(inputs_and_output[0]);
                    sig.inputs_and_output = tcx.intern_type_list(&inputs_and_output);
                    sig
                });
            }
            sig
        }
        ty::Closure(def_id, substs) => {
            let sig = substs.as_closure().sig();

            let env_ty = tcx.closure_env_ty(def_id, substs).unwrap();
            sig.map_bound(|sig| {
                tcx.mk_fn_sig(
                    std::iter::once(env_ty.skip_binder()).chain(sig.inputs().iter().cloned()),
                    sig.output(),
                    sig.c_variadic,
                    sig.unsafety,
                    sig.abi,
                )
            })
        }
        ty::Generator(_, substs, _) => {
            let sig = substs.as_generator().poly_sig();

            let env_region = ty::ReLateBound(ty::INNERMOST, ty::BrEnv);
            let env_ty = tcx.mk_mut_ref(tcx.mk_region(env_region), ty);

            let pin_did = tcx.require_lang_item(rustc_hir::LangItem::Pin, None);
            let pin_adt_ref = tcx.adt_def(pin_did);
            let pin_substs = tcx.intern_substs(&[env_ty.into()]);
            let env_ty = tcx.mk_adt(pin_adt_ref, pin_substs);

            sig.map_bound(|sig| {
                let state_did = tcx.require_lang_item(rustc_hir::LangItem::GeneratorState, None);
                let state_adt_ref = tcx.adt_def(state_did);
                let state_substs =
                    tcx.intern_substs(&[sig.yield_ty.into(), sig.return_ty.into()]);
                let ret_ty = tcx.mk_adt(state_adt_ref, state_substs);

                tcx.mk_fn_sig(
                    [env_ty, sig.resume_ty].iter(),
                    &ret_ty,
                    false,
                    rustc_hir::Unsafety::Normal,
                    rustc_target::spec::abi::Abi::Rust,
                )
            })
        }
        _ => bug!("unexpected type {:?} in Instance::fn_sig", ty),
    }
}

fn clif_sig_from_fn_sig<'tcx>(
    tcx: TyCtxt<'tcx>,
    triple: &target_lexicon::Triple,
    sig: FnSig<'tcx>,
    span: Span,
    is_vtable_fn: bool,
    requires_caller_location: bool,
) -> Signature {
    let abi = match sig.abi {
        Abi::System => Abi::C,
        abi => abi,
    };
    let (call_conv, inputs, output): (CallConv, Vec<Ty<'tcx>>, Ty<'tcx>) = match abi {
        Abi::Rust => (
            CallConv::triple_default(triple),
            sig.inputs().to_vec(),
            sig.output(),
        ),
        Abi::C | Abi::Unadjusted => (
            CallConv::triple_default(triple),
            sig.inputs().to_vec(),
            sig.output(),
        ),
        Abi::SysV64 => (CallConv::SystemV, sig.inputs().to_vec(), sig.output()),
        Abi::RustCall => {
            assert_eq!(sig.inputs().len(), 2);
            let extra_args = match sig.inputs().last().unwrap().kind() {
                ty::Tuple(ref tupled_arguments) => tupled_arguments,
                _ => bug!("argument to function with \"rust-call\" ABI is not a tuple"),
            };
            let mut inputs: Vec<Ty<'tcx>> = vec![sig.inputs()[0]];
            inputs.extend(extra_args.types());
            (CallConv::triple_default(triple), inputs, sig.output())
        }
        Abi::System => unreachable!(),
        Abi::RustIntrinsic => (
            CallConv::triple_default(triple),
            sig.inputs().to_vec(),
            sig.output(),
        ),
        _ => unimplemented!("unsupported abi {:?}", sig.abi),
    };

    let inputs = inputs
        .into_iter()
        .enumerate()
        .map(|(i, ty)| {
            let mut layout = tcx.layout_of(ParamEnv::reveal_all().and(ty)).unwrap();
            if i == 0 && is_vtable_fn {
                // Virtual calls turn their self param into a thin pointer.
                // See https://github.com/rust-lang/rust/blob/37b6a5e5e82497caf5353d9d856e4eb5d14cbe06/src/librustc/ty/layout.rs#L2519-L2572 for more info
                layout = tcx
                    .layout_of(ParamEnv::reveal_all().and(tcx.mk_mut_ptr(tcx.mk_unit())))
                    .unwrap();
            }
            let pass_mode = get_pass_mode(tcx, layout);
            if abi != Abi::Rust && abi != Abi::RustCall && abi != Abi::RustIntrinsic {
                match pass_mode {
                    PassMode::NoPass | PassMode::ByVal(_) => {}
                    PassMode::ByRef { size: Some(size) } => {
                        let purpose = ArgumentPurpose::StructArgument(u32::try_from(size.bytes()).expect("struct too big to pass on stack"));
                        return EmptySinglePair::Single(AbiParam::special(pointer_ty(tcx), purpose)).into_iter();
                    }
                    PassMode::ByValPair(_, _) | PassMode::ByRef { size: None } => {
                        tcx.sess.span_warn(
                            span,
                            &format!(
                                "Argument of type `{:?}` with pass mode `{:?}` is not yet supported \
                                for non-rust abi `{}`. Calling this function may result in a crash.",
                                layout.ty,
                                pass_mode,
                                abi,
                            ),
                        );
                    }
                }
            }
            pass_mode.get_param_ty(tcx).map(AbiParam::new).into_iter()
        })
        .flatten();

    let (mut params, returns): (Vec<_>, Vec<_>) = match get_pass_mode(
        tcx,
        tcx.layout_of(ParamEnv::reveal_all().and(output)).unwrap(),
    ) {
        PassMode::NoPass => (inputs.collect(), vec![]),
        PassMode::ByVal(ret_ty) => (inputs.collect(), vec![AbiParam::new(ret_ty)]),
        PassMode::ByValPair(ret_ty_a, ret_ty_b) => (
            inputs.collect(),
            vec![AbiParam::new(ret_ty_a), AbiParam::new(ret_ty_b)],
        ),
        PassMode::ByRef { size: Some(_) } => {
            (
                Some(pointer_ty(tcx)) // First param is place to put return val
                    .into_iter()
                    .map(|ty| AbiParam::special(ty, ArgumentPurpose::StructReturn))
                    .chain(inputs)
                    .collect(),
                vec![],
            )
        }
        PassMode::ByRef { size: None } => todo!(),
    };

    if requires_caller_location {
        params.push(AbiParam::new(pointer_ty(tcx)));
    }

    Signature {
        params,
        returns,
        call_conv,
    }
}

pub(crate) fn get_function_name_and_sig<'tcx>(
    tcx: TyCtxt<'tcx>,
    triple: &target_lexicon::Triple,
    inst: Instance<'tcx>,
    support_vararg: bool,
) -> (String, Signature) {
    assert!(!inst.substs.needs_infer());
    let fn_sig = tcx.normalize_erasing_late_bound_regions(
        ParamEnv::reveal_all(),
        &fn_sig_for_fn_abi(tcx, inst),
    );
    if fn_sig.c_variadic && !support_vararg {
        tcx.sess.span_fatal(
            tcx.def_span(inst.def_id()),
            "Variadic function definitions are not yet supported",
        );
    }
    let sig = clif_sig_from_fn_sig(
        tcx,
        triple,
        fn_sig,
        tcx.def_span(inst.def_id()),
        false,
        inst.def.requires_caller_location(tcx),
    );
    (tcx.symbol_name(inst).name.to_string(), sig)
}

/// Instance must be monomorphized
pub(crate) fn import_function<'tcx>(
    tcx: TyCtxt<'tcx>,
    module: &mut impl Module,
    inst: Instance<'tcx>,
) -> FuncId {
    let (name, sig) = get_function_name_and_sig(tcx, module.isa().triple(), inst, true);
    module
        .declare_function(&name, Linkage::Import, &sig)
        .unwrap()
}

impl<'tcx, M: Module> FunctionCx<'_, 'tcx, M> {
    /// Instance must be monomorphized
    pub(crate) fn get_function_ref(&mut self, inst: Instance<'tcx>) -> FuncRef {
        let func_id = import_function(self.tcx, &mut self.cx.module, inst);
        let func_ref = self
            .cx
            .module
            .declare_func_in_func(func_id, &mut self.bcx.func);

        #[cfg(debug_assertions)]
        self.add_comment(func_ref, format!("{:?}", inst));

        func_ref
    }

    pub(crate) fn lib_call(
        &mut self,
        name: &str,
        input_tys: Vec<types::Type>,
        output_tys: Vec<types::Type>,
        args: &[Value],
    ) -> &[Value] {
        let sig = Signature {
            params: input_tys.iter().cloned().map(AbiParam::new).collect(),
            returns: output_tys.iter().cloned().map(AbiParam::new).collect(),
            call_conv: CallConv::triple_default(self.triple()),
        };
        let func_id = self
            .cx
            .module
            .declare_function(&name, Linkage::Import, &sig)
            .unwrap();
        let func_ref = self
            .cx
            .module
            .declare_func_in_func(func_id, &mut self.bcx.func);
        let call_inst = self.bcx.ins().call(func_ref, args);
        #[cfg(debug_assertions)]
        {
            self.add_comment(call_inst, format!("easy_call {}", name));
        }
        let results = self.bcx.inst_results(call_inst);
        assert!(results.len() <= 2, "{}", results.len());
        results
    }

    pub(crate) fn easy_call(
        &mut self,
        name: &str,
        args: &[CValue<'tcx>],
        return_ty: Ty<'tcx>,
    ) -> CValue<'tcx> {
        let (input_tys, args): (Vec<_>, Vec<_>) = args
            .iter()
            .map(|arg| {
                (
                    self.clif_type(arg.layout().ty).unwrap(),
                    arg.load_scalar(self),
                )
            })
            .unzip();
        let return_layout = self.layout_of(return_ty);
        let return_tys = if let ty::Tuple(tup) = return_ty.kind() {
            tup.types().map(|ty| self.clif_type(ty).unwrap()).collect()
        } else {
            vec![self.clif_type(return_ty).unwrap()]
        };
        let ret_vals = self.lib_call(name, input_tys, return_tys, &args);
        match *ret_vals {
            [] => CValue::by_ref(
                Pointer::const_addr(self, i64::from(self.pointer_type.bytes())),
                return_layout,
            ),
            [val] => CValue::by_val(val, return_layout),
            [val, extra] => CValue::by_val_pair(val, extra, return_layout),
            _ => unreachable!(),
        }
    }
}

/// Make a [`CPlace`] capable of holding value of the specified type.
fn make_local_place<'tcx>(
    fx: &mut FunctionCx<'_, 'tcx, impl Module>,
    local: Local,
    layout: TyAndLayout<'tcx>,
    is_ssa: bool,
) -> CPlace<'tcx> {
    let place = if is_ssa {
        if let rustc_target::abi::Abi::ScalarPair(_, _) = layout.abi {
            CPlace::new_var_pair(fx, local, layout)
        } else {
            CPlace::new_var(fx, local, layout)
        }
    } else {
        CPlace::new_stack_slot(fx, layout)
    };

    #[cfg(debug_assertions)]
    self::comments::add_local_place_comments(fx, place, local);

    place
}

pub(crate) fn codegen_fn_prelude<'tcx>(
    fx: &mut FunctionCx<'_, 'tcx, impl Module>,
    start_block: Block,
) {
    let ssa_analyzed = crate::analyze::analyze(fx);

    #[cfg(debug_assertions)]
    self::comments::add_args_header_comment(fx);

    let ret_place = self::returning::codegen_return_param(fx, &ssa_analyzed, start_block);
    assert_eq!(fx.local_map.push(ret_place), RETURN_PLACE);

    // None means pass_mode == NoPass
    enum ArgKind<'tcx> {
        Normal(Option<CValue<'tcx>>),
        Spread(Vec<Option<CValue<'tcx>>>),
    }

    let func_params = fx
        .mir
        .args_iter()
        .map(|local| {
            let arg_ty = fx.monomorphize(&fx.mir.local_decls[local].ty);

            // Adapted from https://github.com/rust-lang/rust/blob/145155dc96757002c7b2e9de8489416e2fdbbd57/src/librustc_codegen_llvm/mir/mod.rs#L442-L482
            if Some(local) == fx.mir.spread_arg {
                // This argument (e.g. the last argument in the "rust-call" ABI)
                // is a tuple that was spread at the ABI level and now we have
                // to reconstruct it into a tuple local variable, from multiple
                // individual function arguments.

                let tupled_arg_tys = match arg_ty.kind() {
                    ty::Tuple(ref tys) => tys,
                    _ => bug!("spread argument isn't a tuple?! but {:?}", arg_ty),
                };

                let mut params = Vec::new();
                for (i, arg_ty) in tupled_arg_tys.types().enumerate() {
                    let param = cvalue_for_param(fx, start_block, Some(local), Some(i), arg_ty);
                    params.push(param);
                }

                (local, ArgKind::Spread(params), arg_ty)
            } else {
                let param = cvalue_for_param(fx, start_block, Some(local), None, arg_ty);
                (local, ArgKind::Normal(param), arg_ty)
            }
        })
        .collect::<Vec<(Local, ArgKind<'tcx>, Ty<'tcx>)>>();

    assert!(fx.caller_location.is_none());
    if fx.instance.def.requires_caller_location(fx.tcx) {
        // Store caller location for `#[track_caller]`.
        fx.caller_location = Some(
            cvalue_for_param(fx, start_block, None, None, fx.tcx.caller_location_ty()).unwrap(),
        );
    }

    fx.bcx.switch_to_block(start_block);
    fx.bcx.ins().nop();

    #[cfg(debug_assertions)]
    self::comments::add_locals_header_comment(fx);

    for (local, arg_kind, ty) in func_params {
        let layout = fx.layout_of(ty);

        let is_ssa = ssa_analyzed[local] == crate::analyze::SsaKind::Ssa;

        // While this is normally an optimization to prevent an unnecessary copy when an argument is
        // not mutated by the current function, this is necessary to support unsized arguments.
        if let ArgKind::Normal(Some(val)) = arg_kind {
            if let Some((addr, meta)) = val.try_to_ptr() {
                let local_decl = &fx.mir.local_decls[local];
                //                       v this ! is important
                let internally_mutable = !val.layout().ty.is_freeze(
                    fx.tcx.at(local_decl.source_info.span),
                    ParamEnv::reveal_all(),
                );
                if local_decl.mutability == mir::Mutability::Not && !internally_mutable {
                    // We wont mutate this argument, so it is fine to borrow the backing storage
                    // of this argument, to prevent a copy.

                    let place = if let Some(meta) = meta {
                        CPlace::for_ptr_with_extra(addr, meta, val.layout())
                    } else {
                        CPlace::for_ptr(addr, val.layout())
                    };

                    #[cfg(debug_assertions)]
                    self::comments::add_local_place_comments(fx, place, local);

                    assert_eq!(fx.local_map.push(place), local);
                    continue;
                }
            }
        }

        let place = make_local_place(fx, local, layout, is_ssa);
        assert_eq!(fx.local_map.push(place), local);

        match arg_kind {
            ArgKind::Normal(param) => {
                if let Some(param) = param {
                    place.write_cvalue(fx, param);
                }
            }
            ArgKind::Spread(params) => {
                for (i, param) in params.into_iter().enumerate() {
                    if let Some(param) = param {
                        place
                            .place_field(fx, mir::Field::new(i))
                            .write_cvalue(fx, param);
                    }
                }
            }
        }
    }

    for local in fx.mir.vars_and_temps_iter() {
        let ty = fx.monomorphize(&fx.mir.local_decls[local].ty);
        let layout = fx.layout_of(ty);

        let is_ssa = ssa_analyzed[local] == crate::analyze::SsaKind::Ssa;

        let place = make_local_place(fx, local, layout, is_ssa);
        assert_eq!(fx.local_map.push(place), local);
    }

    fx.bcx
        .ins()
        .jump(*fx.block_map.get(START_BLOCK).unwrap(), &[]);
}

pub(crate) fn codegen_terminator_call<'tcx>(
    fx: &mut FunctionCx<'_, 'tcx, impl Module>,
    span: Span,
    current_block: Block,
    func: &Operand<'tcx>,
    args: &[Operand<'tcx>],
    destination: Option<(Place<'tcx>, BasicBlock)>,
) {
    let fn_ty = fx.monomorphize(&func.ty(fx.mir, fx.tcx));
    let fn_sig = fx
        .tcx
        .normalize_erasing_late_bound_regions(ParamEnv::reveal_all(), &fn_ty.fn_sig(fx.tcx));

    let destination = destination.map(|(place, bb)| (codegen_place(fx, place), bb));

    // Handle special calls like instrinsics and empty drop glue.
    let instance = if let ty::FnDef(def_id, substs) = *fn_ty.kind() {
        let instance = ty::Instance::resolve(fx.tcx, ty::ParamEnv::reveal_all(), def_id, substs)
            .unwrap()
            .unwrap()
            .polymorphize(fx.tcx);

        if fx.tcx.symbol_name(instance).name.starts_with("llvm.") {
            crate::intrinsics::codegen_llvm_intrinsic_call(
                fx,
                &fx.tcx.symbol_name(instance).name,
                substs,
                args,
                destination,
            );
            return;
        }

        match instance.def {
            InstanceDef::Intrinsic(_) => {
                crate::intrinsics::codegen_intrinsic_call(fx, instance, args, destination, span);
                return;
            }
            InstanceDef::DropGlue(_, None) => {
                // empty drop glue - a nop.
                let (_, dest) = destination.expect("Non terminating drop_in_place_real???");
                let ret_block = fx.get_block(dest);
                fx.bcx.ins().jump(ret_block, &[]);
                return;
            }
            _ => Some(instance),
        }
    } else {
        None
    };

    let is_cold = instance
        .map(|inst| {
            fx.tcx
                .codegen_fn_attrs(inst.def_id())
                .flags
                .contains(CodegenFnAttrFlags::COLD)
        })
        .unwrap_or(false);
    if is_cold {
        fx.cold_blocks.insert(current_block);
    }

    // Unpack arguments tuple for closures
    let args = if fn_sig.abi == Abi::RustCall {
        assert_eq!(args.len(), 2, "rust-call abi requires two arguments");
        let self_arg = codegen_operand(fx, &args[0]);
        let pack_arg = codegen_operand(fx, &args[1]);

        let tupled_arguments = match pack_arg.layout().ty.kind() {
            ty::Tuple(ref tupled_arguments) => tupled_arguments,
            _ => bug!("argument to function with \"rust-call\" ABI is not a tuple"),
        };

        let mut args = Vec::with_capacity(1 + tupled_arguments.len());
        args.push(self_arg);
        for i in 0..tupled_arguments.len() {
            args.push(pack_arg.value_field(fx, mir::Field::new(i)));
        }
        args
    } else {
        args.iter()
            .map(|arg| codegen_operand(fx, arg))
            .collect::<Vec<_>>()
    };

    //   | indirect call target
    //   |         | the first argument to be passed
    //   v         v          v virtual calls are special cased below
    let (func_ref, first_arg, is_virtual_call) = match instance {
        // Trait object call
        Some(Instance {
            def: InstanceDef::Virtual(_, idx),
            ..
        }) => {
            #[cfg(debug_assertions)]
            {
                let nop_inst = fx.bcx.ins().nop();
                fx.add_comment(
                    nop_inst,
                    format!(
                        "virtual call; self arg pass mode: {:?}",
                        get_pass_mode(fx.tcx, args[0].layout())
                    ),
                );
            }
            let (ptr, method) = crate::vtable::get_ptr_and_method_ref(fx, args[0], idx);
            (Some(method), Single(ptr), true)
        }

        // Normal call
        Some(_) => (
            None,
            args.get(0)
                .map(|arg| adjust_arg_for_abi(fx, *arg))
                .unwrap_or(Empty),
            false,
        ),

        // Indirect call
        None => {
            #[cfg(debug_assertions)]
            {
                let nop_inst = fx.bcx.ins().nop();
                fx.add_comment(nop_inst, "indirect call");
            }
            let func = codegen_operand(fx, func).load_scalar(fx);
            (
                Some(func),
                args.get(0)
                    .map(|arg| adjust_arg_for_abi(fx, *arg))
                    .unwrap_or(Empty),
                false,
            )
        }
    };

    let ret_place = destination.map(|(place, _)| place);
    let (call_inst, call_args) =
        self::returning::codegen_with_call_return_arg(fx, fn_sig, ret_place, |fx, return_ptr| {
            let mut call_args: Vec<Value> = return_ptr
                .into_iter()
                .chain(first_arg.into_iter())
                .chain(
                    args.into_iter()
                        .skip(1)
                        .map(|arg| adjust_arg_for_abi(fx, arg).into_iter())
                        .flatten(),
                )
                .collect::<Vec<_>>();

            if instance
                .map(|inst| inst.def.requires_caller_location(fx.tcx))
                .unwrap_or(false)
            {
                // Pass the caller location for `#[track_caller]`.
                let caller_location = fx.get_caller_location(span);
                call_args.extend(adjust_arg_for_abi(fx, caller_location).into_iter());
            }

            let call_inst = if let Some(func_ref) = func_ref {
                let sig = clif_sig_from_fn_sig(
                    fx.tcx,
                    fx.triple(),
                    fn_sig,
                    span,
                    is_virtual_call,
                    false, // calls through function pointers never pass the caller location
                );
                let sig = fx.bcx.import_signature(sig);
                fx.bcx.ins().call_indirect(sig, func_ref, &call_args)
            } else {
                let func_ref =
                    fx.get_function_ref(instance.expect("non-indirect call on non-FnDef type"));
                fx.bcx.ins().call(func_ref, &call_args)
            };

            (call_inst, call_args)
        });

    // FIXME find a cleaner way to support varargs
    if fn_sig.c_variadic {
        if fn_sig.abi != Abi::C {
            fx.tcx.sess.span_fatal(
                span,
                &format!("Variadic call for non-C abi {:?}", fn_sig.abi),
            );
        }
        let sig_ref = fx.bcx.func.dfg.call_signature(call_inst).unwrap();
        let abi_params = call_args
            .into_iter()
            .map(|arg| {
                let ty = fx.bcx.func.dfg.value_type(arg);
                if !ty.is_int() {
                    // FIXME set %al to upperbound on float args once floats are supported
                    fx.tcx
                        .sess
                        .span_fatal(span, &format!("Non int ty {:?} for variadic call", ty));
                }
                AbiParam::new(ty)
            })
            .collect::<Vec<AbiParam>>();
        fx.bcx.func.dfg.signatures[sig_ref].params = abi_params;
    }

    if let Some((_, dest)) = destination {
        let ret_block = fx.get_block(dest);
        fx.bcx.ins().jump(ret_block, &[]);
    } else {
        trap_unreachable(fx, "[corruption] Diverging function returned");
    }
}

pub(crate) fn codegen_drop<'tcx>(
    fx: &mut FunctionCx<'_, 'tcx, impl Module>,
    span: Span,
    drop_place: CPlace<'tcx>,
) {
    let ty = drop_place.layout().ty;
    let drop_fn = Instance::resolve_drop_in_place(fx.tcx, ty).polymorphize(fx.tcx);

    if let ty::InstanceDef::DropGlue(_, None) = drop_fn.def {
        // we don't actually need to drop anything
    } else {
        let drop_fn_ty = drop_fn.ty(fx.tcx, ParamEnv::reveal_all());
        let fn_sig = fx.tcx.normalize_erasing_late_bound_regions(
            ParamEnv::reveal_all(),
            &drop_fn_ty.fn_sig(fx.tcx),
        );
        assert_eq!(fn_sig.output(), fx.tcx.mk_unit());

        match ty.kind() {
            ty::Dynamic(..) => {
                let (ptr, vtable) = drop_place.to_ptr_maybe_unsized();
                let ptr = ptr.get_addr(fx);
                let drop_fn = crate::vtable::drop_fn_of_obj(fx, vtable.unwrap());

                let sig = clif_sig_from_fn_sig(
                    fx.tcx,
                    fx.triple(),
                    fn_sig,
                    span,
                    true,
                    false, // `drop_in_place` is never `#[track_caller]`
                );
                let sig = fx.bcx.import_signature(sig);
                fx.bcx.ins().call_indirect(sig, drop_fn, &[ptr]);
            }
            _ => {
                assert!(!matches!(drop_fn.def, InstanceDef::Virtual(_, _)));

                let arg_value = drop_place.place_ref(
                    fx,
                    fx.layout_of(fx.tcx.mk_ref(
                        &ty::RegionKind::ReErased,
                        TypeAndMut {
                            ty,
                            mutbl: crate::rustc_hir::Mutability::Mut,
                        },
                    )),
                );
                let arg_value = adjust_arg_for_abi(fx, arg_value);

                let mut call_args: Vec<Value> = arg_value.into_iter().collect::<Vec<_>>();

                if drop_fn.def.requires_caller_location(fx.tcx) {
                    // Pass the caller location for `#[track_caller]`.
                    let caller_location = fx.get_caller_location(span);
                    call_args.extend(adjust_arg_for_abi(fx, caller_location).into_iter());
                }

                let func_ref = fx.get_function_ref(drop_fn);
                fx.bcx.ins().call(func_ref, &call_args);
            }
        }
    }
}