compiler/rustc_middle/src/ty/vtable.rs

   1 use std::convert::TryFrom;
   2
   3 use crate::mir::interpret::{alloc_range, AllocId, Allocation, Pointer, Scalar, ScalarMaybeUninit};
   4 use crate::ty::fold::TypeFoldable;
   5 use crate::ty::{self, DefId, PolyExistentialTraitRef, SubstsRef, Ty, TyCtxt};
   6 use rustc_ast::Mutability;
   7
   8 #[derive(Clone, Copy, Debug, PartialEq, HashStable)]
   9 pub enum VtblEntry<'tcx> {
  10     MetadataDropInPlace,
  11     MetadataSize,
  12     MetadataAlign,
  13     Vacant,
  14     Method(DefId, SubstsRef<'tcx>),
  15     TraitVPtr(PolyExistentialTraitRef<'tcx>),
  16 }
  17
  18 pub const COMMON_VTABLE_ENTRIES: &[VtblEntry<'_>] =
  19     &[VtblEntry::MetadataDropInPlace, VtblEntry::MetadataSize, VtblEntry::MetadataAlign];
  20
  21 pub const COMMON_VTABLE_ENTRIES_DROPINPLACE: usize = 0;
  22 pub const COMMON_VTABLE_ENTRIES_SIZE: usize = 1;
  23 pub const COMMON_VTABLE_ENTRIES_ALIGN: usize = 2;
  24
  25 impl<'tcx> TyCtxt<'tcx> {
  26     /// Retrieves an allocation that represents the contents of a vtable.
  27     /// There's a cache within `TyCtxt` so it will be deduplicated.
  28     pub fn vtable_allocation(
  29         self,
  30         ty: Ty<'tcx>,
  31         poly_trait_ref: Option<ty::PolyExistentialTraitRef<'tcx>>,
  32     ) -> AllocId {
  33         let tcx = self;
  34         let vtables_cache = tcx.vtables_cache.lock();
  35         if let Some(alloc_id) = vtables_cache.get(&(ty, poly_trait_ref)).cloned() {
  36             return alloc_id;
  37         }
  38         drop(vtables_cache);
  39
  40         // See https://github.com/rust-lang/rust/pull/86475#discussion_r655162674
  41         assert!(
  42             !ty.needs_subst() && !poly_trait_ref.map_or(false, |trait_ref| trait_ref.needs_subst())
  43         );
  44         let param_env = ty::ParamEnv::reveal_all();
  45         let vtable_entries = if let Some(poly_trait_ref) = poly_trait_ref {
  46             let trait_ref = poly_trait_ref.with_self_ty(tcx, ty);
  47             let trait_ref = tcx.erase_regions(trait_ref);
  48
  49             tcx.vtable_entries(trait_ref)
  50         } else {
  51             COMMON_VTABLE_ENTRIES
  52         };
  53
  54         let layout =
  55             tcx.layout_of(param_env.and(ty)).expect("failed to build vtable representation");
  56         assert!(!layout.is_unsized(), "can't create a vtable for an unsized type");
  57         let size = layout.size.bytes();
  58         let align = layout.align.abi.bytes();
  59
  60         let ptr_size = tcx.data_layout.pointer_size;
  61         let ptr_align = tcx.data_layout.pointer_align.abi;
  62
  63         let vtable_size = ptr_size * u64::try_from(vtable_entries.len()).unwrap();
  64         let mut vtable =
  65             Allocation::uninit(vtable_size, ptr_align, /* panic_on_fail */ true).unwrap();
  66
  67         // No need to do any alignment checks on the memory accesses below, because we know the
  68         // allocation is correctly aligned as we created it above. Also we're only offsetting by
  69         // multiples of `ptr_align`, which means that it will stay aligned to `ptr_align`.
  70
  71         for (idx, entry) in vtable_entries.iter().enumerate() {
  72             let idx: u64 = u64::try_from(idx).unwrap();
  73             let scalar = match entry {
  74                 VtblEntry::MetadataDropInPlace => {
  75                     let instance = ty::Instance::resolve_drop_in_place(tcx, ty);
  76                     let fn_alloc_id = tcx.create_fn_alloc(instance);
  77                     let fn_ptr = Pointer::from(fn_alloc_id);
  78                     ScalarMaybeUninit::from_pointer(fn_ptr, &tcx)
  79                 }
  80                 VtblEntry::MetadataSize => Scalar::from_uint(size, ptr_size).into(),
  81                 VtblEntry::MetadataAlign => Scalar::from_uint(align, ptr_size).into(),
  82                 VtblEntry::Vacant => continue,
  83                 VtblEntry::Method(def_id, substs) => {
  84                     // See https://github.com/rust-lang/rust/pull/86475#discussion_r655162674
  85                     assert!(!substs.needs_subst());
  86
  87                     // Prepare the fn ptr we write into the vtable.
  88                     let instance =
  89                         ty::Instance::resolve_for_vtable(tcx, param_env, *def_id, substs)
  90                             .expect("resolution failed during building vtable representation")
  91                             .polymorphize(tcx);
  92                     let fn_alloc_id = tcx.create_fn_alloc(instance);
  93                     let fn_ptr = Pointer::from(fn_alloc_id);
  94                     ScalarMaybeUninit::from_pointer(fn_ptr, &tcx)
  95                 }
  96                 VtblEntry::TraitVPtr(trait_ref) => {
  97                     let supertrait_alloc_id = self.vtable_allocation(ty, Some(*trait_ref));
  98                     let vptr = Pointer::from(supertrait_alloc_id);
  99                     ScalarMaybeUninit::from_pointer(vptr, &tcx)
 100                 }
 101             };
 102             vtable
 103                 .write_scalar(&tcx, alloc_range(ptr_size * idx, ptr_size), scalar)
 104                 .expect("failed to build vtable representation");
 105         }
 106
 107         vtable.mutability = Mutability::Not;
 108         let alloc_id = tcx.create_memory_alloc(tcx.intern_const_alloc(vtable));
 109         let mut vtables_cache = self.vtables_cache.lock();
 110         vtables_cache.insert((ty, poly_trait_ref), alloc_id);
 111         alloc_id
 112     }
 113 }