Copy Qualif to start work on new const validator

This is an exact copy of the `Qualif` trait from `qualify_consts.rs` and
its first two implementers, `HasMutInterior` and `NeedsDrop`.

Author: ecstatic-morse

src/librustc_mir/transform/check_consts/qualifs.rs

@@ -0,0 +1,271 @@
+/// A "qualif"(-ication) is a way to look for something "bad" in the MIR that would disqualify some
+/// code for promotion or prevent it from evaluating at compile time. So `return true` means
+/// "I found something bad, no reason to go on searching". `false` is only returned if we
+/// definitely cannot find anything bad anywhere.
+///
+/// The default implementations proceed structurally.
+trait Qualif {
+    const IDX: usize;
+
+    /// Return the qualification that is (conservatively) correct for any value
+    /// of the type, or `None` if the qualification is not value/type-based.
+    fn in_any_value_of_ty(_cx: &ConstCx<'_, 'tcx>, _ty: Ty<'tcx>) -> Option<bool> {
+        None
+    }
+
+    /// Return a mask for the qualification, given a type. This is `false` iff
+    /// no value of that type can have the qualification.
+    fn mask_for_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
+        Self::in_any_value_of_ty(cx, ty).unwrap_or(true)
+    }
+
+    fn in_local(cx: &ConstCx<'_, '_>, local: Local) -> bool {
+        cx.per_local.0[Self::IDX].contains(local)
+    }
+
+    fn in_static(_cx: &ConstCx<'_, 'tcx>, _static: &Static<'tcx>) -> bool {
+        // FIXME(eddyb) should we do anything here for value properties?
+        false
+    }
+
+    fn in_projection_structurally(
+        cx: &ConstCx<'_, 'tcx>,
+        place: PlaceRef<'_, 'tcx>,
+    ) -> bool {
+        if let [proj_base @ .., elem] = place.projection {
+            let base_qualif = Self::in_place(cx, PlaceRef {
+                base: place.base,
+                projection: proj_base,
+            });
+            let qualif = base_qualif && Self::mask_for_ty(
+                cx,
+                Place::ty_from(place.base, proj_base, cx.body, cx.tcx)
+                    .projection_ty(cx.tcx, elem)
+                    .ty,
+            );
+            match elem {
+                ProjectionElem::Deref |
+                ProjectionElem::Subslice { .. } |
+                ProjectionElem::Field(..) |
+                ProjectionElem::ConstantIndex { .. } |
+                ProjectionElem::Downcast(..) => qualif,
+
+                ProjectionElem::Index(local) => qualif || Self::in_local(cx, *local),
+            }
+        } else {
+            bug!("This should be called if projection is not empty");
+        }
+    }
+
+    fn in_projection(
+        cx: &ConstCx<'_, 'tcx>,
+        place: PlaceRef<'_, 'tcx>,
+    ) -> bool {
+        Self::in_projection_structurally(cx, place)
+    }
+
+    fn in_place(cx: &ConstCx<'_, 'tcx>, place: PlaceRef<'_, 'tcx>) -> bool {
+        match place {
+            PlaceRef {
+                base: PlaceBase::Local(local),
+                projection: [],
+            } => Self::in_local(cx, *local),
+            PlaceRef {
+                base: PlaceBase::Static(box Static {
+                    kind: StaticKind::Promoted(..),
+                    ..
+                }),
+                projection: [],
+            } => bug!("qualifying already promoted MIR"),
+            PlaceRef {
+                base: PlaceBase::Static(static_),
+                projection: [],
+            } => {
+                Self::in_static(cx, static_)
+            },
+            PlaceRef {
+                base: _,
+                projection: [.., _],
+            } => Self::in_projection(cx, place),
+        }
+    }
+
+    fn in_operand(cx: &ConstCx<'_, 'tcx>, operand: &Operand<'tcx>) -> bool {
+        match *operand {
+            Operand::Copy(ref place) |
+            Operand::Move(ref place) => Self::in_place(cx, place.as_ref()),
+
+            Operand::Constant(ref constant) => {
+                if let ConstValue::Unevaluated(def_id, _) = constant.literal.val {
+                    // Don't peek inside trait associated constants.
+                    if cx.tcx.trait_of_item(def_id).is_some() {
+                        Self::in_any_value_of_ty(cx, constant.literal.ty).unwrap_or(false)
+                    } else {
+                        let (bits, _) = cx.tcx.at(constant.span).mir_const_qualif(def_id);
+
+                        let qualif = PerQualif::decode_from_bits(bits).0[Self::IDX];
+
+                        // Just in case the type is more specific than
+                        // the definition, e.g., impl associated const
+                        // with type parameters, take it into account.
+                        qualif && Self::mask_for_ty(cx, constant.literal.ty)
+                    }
+                } else {
+                    false
+                }
+            }
+        }
+    }
+
+    fn in_rvalue_structurally(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
+        match *rvalue {
+            Rvalue::NullaryOp(..) => false,
+
+            Rvalue::Discriminant(ref place) |
+            Rvalue::Len(ref place) => Self::in_place(cx, place.as_ref()),
+
+            Rvalue::Use(ref operand) |
+            Rvalue::Repeat(ref operand, _) |
+            Rvalue::UnaryOp(_, ref operand) |
+            Rvalue::Cast(_, ref operand, _) => Self::in_operand(cx, operand),
+
+            Rvalue::BinaryOp(_, ref lhs, ref rhs) |
+            Rvalue::CheckedBinaryOp(_, ref lhs, ref rhs) => {
+                Self::in_operand(cx, lhs) || Self::in_operand(cx, rhs)
+            }
+
+            Rvalue::Ref(_, _, ref place) => {
+                // Special-case reborrows to be more like a copy of the reference.
+                if let box [proj_base @ .., elem] = &place.projection {
+                    if ProjectionElem::Deref == *elem {
+                        let base_ty = Place::ty_from(&place.base, proj_base, cx.body, cx.tcx).ty;
+                        if let ty::Ref(..) = base_ty.sty {
+                            return Self::in_place(cx, PlaceRef {
+                                base: &place.base,
+                                projection: proj_base,
+                            });
+                        }
+                    }
+                }
+
+                Self::in_place(cx, place.as_ref())
+            }
+
+            Rvalue::Aggregate(_, ref operands) => {
+                operands.iter().any(|o| Self::in_operand(cx, o))
+            }
+        }
+    }
+
+    fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
+        Self::in_rvalue_structurally(cx, rvalue)
+    }
+
+    fn in_call(
+        cx: &ConstCx<'_, 'tcx>,
+        _callee: &Operand<'tcx>,
+        _args: &[Operand<'tcx>],
+        return_ty: Ty<'tcx>,
+    ) -> bool {
+        // Be conservative about the returned value of a const fn.
+        Self::in_any_value_of_ty(cx, return_ty).unwrap_or(false)
+    }
+
+    fn in_value(cx: &ConstCx<'_, 'tcx>, source: ValueSource<'_, 'tcx>) -> bool {
+        match source {
+            ValueSource::Rvalue(rvalue) => Self::in_rvalue(cx, rvalue),
+            ValueSource::DropAndReplace(source) => Self::in_operand(cx, source),
+            ValueSource::Call { callee, args, return_ty } => {
+                Self::in_call(cx, callee, args, return_ty)
+            }
+        }
+    }
+}
+
+/// Constant containing interior mutability (`UnsafeCell<T>`).
+/// This must be ruled out to make sure that evaluating the constant at compile-time
+/// and at *any point* during the run-time would produce the same result. In particular,
+/// promotion of temporaries must not change program behavior; if the promoted could be
+/// written to, that would be a problem.
+struct HasMutInterior;
+
+impl Qualif for HasMutInterior {
+    const IDX: usize = 0;
+
+    fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> Option<bool> {
+        Some(!ty.is_freeze(cx.tcx, cx.param_env, DUMMY_SP))
+    }
+
+    fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
+        match *rvalue {
+            // Returning `true` for `Rvalue::Ref` indicates the borrow isn't
+            // allowed in constants (and the `Checker` will error), and/or it
+            // won't be promoted, due to `&mut ...` or interior mutability.
+            Rvalue::Ref(_, kind, ref place) => {
+                let ty = place.ty(cx.body, cx.tcx).ty;
+
+                if let BorrowKind::Mut { .. } = kind {
+                    // In theory, any zero-sized value could be borrowed
+                    // mutably without consequences. However, only &mut []
+                    // is allowed right now, and only in functions.
+                    if cx.mode == Mode::StaticMut {
+                        // Inside a `static mut`, &mut [...] is also allowed.
+                        match ty.sty {
+                            ty::Array(..) | ty::Slice(_) => {}
+                            _ => return true,
+                        }
+                    } else if let ty::Array(_, len) = ty.sty {
+                        // FIXME(eddyb) the `cx.mode == Mode::NonConstFn` condition
+                        // seems unnecessary, given that this is merely a ZST.
+                        match len.try_eval_usize(cx.tcx, cx.param_env) {
+                            Some(0) if cx.mode == Mode::NonConstFn => {},
+                            _ => return true,
+                        }
+                    } else {
+                        return true;
+                    }
+                }
+            }
+
+            Rvalue::Aggregate(ref kind, _) => {
+                if let AggregateKind::Adt(def, ..) = **kind {
+                    if Some(def.did) == cx.tcx.lang_items().unsafe_cell_type() {
+                        let ty = rvalue.ty(cx.body, cx.tcx);
+                        assert_eq!(Self::in_any_value_of_ty(cx, ty), Some(true));
+                        return true;
+                    }
+                }
+            }
+
+            _ => {}
+        }
+
+        Self::in_rvalue_structurally(cx, rvalue)
+    }
+}
+
+/// Constant containing an ADT that implements `Drop`.
+/// This must be ruled out (a) because we cannot run `Drop` during compile-time
+/// as that might not be a `const fn`, and (b) because implicit promotion would
+/// remove side-effects that occur as part of dropping that value.
+struct NeedsDrop;
+
+impl Qualif for NeedsDrop {
+    const IDX: usize = 1;
+
+    fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> Option<bool> {
+        Some(ty.needs_drop(cx.tcx, cx.param_env))
+    }
+
+    fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
+        if let Rvalue::Aggregate(ref kind, _) = *rvalue {
+            if let AggregateKind::Adt(def, ..) = **kind {
+                if def.has_dtor(cx.tcx) {
+                    return true;
+                }
+            }
+        }
+
+        Self::in_rvalue_structurally(cx, rvalue)
+    }
+}