rust/compiler/rustc_mir/src/interpret/traits.rs

use std::convert::TryFrom;

use rustc_middle::mir::interpret::{
    AllocError, InterpError, InterpResult, Pointer, PointerArithmetic, Scalar,
    UndefinedBehaviorInfo, UnsupportedOpInfo,
};
use rustc_middle::ty::{
    self, Instance, Ty, VtblEntry, COMMON_VTABLE_ENTRIES, COMMON_VTABLE_ENTRIES_ALIGN,
    COMMON_VTABLE_ENTRIES_DROPINPLACE, COMMON_VTABLE_ENTRIES_SIZE,
};
use rustc_target::abi::{Align, LayoutOf, Size};

use super::alloc_range;
use super::util::ensure_monomorphic_enough;
use super::{Allocation, FnVal, InterpCx, Machine};

fn vtable_alloc_error_to_interp_error<'tcx>(error: AllocError) -> InterpError<'tcx> {
    match error {
        AllocError::ReadPointerAsBytes => {
            InterpError::Unsupported(UnsupportedOpInfo::ReadPointerAsBytes)
        }
        AllocError::InvalidUninitBytes(_info) => {
            InterpError::UndefinedBehavior(UndefinedBehaviorInfo::InvalidUninitBytes(None))
        }
    }
}

impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
    /// Creates a dynamic vtable for the given type and vtable origin. This is used only for
    /// objects.
    ///
    /// The `trait_ref` encodes the erased self type. Hence, if we are
    /// making an object `Foo<Trait>` from a value of type `Foo<T>`, then
    /// `trait_ref` would map `T: Trait`.
    pub fn get_vtable(
        &mut self,
        ty: Ty<'tcx>,
        poly_trait_ref: Option<ty::PolyExistentialTraitRef<'tcx>>,
    ) -> InterpResult<'tcx, Pointer<M::PointerTag>> {
        trace!("get_vtable(trait_ref={:?})", poly_trait_ref);

        let (ty, poly_trait_ref) = self.tcx.erase_regions((ty, poly_trait_ref));

        // All vtables must be monomorphic, bail out otherwise.
        ensure_monomorphic_enough(*self.tcx, ty)?;
        ensure_monomorphic_enough(*self.tcx, poly_trait_ref)?;

        if let Some(&vtable) = self.vtables.get(&(ty, poly_trait_ref)) {
            // This means we guarantee that there are no duplicate vtables, we will
            // always use the same vtable for the same (Type, Trait) combination.
            // That's not what happens in rustc, but emulating per-crate deduplication
            // does not sound like it actually makes anything any better.
            return Ok(vtable);
        }

        let vtable_entries = if let Some(poly_trait_ref) = poly_trait_ref {
            let trait_ref = poly_trait_ref.with_self_ty(*self.tcx, ty);
            let trait_ref = self.tcx.erase_regions(trait_ref);

            self.tcx.vtable_entries(trait_ref)
        } else {
            COMMON_VTABLE_ENTRIES
        };

        let layout = self.layout_of(ty)?;
        assert!(!layout.is_unsized(), "can't create a vtable for an unsized type");
        let size = layout.size.bytes();
        let align = layout.align.abi.bytes();

        let tcx = *self.tcx;
        let ptr_size = self.pointer_size();
        let ptr_align = tcx.data_layout.pointer_align.abi;
        // /////////////////////////////////////////////////////////////////////////////////////////
        // If you touch this code, be sure to also make the corresponding changes to
        // `get_vtable` in `rust_codegen_llvm/meth.rs`.
        // /////////////////////////////////////////////////////////////////////////////////////////
        let vtable_size = ptr_size * u64::try_from(vtable_entries.len()).unwrap();
        let mut vtable = Allocation::uninit(vtable_size, ptr_align);

        // No need to do any alignment checks on the memory accesses below, because we know the
        // allocation is correctly aligned as we created it above. Also we're only offsetting by
        // multiples of `ptr_align`, which means that it will stay aligned to `ptr_align`.
        let scalars = vtable_entries
            .iter()
            .map(|entry| -> InterpResult<'tcx, _> {
                match entry {
                    VtblEntry::MetadataDropInPlace => {
                        let instance = Instance::resolve_drop_in_place(tcx, ty);
                        let fn_alloc_id = tcx.create_fn_alloc(instance);
                        let fn_ptr = Pointer::from(fn_alloc_id);
                        Ok(Some(fn_ptr.into()))
                    }
                    VtblEntry::MetadataSize => Ok(Some(Scalar::from_uint(size, ptr_size).into())),
                    VtblEntry::MetadataAlign => Ok(Some(Scalar::from_uint(align, ptr_size).into())),
                    VtblEntry::Vacant => Ok(None),
                    VtblEntry::Method(def_id, substs) => {
                        // Prepare the fn ptr we write into the vtable.
                        let instance =
                            Instance::resolve_for_vtable(tcx, self.param_env, *def_id, substs)
                                .ok_or_else(|| err_inval!(TooGeneric))?;
                        let fn_alloc_id = tcx.create_fn_alloc(instance);
                        let fn_ptr = Pointer::from(fn_alloc_id);
                        Ok(Some(fn_ptr.into()))
                    }
                }
            })
            .collect::<Result<Vec<_>, _>>()?;
        for (idx, scalar) in scalars.into_iter().enumerate() {
            if let Some(scalar) = scalar {
                let idx: u64 = u64::try_from(idx).unwrap();
                vtable
                    .write_scalar(self, alloc_range(ptr_size * idx, ptr_size), scalar)
                    .map_err(vtable_alloc_error_to_interp_error)?;
            }
        }

        let vtable_id = tcx.create_memory_alloc(tcx.intern_const_alloc(vtable));
        let vtable_ptr = self.memory.global_base_pointer(Pointer::from(vtable_id))?;

        assert!(self.vtables.insert((ty, poly_trait_ref), vtable_ptr).is_none());

        Ok(vtable_ptr)
    }

    /// Resolves the function at the specified slot in the provided
    /// vtable. Currently an index of '3' (`COMMON_VTABLE_ENTRIES.len()`)
    /// corresponds to the first method declared in the trait of the provided vtable.
    pub fn get_vtable_slot(
        &self,
        vtable: Scalar<M::PointerTag>,
        idx: u64,
    ) -> InterpResult<'tcx, FnVal<'tcx, M::ExtraFnVal>> {
        let ptr_size = self.pointer_size();
        let vtable_slot = vtable.ptr_offset(ptr_size * idx, self)?;
        let vtable_slot = self
            .memory
            .get(vtable_slot, ptr_size, self.tcx.data_layout.pointer_align.abi)?
            .expect("cannot be a ZST");
        let fn_ptr = vtable_slot.read_ptr_sized(Size::ZERO)?.check_init()?;
        self.memory.get_fn(fn_ptr)
    }

    /// Returns the drop fn instance as well as the actual dynamic type.
    pub fn read_drop_type_from_vtable(
        &self,
        vtable: Scalar<M::PointerTag>,
    ) -> InterpResult<'tcx, (ty::Instance<'tcx>, Ty<'tcx>)> {
        let pointer_size = self.pointer_size();
        // We don't care about the pointee type; we just want a pointer.
        let vtable = self
            .memory
            .get(
                vtable,
                pointer_size * u64::try_from(COMMON_VTABLE_ENTRIES.len()).unwrap(),
                self.tcx.data_layout.pointer_align.abi,
            )?
            .expect("cannot be a ZST");
        let drop_fn = vtable
            .read_ptr_sized(
                pointer_size * u64::try_from(COMMON_VTABLE_ENTRIES_DROPINPLACE).unwrap(),
            )?
            .check_init()?;
        // We *need* an instance here, no other kind of function value, to be able
        // to determine the type.
        let drop_instance = self.memory.get_fn(drop_fn)?.as_instance()?;
        trace!("Found drop fn: {:?}", drop_instance);
        let fn_sig = drop_instance.ty(*self.tcx, self.param_env).fn_sig(*self.tcx);
        let fn_sig = self.tcx.normalize_erasing_late_bound_regions(self.param_env, fn_sig);
        // The drop function takes `*mut T` where `T` is the type being dropped, so get that.
        let args = fn_sig.inputs();
        if args.len() != 1 {
            throw_ub!(InvalidVtableDropFn(fn_sig));
        }
        let ty =
            args[0].builtin_deref(true).ok_or_else(|| err_ub!(InvalidVtableDropFn(fn_sig)))?.ty;
        Ok((drop_instance, ty))
    }

    pub fn read_size_and_align_from_vtable(
        &self,
        vtable: Scalar<M::PointerTag>,
    ) -> InterpResult<'tcx, (Size, Align)> {
        let pointer_size = self.pointer_size();
        // We check for `size = 3 * ptr_size`, which covers the drop fn (unused here),
        // the size, and the align (which we read below).
        let vtable = self
            .memory
            .get(
                vtable,
                pointer_size * u64::try_from(COMMON_VTABLE_ENTRIES.len()).unwrap(),
                self.tcx.data_layout.pointer_align.abi,
            )?
            .expect("cannot be a ZST");
        let size = vtable
            .read_ptr_sized(pointer_size * u64::try_from(COMMON_VTABLE_ENTRIES_SIZE).unwrap())?
            .check_init()?;
        let size = u64::try_from(self.force_bits(size, pointer_size)?).unwrap();
        let align = vtable
            .read_ptr_sized(pointer_size * u64::try_from(COMMON_VTABLE_ENTRIES_ALIGN).unwrap())?
            .check_init()?;
        let align = u64::try_from(self.force_bits(align, pointer_size)?).unwrap();
        let align = Align::from_bytes(align).map_err(|e| err_ub!(InvalidVtableAlignment(e)))?;

        if size >= self.tcx.data_layout.obj_size_bound() {
            throw_ub!(InvalidVtableSize);
        }
        Ok((Size::from_bytes(size), align))
    }
}