src/librustc_mir/transform/check_consts/qualifs.rs

   1 //! Structural const qualification.
   2 //!
   3 //! See the `Qualif` trait for more info.
   4
   5 use rustc_middle::mir::*;
   6 use rustc_middle::ty::{self, AdtDef, Ty};
   7 use rustc_span::DUMMY_SP;
   8
   9 use super::Item as ConstCx;
  10
  11 pub fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> ConstQualifs {
  12     ConstQualifs {
  13         has_mut_interior: HasMutInterior::in_any_value_of_ty(cx, ty),
  14         needs_drop: NeedsDrop::in_any_value_of_ty(cx, ty),
  15     }
  16 }
  17
  18 /// A "qualif"(-ication) is a way to look for something "bad" in the MIR that would disqualify some
  19 /// code for promotion or prevent it from evaluating at compile time.
  20 ///
  21 /// Normally, we would determine what qualifications apply to each type and error when an illegal
  22 /// operation is performed on such a type. However, this was found to be too imprecise, especially
  23 /// in the presence of `enum`s. If only a single variant of an enum has a certain qualification, we
  24 /// needn't reject code unless it actually constructs and operates on the qualifed variant.
  25 ///
  26 /// To accomplish this, const-checking and promotion use a value-based analysis (as opposed to a
  27 /// type-based one). Qualifications propagate structurally across variables: If a local (or a
  28 /// projection of a local) is assigned a qualifed value, that local itself becomes qualifed.
  29 pub trait Qualif {
  30     /// The name of the file used to debug the dataflow analysis that computes this qualif.
  31     const ANALYSIS_NAME: &'static str;
  32
  33     /// Whether this `Qualif` is cleared when a local is moved from.
  34     const IS_CLEARED_ON_MOVE: bool = false;
  35
  36     /// Extracts the field of `ConstQualifs` that corresponds to this `Qualif`.
  37     fn in_qualifs(qualifs: &ConstQualifs) -> bool;
  38
  39     /// Returns `true` if *any* value of the given type could possibly have this `Qualif`.
  40     ///
  41     /// This function determines `Qualif`s when we cannot do a value-based analysis. Since qualif
  42     /// propagation is context-insenstive, this includes function arguments and values returned
  43     /// from a call to another function.
  44     ///
  45     /// It also determines the `Qualif`s for primitive types.
  46     fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool;
  47
  48     /// Returns `true` if this `Qualif` is inherent to the given struct or enum.
  49     ///
  50     /// By default, `Qualif`s propagate into ADTs in a structural way: An ADT only becomes
  51     /// qualified if part of it is assigned a value with that `Qualif`. However, some ADTs *always*
  52     /// have a certain `Qualif`, regardless of whether their fields have it. For example, a type
  53     /// with a custom `Drop` impl is inherently `NeedsDrop`.
  54     ///
  55     /// Returning `true` for `in_adt_inherently` but `false` for `in_any_value_of_ty` is unsound.
  56     fn in_adt_inherently(cx: &ConstCx<'_, 'tcx>, adt: &AdtDef) -> bool;
  57 }
  58
  59 /// Constant containing interior mutability (`UnsafeCell<T>`).
  60 /// This must be ruled out to make sure that evaluating the constant at compile-time
  61 /// and at *any point* during the run-time would produce the same result. In particular,
  62 /// promotion of temporaries must not change program behavior; if the promoted could be
  63 /// written to, that would be a problem.
  64 pub struct HasMutInterior;
  65
  66 impl Qualif for HasMutInterior {
  67     const ANALYSIS_NAME: &'static str = "flow_has_mut_interior";
  68
  69     fn in_qualifs(qualifs: &ConstQualifs) -> bool {
  70         qualifs.has_mut_interior
  71     }
  72
  73     fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
  74         !ty.is_freeze(cx.tcx, cx.param_env, DUMMY_SP)
  75     }
  76
  77     fn in_adt_inherently(cx: &ConstCx<'_, 'tcx>, adt: &AdtDef) -> bool {
  78         // Exactly one type, `UnsafeCell`, has the `HasMutInterior` qualif inherently.
  79         // It arises structurally for all other types.
  80         Some(adt.did) == cx.tcx.lang_items().unsafe_cell_type()
  81     }
  82 }
  83
  84 /// Constant containing an ADT that implements `Drop`.
  85 /// This must be ruled out (a) because we cannot run `Drop` during compile-time
  86 /// as that might not be a `const fn`, and (b) because implicit promotion would
  87 /// remove side-effects that occur as part of dropping that value.
  88 pub struct NeedsDrop;
  89
  90 impl Qualif for NeedsDrop {
  91     const ANALYSIS_NAME: &'static str = "flow_needs_drop";
  92     const IS_CLEARED_ON_MOVE: bool = true;
  93
  94     fn in_qualifs(qualifs: &ConstQualifs) -> bool {
  95         qualifs.needs_drop
  96     }
  97
  98     fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
  99         ty.needs_drop(cx.tcx, cx.param_env)
 100     }
 101
 102     fn in_adt_inherently(cx: &ConstCx<'_, 'tcx>, adt: &AdtDef) -> bool {
 103         adt.has_dtor(cx.tcx)
 104     }
 105 }
 106
 107 // FIXME: Use `mir::visit::Visitor` for the `in_*` functions if/when it supports early return.
 108
 109 /// Returns `true` if this `Rvalue` contains qualif `Q`.
 110 pub fn in_rvalue<Q, F>(cx: &ConstCx<'_, 'tcx>, in_local: &mut F, rvalue: &Rvalue<'tcx>) -> bool
 111 where
 112     Q: Qualif,
 113     F: FnMut(Local) -> bool,
 114 {
 115     match rvalue {
 116         Rvalue::NullaryOp(..) => Q::in_any_value_of_ty(cx, rvalue.ty(*cx.body, cx.tcx)),
 117
 118         Rvalue::Discriminant(place) | Rvalue::Len(place) => {
 119             in_place::<Q, _>(cx, in_local, place.as_ref())
 120         }
 121
 122         Rvalue::Use(operand)
 123         | Rvalue::Repeat(operand, _)
 124         | Rvalue::UnaryOp(_, operand)
 125         | Rvalue::Cast(_, operand, _) => in_operand::<Q, _>(cx, in_local, operand),
 126
 127         Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
 128             in_operand::<Q, _>(cx, in_local, lhs) || in_operand::<Q, _>(cx, in_local, rhs)
 129         }
 130
 131         Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
 132             // Special-case reborrows to be more like a copy of the reference.
 133             if let &[ref proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
 134                 let base_ty = Place::ty_from(place.local, proj_base, *cx.body, cx.tcx).ty;
 135                 if let ty::Ref(..) = base_ty.kind {
 136                     return in_place::<Q, _>(
 137                         cx,
 138                         in_local,
 139                         PlaceRef { local: place.local, projection: proj_base },
 140                     );
 141                 }
 142             }
 143
 144             in_place::<Q, _>(cx, in_local, place.as_ref())
 145         }
 146
 147         Rvalue::Aggregate(kind, operands) => {
 148             // Return early if we know that the struct or enum being constructed is always
 149             // qualified.
 150             if let AggregateKind::Adt(def, ..) = **kind {
 151                 if Q::in_adt_inherently(cx, def) {
 152                     return true;
 153                 }
 154             }
 155
 156             // Otherwise, proceed structurally...
 157             operands.iter().any(|o| in_operand::<Q, _>(cx, in_local, o))
 158         }
 159     }
 160 }
 161
 162 /// Returns `true` if this `Place` contains qualif `Q`.
 163 pub fn in_place<Q, F>(cx: &ConstCx<'_, 'tcx>, in_local: &mut F, place: PlaceRef<'tcx>) -> bool
 164 where
 165     Q: Qualif,
 166     F: FnMut(Local) -> bool,
 167 {
 168     let mut projection = place.projection;
 169     while let [ref proj_base @ .., proj_elem] = projection {
 170         match *proj_elem {
 171             ProjectionElem::Index(index) if in_local(index) => return true,
 172
 173             ProjectionElem::Deref
 174             | ProjectionElem::Field(_, _)
 175             | ProjectionElem::ConstantIndex { .. }
 176             | ProjectionElem::Subslice { .. }
 177             | ProjectionElem::Downcast(_, _)
 178             | ProjectionElem::Index(_) => {}
 179         }
 180
 181         let base_ty = Place::ty_from(place.local, proj_base, *cx.body, cx.tcx);
 182         let proj_ty = base_ty.projection_ty(cx.tcx, proj_elem).ty;
 183         if !Q::in_any_value_of_ty(cx, proj_ty) {
 184             return false;
 185         }
 186
 187         projection = proj_base;
 188     }
 189
 190     assert!(projection.is_empty());
 191     in_local(place.local)
 192 }
 193
 194 /// Returns `true` if this `Operand` contains qualif `Q`.
 195 pub fn in_operand<Q, F>(cx: &ConstCx<'_, 'tcx>, in_local: &mut F, operand: &Operand<'tcx>) -> bool
 196 where
 197     Q: Qualif,
 198     F: FnMut(Local) -> bool,
 199 {
 200     let constant = match operand {
 201         Operand::Copy(place) | Operand::Move(place) => {
 202             return in_place::<Q, _>(cx, in_local, place.as_ref());
 203         }
 204
 205         Operand::Constant(c) => c,
 206     };
 207
 208     // Check the qualifs of the value of `const` items.
 209     if let ty::ConstKind::Unevaluated(def_id, _, promoted) = constant.literal.val {
 210         assert!(promoted.is_none());
 211         // Don't peek inside trait associated constants.
 212         if cx.tcx.trait_of_item(def_id).is_none() {
 213             let qualifs = cx.tcx.at(constant.span).mir_const_qualif(def_id);
 214             if !Q::in_qualifs(&qualifs) {
 215                 return false;
 216             }
 217
 218             // Just in case the type is more specific than
 219             // the definition, e.g., impl associated const
 220             // with type parameters, take it into account.
 221         }
 222     }
 223     // Otherwise use the qualifs of the type.
 224     Q::in_any_value_of_ty(cx, constant.literal.ty)
 225 }