src/librustc_mir/interpret/visitor.rs

   1 //! Visitor for a run-time value with a given layout: Traverse enums, structs and other compound
   2 //! types until we arrive at the leaves, with custom handling for primitive types.
   3
   4 use rustc::mir::interpret::InterpResult;
   5 use rustc::ty;
   6 use rustc::ty::layout::{self, TyLayout, VariantIdx};
   7
   8 use super::{InterpCx, MPlaceTy, Machine, OpTy};
   9
  10 // A thing that we can project into, and that has a layout.
  11 // This wouldn't have to depend on `Machine` but with the current type inference,
  12 // that's just more convenient to work with (avoids repeating all the `Machine` bounds).
  13 pub trait Value<'mir, 'tcx, M: Machine<'mir, 'tcx>>: Copy {
  14     /// Gets this value's layout.
  15     fn layout(&self) -> TyLayout<'tcx>;
  16
  17     /// Makes this into an `OpTy`.
  18     fn to_op(self, ecx: &InterpCx<'mir, 'tcx, M>) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>>;
  19
  20     /// Creates this from an `MPlaceTy`.
  21     fn from_mem_place(mplace: MPlaceTy<'tcx, M::PointerTag>) -> Self;
  22
  23     /// Projects to the given enum variant.
  24     fn project_downcast(
  25         self,
  26         ecx: &InterpCx<'mir, 'tcx, M>,
  27         variant: VariantIdx,
  28     ) -> InterpResult<'tcx, Self>;
  29
  30     /// Projects to the n-th field.
  31     fn project_field(self, ecx: &InterpCx<'mir, 'tcx, M>, field: u64) -> InterpResult<'tcx, Self>;
  32 }
  33
  34 // Operands and memory-places are both values.
  35 // Places in general are not due to `place_field` having to do `force_allocation`.
  36 impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Value<'mir, 'tcx, M> for OpTy<'tcx, M::PointerTag> {
  37     #[inline(always)]
  38     fn layout(&self) -> TyLayout<'tcx> {
  39         self.layout
  40     }
  41
  42     #[inline(always)]
  43     fn to_op(
  44         self,
  45         _ecx: &InterpCx<'mir, 'tcx, M>,
  46     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
  47         Ok(self)
  48     }
  49
  50     #[inline(always)]
  51     fn from_mem_place(mplace: MPlaceTy<'tcx, M::PointerTag>) -> Self {
  52         mplace.into()
  53     }
  54
  55     #[inline(always)]
  56     fn project_downcast(
  57         self,
  58         ecx: &InterpCx<'mir, 'tcx, M>,
  59         variant: VariantIdx,
  60     ) -> InterpResult<'tcx, Self> {
  61         ecx.operand_downcast(self, variant)
  62     }
  63
  64     #[inline(always)]
  65     fn project_field(self, ecx: &InterpCx<'mir, 'tcx, M>, field: u64) -> InterpResult<'tcx, Self> {
  66         ecx.operand_field(self, field)
  67     }
  68 }
  69
  70 impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Value<'mir, 'tcx, M> for MPlaceTy<'tcx, M::PointerTag> {
  71     #[inline(always)]
  72     fn layout(&self) -> TyLayout<'tcx> {
  73         self.layout
  74     }
  75
  76     #[inline(always)]
  77     fn to_op(
  78         self,
  79         _ecx: &InterpCx<'mir, 'tcx, M>,
  80     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
  81         Ok(self.into())
  82     }
  83
  84     #[inline(always)]
  85     fn from_mem_place(mplace: MPlaceTy<'tcx, M::PointerTag>) -> Self {
  86         mplace
  87     }
  88
  89     #[inline(always)]
  90     fn project_downcast(
  91         self,
  92         ecx: &InterpCx<'mir, 'tcx, M>,
  93         variant: VariantIdx,
  94     ) -> InterpResult<'tcx, Self> {
  95         ecx.mplace_downcast(self, variant)
  96     }
  97
  98     #[inline(always)]
  99     fn project_field(self, ecx: &InterpCx<'mir, 'tcx, M>, field: u64) -> InterpResult<'tcx, Self> {
 100         ecx.mplace_field(self, field)
 101     }
 102 }
 103
 104 macro_rules! make_value_visitor {
 105     ($visitor_trait_name:ident, $($mutability:ident)?) => {
 106         // How to traverse a value and what to do when we are at the leaves.
 107         pub trait $visitor_trait_name<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>>: Sized {
 108             type V: Value<'mir, 'tcx, M>;
 109
 110             /// The visitor must have an `InterpCx` in it.
 111             fn ecx(&$($mutability)? self)
 112                 -> &$($mutability)? InterpCx<'mir, 'tcx, M>;
 113
 114             // Recursive actions, ready to be overloaded.
 115             /// Visits the given value, dispatching as appropriate to more specialized visitors.
 116             #[inline(always)]
 117             fn visit_value(&mut self, v: Self::V) -> InterpResult<'tcx>
 118             {
 119                 self.walk_value(v)
 120             }
 121             /// Visits the given value as a union. No automatic recursion can happen here.
 122             #[inline(always)]
 123             fn visit_union(&mut self, _v: Self::V, _fields: usize) -> InterpResult<'tcx>
 124             {
 125                 Ok(())
 126             }
 127             /// Visits this value as an aggregate, you are getting an iterator yielding
 128             /// all the fields (still in an `InterpResult`, you have to do error handling yourself).
 129             /// Recurses into the fields.
 130             #[inline(always)]
 131             fn visit_aggregate(
 132                 &mut self,
 133                 v: Self::V,
 134                 fields: impl Iterator<Item=InterpResult<'tcx, Self::V>>,
 135             ) -> InterpResult<'tcx> {
 136                 self.walk_aggregate(v, fields)
 137             }
 138
 139             /// Called each time we recurse down to a field of a "product-like" aggregate
 140             /// (structs, tuples, arrays and the like, but not enums), passing in old (outer)
 141             /// and new (inner) value.
 142             /// This gives the visitor the chance to track the stack of nested fields that
 143             /// we are descending through.
 144             #[inline(always)]
 145             fn visit_field(
 146                 &mut self,
 147                 _old_val: Self::V,
 148                 _field: usize,
 149                 new_val: Self::V,
 150             ) -> InterpResult<'tcx> {
 151                 self.visit_value(new_val)
 152             }
 153             /// Called when recursing into an enum variant.
 154             /// This gives the visitor the chance to track the stack of nested fields that
 155             /// we are descending through.
 156             #[inline(always)]
 157             fn visit_variant(
 158                 &mut self,
 159                 _old_val: Self::V,
 160                 _variant: VariantIdx,
 161                 new_val: Self::V,
 162             ) -> InterpResult<'tcx> {
 163                 self.visit_value(new_val)
 164             }
 165
 166             // Default recursors. Not meant to be overloaded.
 167             fn walk_aggregate(
 168                 &mut self,
 169                 v: Self::V,
 170                 fields: impl Iterator<Item=InterpResult<'tcx, Self::V>>,
 171             ) -> InterpResult<'tcx> {
 172                 // Now iterate over it.
 173                 for (idx, field_val) in fields.enumerate() {
 174                     self.visit_field(v, idx, field_val?)?;
 175                 }
 176                 Ok(())
 177             }
 178             fn walk_value(&mut self, v: Self::V) -> InterpResult<'tcx>
 179             {
 180                 trace!("walk_value: type: {}", v.layout().ty);
 181
 182                 // Special treatment for special types, where the (static) layout is not sufficient.
 183                 match v.layout().ty.kind {
 184                     // If it is a trait object, switch to the real type that was used to create it.
 185                     ty::Dynamic(..) => {
 186                         // immediate trait objects are not a thing
 187                         let dest = v.to_op(self.ecx())?.assert_mem_place(self.ecx());
 188                         let inner = self.ecx().unpack_dyn_trait(dest)?.1;
 189                         trace!("walk_value: dyn object layout: {:#?}", inner.layout);
 190                         // recurse with the inner type
 191                         return self.visit_field(v, 0, Value::from_mem_place(inner));
 192                     },
 193                     // Slices do not need special handling here: they have `Array` field
 194                     // placement with length 0, so we enter the `Array` case below which
 195                     // indirectly uses the metadata to determine the actual length.
 196                     _ => {},
 197                 };
 198
 199                 // Visit the fields of this value.
 200                 match v.layout().fields {
 201                     layout::FieldPlacement::Union(fields) => {
 202                         self.visit_union(v, fields)?;
 203                     },
 204                     layout::FieldPlacement::Arbitrary { ref offsets, .. } => {
 205                         // FIXME: We collect in a vec because otherwise there are lifetime
 206                         // errors: Projecting to a field needs access to `ecx`.
 207                         let fields: Vec<InterpResult<'tcx, Self::V>> =
 208                             (0..offsets.len()).map(|i| {
 209                                 v.project_field(self.ecx(), i as u64)
 210                             })
 211                             .collect();
 212                         self.visit_aggregate(v, fields.into_iter())?;
 213                     },
 214                     layout::FieldPlacement::Array { .. } => {
 215                         // Let's get an mplace first.
 216                         let mplace = v.to_op(self.ecx())?.assert_mem_place(self.ecx());
 217                         // Now we can go over all the fields.
 218                         // This uses the *run-time length*, i.e., if we are a slice,
 219                         // the dynamic info from the metadata is used.
 220                         let iter = self.ecx().mplace_array_fields(mplace)?
 221                             .map(|f| f.and_then(|f| {
 222                                 Ok(Value::from_mem_place(f))
 223                             }));
 224                         self.visit_aggregate(v, iter)?;
 225                     }
 226                 }
 227
 228                 match v.layout().variants {
 229                     // If this is a multi-variant layout, find the right variant and proceed
 230                     // with *its* fields.
 231                     layout::Variants::Multiple { .. } => {
 232                         let op = v.to_op(self.ecx())?;
 233                         let idx = self.ecx().read_discriminant(op)?.1;
 234                         let inner = v.project_downcast(self.ecx(), idx)?;
 235                         trace!("walk_value: variant layout: {:#?}", inner.layout());
 236                         // recurse with the inner type
 237                         self.visit_variant(v, idx, inner)
 238                     }
 239                     // For single-variant layouts, we already did anything there is to do.
 240                     layout::Variants::Single { .. } => Ok(())
 241                 }
 242             }
 243         }
 244     }
 245 }
 246
 247 make_value_visitor!(ValueVisitor,);
 248 make_value_visitor!(MutValueVisitor, mut);