compiler/rustc_middle/src/ty/vtable.rs

   1 use std::convert::TryFrom;
   2 use std::fmt;
   3
   4 use crate::mir::interpret::{alloc_range, AllocId, Allocation, Pointer, Scalar, ScalarMaybeUninit};
   5 use crate::ty::{self, Instance, PolyTraitRef, Ty, TyCtxt};
   6 use rustc_ast::Mutability;
   7
   8 #[derive(Clone, Copy, PartialEq, HashStable)]
   9 pub enum VtblEntry<'tcx> {
  10     /// destructor of this type (used in vtable header)
  11     MetadataDropInPlace,
  12     /// layout size of this type (used in vtable header)
  13     MetadataSize,
  14     /// layout align of this type (used in vtable header)
  15     MetadataAlign,
  16     /// non-dispatchable associated function that is excluded from trait object
  17     Vacant,
  18     /// dispatchable associated function
  19     Method(Instance<'tcx>),
  20     /// pointer to a separate supertrait vtable, can be used by trait upcasting coercion
  21     TraitVPtr(PolyTraitRef<'tcx>),
  22 }
  23
  24 impl<'tcx> fmt::Debug for VtblEntry<'tcx> {
  25     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  26         // We want to call `Display` on `Instance` and `PolyTraitRef`,
  27         // so we implement this manually.
  28         match self {
  29             VtblEntry::MetadataDropInPlace => write!(f, "MetadataDropInPlace"),
  30             VtblEntry::MetadataSize => write!(f, "MetadataSize"),
  31             VtblEntry::MetadataAlign => write!(f, "MetadataAlign"),
  32             VtblEntry::Vacant => write!(f, "Vacant"),
  33             VtblEntry::Method(instance) => write!(f, "Method({})", instance),
  34             VtblEntry::TraitVPtr(trait_ref) => write!(f, "TraitVPtr({})", trait_ref),
  35         }
  36     }
  37 }
  38
  39 pub const COMMON_VTABLE_ENTRIES: &[VtblEntry<'_>] =
  40     &[VtblEntry::MetadataDropInPlace, VtblEntry::MetadataSize, VtblEntry::MetadataAlign];
  41
  42 pub const COMMON_VTABLE_ENTRIES_DROPINPLACE: usize = 0;
  43 pub const COMMON_VTABLE_ENTRIES_SIZE: usize = 1;
  44 pub const COMMON_VTABLE_ENTRIES_ALIGN: usize = 2;
  45
  46 impl<'tcx> TyCtxt<'tcx> {
  47     /// Retrieves an allocation that represents the contents of a vtable.
  48     /// There's a cache within `TyCtxt` so it will be deduplicated.
  49     pub fn vtable_allocation(
  50         self,
  51         ty: Ty<'tcx>,
  52         poly_trait_ref: Option<ty::PolyExistentialTraitRef<'tcx>>,
  53     ) -> AllocId {
  54         let tcx = self;
  55         let vtables_cache = tcx.vtables_cache.lock();
  56         if let Some(alloc_id) = vtables_cache.get(&(ty, poly_trait_ref)).cloned() {
  57             return alloc_id;
  58         }
  59         drop(vtables_cache);
  60
  61         let vtable_entries = if let Some(poly_trait_ref) = poly_trait_ref {
  62             let trait_ref = poly_trait_ref.with_self_ty(tcx, ty);
  63             let trait_ref = tcx.erase_regions(trait_ref);
  64
  65             tcx.vtable_entries(trait_ref)
  66         } else {
  67             COMMON_VTABLE_ENTRIES
  68         };
  69
  70         let layout = tcx
  71             .layout_of(ty::ParamEnv::reveal_all().and(ty))
  72             .expect("failed to build vtable representation");
  73         assert!(!layout.is_unsized(), "can't create a vtable for an unsized type");
  74         let size = layout.size.bytes();
  75         let align = layout.align.abi.bytes();
  76
  77         let ptr_size = tcx.data_layout.pointer_size;
  78         let ptr_align = tcx.data_layout.pointer_align.abi;
  79
  80         let vtable_size = ptr_size * u64::try_from(vtable_entries.len()).unwrap();
  81         let mut vtable =
  82             Allocation::uninit(vtable_size, ptr_align, /* panic_on_fail */ true).unwrap();
  83
  84         // No need to do any alignment checks on the memory accesses below, because we know the
  85         // allocation is correctly aligned as we created it above. Also we're only offsetting by
  86         // multiples of `ptr_align`, which means that it will stay aligned to `ptr_align`.
  87
  88         for (idx, entry) in vtable_entries.iter().enumerate() {
  89             let idx: u64 = u64::try_from(idx).unwrap();
  90             let scalar = match entry {
  91                 VtblEntry::MetadataDropInPlace => {
  92                     let instance = ty::Instance::resolve_drop_in_place(tcx, ty);
  93                     let fn_alloc_id = tcx.create_fn_alloc(instance);
  94                     let fn_ptr = Pointer::from(fn_alloc_id);
  95                     ScalarMaybeUninit::from_pointer(fn_ptr, &tcx)
  96                 }
  97                 VtblEntry::MetadataSize => Scalar::from_uint(size, ptr_size).into(),
  98                 VtblEntry::MetadataAlign => Scalar::from_uint(align, ptr_size).into(),
  99                 VtblEntry::Vacant => continue,
 100                 VtblEntry::Method(instance) => {
 101                     // Prepare the fn ptr we write into the vtable.
 102                     let instance = instance.polymorphize(tcx);
 103                     let fn_alloc_id = tcx.create_fn_alloc(instance);
 104                     let fn_ptr = Pointer::from(fn_alloc_id);
 105                     ScalarMaybeUninit::from_pointer(fn_ptr, &tcx)
 106                 }
 107                 VtblEntry::TraitVPtr(trait_ref) => {
 108                     let super_trait_ref = trait_ref.map_bound(|trait_ref| {
 109                         ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
 110                     });
 111                     let supertrait_alloc_id = self.vtable_allocation(ty, Some(super_trait_ref));
 112                     let vptr = Pointer::from(supertrait_alloc_id);
 113                     ScalarMaybeUninit::from_pointer(vptr, &tcx)
 114                 }
 115             };
 116             vtable
 117                 .write_scalar(&tcx, alloc_range(ptr_size * idx, ptr_size), scalar)
 118                 .expect("failed to build vtable representation");
 119         }
 120
 121         vtable.mutability = Mutability::Not;
 122         let alloc_id = tcx.create_memory_alloc(tcx.intern_const_alloc(vtable));
 123         let mut vtables_cache = self.vtables_cache.lock();
 124         vtables_cache.insert((ty, poly_trait_ref), alloc_id);
 125         alloc_id
 126     }
 127 }