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Rollup merge of #67335 - ecstatic-morse:qualif-refactor, r=eddyb
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Refactor the `Qualif` trait

This PR attempts to preserve the existing semantics of the `Qualif` trait while reducing its API to two significant methods with descriptive names, `in_any_value_of_ty` and `in_adt_inherently`. The other `in_*` methods have been made into free functions, since they should never be overloaded. Finally, I changed the bounds on the `in_local` argument to be less restrictive (`FnMut` instead of `Fn`), which addresses a FIXME in the const-checker.

r? @eddyb
cc @pnkfelix @oli-obk
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@Dylan-DPC
Dylan-DPC authored Mar 16, 2020
2 parents 8e6de32 + 6f75d3f commit 59b0058
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335 changes: 152 additions & 183 deletions src/librustc_mir/transform/check_consts/qualifs.rs
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Original file line number Diff line number Diff line change
@@ -1,7 +1,9 @@
//! A copy of the `Qualif` trait in `qualify_consts.rs` that is suitable for the new validator.
//! Structural const qualification.
//!
//! See the `Qualif` trait for more info.

use rustc::mir::*;
use rustc::ty::{self, Ty};
use rustc::ty::{self, AdtDef, Ty};
use rustc_span::DUMMY_SP;

use super::Item as ConstCx;
Expand All @@ -14,169 +16,44 @@ pub fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> ConstQualifs
}

/// 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.
/// code for promotion or prevent it from evaluating at compile time.
///
/// The default implementations proceed structurally.
/// Normally, we would determine what qualifications apply to each type and error when an illegal
/// operation is performed on such a type. However, this was found to be too imprecise, especially
/// in the presence of `enum`s. If only a single variant of an enum has a certain qualification, we
/// needn't reject code unless it actually constructs and operates on the qualifed variant.
///
/// To accomplish this, const-checking and promotion use a value-based analysis (as opposed to a
/// type-based one). Qualifications propagate structurally across variables: If a local (or a
/// projection of a local) is assigned a qualifed value, that local itself becomes qualifed.
pub trait Qualif {
/// The name of the file used to debug the dataflow analysis that computes this qualif.
const ANALYSIS_NAME: &'static str;

/// Whether this `Qualif` is cleared when a local is moved from.
const IS_CLEARED_ON_MOVE: bool = false;

/// Extracts the field of `ConstQualifs` that corresponds to this `Qualif`.
fn in_qualifs(qualifs: &ConstQualifs) -> bool;

/// Return the qualification that is (conservatively) correct for any value
/// of the type.
fn in_any_value_of_ty(_cx: &ConstCx<'_, 'tcx>, _ty: Ty<'tcx>) -> bool;

fn in_projection_structurally(
cx: &ConstCx<'_, 'tcx>,
per_local: &mut impl FnMut(Local) -> bool,
place: PlaceRef<'tcx>,
) -> bool {
if let [proj_base @ .., elem] = place.projection {
let base_qualif = Self::in_place(
cx,
per_local,
PlaceRef { local: place.local, projection: proj_base },
);
let qualif = base_qualif
&& Self::in_any_value_of_ty(
cx,
Place::ty_from(place.local, 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 || per_local(*local),
}
} else {
bug!("This should be called if projection is not empty");
}
}

fn in_projection(
cx: &ConstCx<'_, 'tcx>,
per_local: &mut impl FnMut(Local) -> bool,
place: PlaceRef<'tcx>,
) -> bool {
Self::in_projection_structurally(cx, per_local, place)
}

fn in_place(
cx: &ConstCx<'_, 'tcx>,
per_local: &mut impl FnMut(Local) -> bool,
place: PlaceRef<'tcx>,
) -> bool {
match place {
PlaceRef { local, projection: [] } => per_local(local),
PlaceRef { local: _, projection: [.., _] } => Self::in_projection(cx, per_local, place),
}
}

fn in_operand(
cx: &ConstCx<'_, 'tcx>,
per_local: &mut impl FnMut(Local) -> bool,
operand: &Operand<'tcx>,
) -> bool {
match *operand {
Operand::Copy(ref place) | Operand::Move(ref place) => {
Self::in_place(cx, per_local, place.as_ref())
}

Operand::Constant(ref constant) => {
// Check the qualifs of the value of `const` items.
if let ty::ConstKind::Unevaluated(def_id, _, promoted) = constant.literal.val {
assert!(promoted.is_none());
// Don't peek inside trait associated constants.
if cx.tcx.trait_of_item(def_id).is_none() {
let qualifs = cx.tcx.at(constant.span).mir_const_qualif(def_id);
if !Self::in_qualifs(&qualifs) {
return false;
}

// Just in case the type is more specific than
// the definition, e.g., impl associated const
// with type parameters, take it into account.
}
}
// Otherwise use the qualifs of the type.
Self::in_any_value_of_ty(cx, constant.literal.ty)
}
}
}

fn in_rvalue_structurally(
cx: &ConstCx<'_, 'tcx>,
per_local: &mut impl FnMut(Local) -> bool,
rvalue: &Rvalue<'tcx>,
) -> bool {
match *rvalue {
Rvalue::NullaryOp(..) => false,

Rvalue::Discriminant(ref place) | Rvalue::Len(ref place) => {
Self::in_place(cx, per_local, place.as_ref())
}

Rvalue::Use(ref operand)
| Rvalue::Repeat(ref operand, _)
| Rvalue::UnaryOp(_, ref operand)
| Rvalue::Cast(_, ref operand, _) => Self::in_operand(cx, per_local, operand),

Rvalue::BinaryOp(_, ref lhs, ref rhs)
| Rvalue::CheckedBinaryOp(_, ref lhs, ref rhs) => {
Self::in_operand(cx, per_local, lhs) || Self::in_operand(cx, per_local, rhs)
}

Rvalue::Ref(_, _, ref place) | Rvalue::AddressOf(_, ref place) => {
// Special-case reborrows to be more like a copy of the reference.
if let [proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
let base_ty = Place::ty_from(place.local, proj_base, *cx.body, cx.tcx).ty;
if let ty::Ref(..) = base_ty.kind {
return Self::in_place(
cx,
per_local,
PlaceRef { local: place.local, projection: proj_base },
);
}
}

Self::in_place(cx, per_local, place.as_ref())
}

Rvalue::Aggregate(_, ref operands) => {
operands.iter().any(|o| Self::in_operand(cx, per_local, o))
}
}
}

fn in_rvalue(
cx: &ConstCx<'_, 'tcx>,
per_local: &mut impl FnMut(Local) -> bool,
rvalue: &Rvalue<'tcx>,
) -> bool {
Self::in_rvalue_structurally(cx, per_local, rvalue)
}

fn in_call(
cx: &ConstCx<'_, 'tcx>,
_per_local: &mut impl FnMut(Local) -> bool,
_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)
}
/// Returns `true` if *any* value of the given type could possibly have this `Qualif`.
///
/// This function determines `Qualif`s when we cannot do a value-based analysis. Since qualif
/// propagation is context-insenstive, this includes function arguments and values returned
/// from a call to another function.
///
/// It also determines the `Qualif`s for primitive types.
fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool;

/// Returns `true` if this `Qualif` is inherent to the given struct or enum.
///
/// By default, `Qualif`s propagate into ADTs in a structural way: An ADT only becomes
/// qualified if part of it is assigned a value with that `Qualif`. However, some ADTs *always*
/// have a certain `Qualif`, regardless of whether their fields have it. For example, a type
/// with a custom `Drop` impl is inherently `NeedsDrop`.
///
/// Returning `true` for `in_adt_inherently` but `false` for `in_any_value_of_ty` is unsound.
fn in_adt_inherently(cx: &ConstCx<'_, 'tcx>, adt: &AdtDef) -> bool;
}

/// Constant containing interior mutability (`UnsafeCell<T>`).
Expand All @@ -197,26 +74,10 @@ impl Qualif for HasMutInterior {
!ty.is_freeze(cx.tcx, cx.param_env, DUMMY_SP)
}

fn in_rvalue(
cx: &ConstCx<'_, 'tcx>,
per_local: &mut impl FnMut(Local) -> bool,
rvalue: &Rvalue<'tcx>,
) -> bool {
match *rvalue {
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), true);
return true;
}
}
}

_ => {}
}

Self::in_rvalue_structurally(cx, per_local, rvalue)
fn in_adt_inherently(cx: &ConstCx<'_, 'tcx>, adt: &AdtDef) -> bool {
// Exactly one type, `UnsafeCell`, has the `HasMutInterior` qualif inherently.
// It arises structurally for all other types.
Some(adt.did) == cx.tcx.lang_items().unsafe_cell_type()
}
}

Expand All @@ -238,19 +99,127 @@ impl Qualif for NeedsDrop {
ty.needs_drop(cx.tcx, cx.param_env)
}

fn in_rvalue(
cx: &ConstCx<'_, 'tcx>,
per_local: &mut impl FnMut(Local) -> bool,
rvalue: &Rvalue<'tcx>,
) -> bool {
if let Rvalue::Aggregate(ref kind, _) = *rvalue {
fn in_adt_inherently(cx: &ConstCx<'_, 'tcx>, adt: &AdtDef) -> bool {
adt.has_dtor(cx.tcx)
}
}

// FIXME: Use `mir::visit::Visitor` for the `in_*` functions if/when it supports early return.

/// Returns `true` if this `Rvalue` contains qualif `Q`.
pub fn in_rvalue<Q, F>(cx: &ConstCx<'_, 'tcx>, in_local: &mut F, rvalue: &Rvalue<'tcx>) -> bool
where
Q: Qualif,
F: FnMut(Local) -> bool,
{
match rvalue {
Rvalue::NullaryOp(..) => Q::in_any_value_of_ty(cx, rvalue.ty(*cx.body, cx.tcx)),

Rvalue::Discriminant(place) | Rvalue::Len(place) => {
in_place::<Q, _>(cx, in_local, place.as_ref())
}

Rvalue::Use(operand)
| Rvalue::Repeat(operand, _)
| Rvalue::UnaryOp(_, operand)
| Rvalue::Cast(_, operand, _) => in_operand::<Q, _>(cx, in_local, operand),

Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
in_operand::<Q, _>(cx, in_local, lhs) || in_operand::<Q, _>(cx, in_local, rhs)
}

Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
// Special-case reborrows to be more like a copy of the reference.
if let &[ref proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
let base_ty = Place::ty_from(place.local, proj_base, *cx.body, cx.tcx).ty;
if let ty::Ref(..) = base_ty.kind {
return in_place::<Q, _>(
cx,
in_local,
PlaceRef { local: place.local, projection: proj_base },
);
}
}

in_place::<Q, _>(cx, in_local, place.as_ref())
}

Rvalue::Aggregate(kind, operands) => {
// Return early if we know that the struct or enum being constructed is always
// qualified.
if let AggregateKind::Adt(def, ..) = **kind {
if def.has_dtor(cx.tcx) {
if Q::in_adt_inherently(cx, def) {
return true;
}
}

// Otherwise, proceed structurally...
operands.iter().any(|o| in_operand::<Q, _>(cx, in_local, o))
}
}
}

Self::in_rvalue_structurally(cx, per_local, rvalue)
/// Returns `true` if this `Place` contains qualif `Q`.
pub fn in_place<Q, F>(cx: &ConstCx<'_, 'tcx>, in_local: &mut F, place: PlaceRef<'tcx>) -> bool
where
Q: Qualif,
F: FnMut(Local) -> bool,
{
let mut projection = place.projection;
while let [ref proj_base @ .., proj_elem] = projection {
match *proj_elem {
ProjectionElem::Index(index) if in_local(index) => return true,

ProjectionElem::Deref
| ProjectionElem::Field(_, _)
| ProjectionElem::ConstantIndex { .. }
| ProjectionElem::Subslice { .. }
| ProjectionElem::Downcast(_, _)
| ProjectionElem::Index(_) => {}
}

let base_ty = Place::ty_from(place.local, proj_base, *cx.body, cx.tcx);
let proj_ty = base_ty.projection_ty(cx.tcx, proj_elem).ty;
if !Q::in_any_value_of_ty(cx, proj_ty) {
return false;
}

projection = proj_base;
}

assert!(projection.is_empty());
in_local(place.local)
}

/// Returns `true` if this `Operand` contains qualif `Q`.
pub fn in_operand<Q, F>(cx: &ConstCx<'_, 'tcx>, in_local: &mut F, operand: &Operand<'tcx>) -> bool
where
Q: Qualif,
F: FnMut(Local) -> bool,
{
let constant = match operand {
Operand::Copy(place) | Operand::Move(place) => {
return in_place::<Q, _>(cx, in_local, place.as_ref());
}

Operand::Constant(c) => c,
};

// Check the qualifs of the value of `const` items.
if let ty::ConstKind::Unevaluated(def_id, _, promoted) = constant.literal.val {
assert!(promoted.is_none());
// Don't peek inside trait associated constants.
if cx.tcx.trait_of_item(def_id).is_none() {
let qualifs = cx.tcx.at(constant.span).mir_const_qualif(def_id);
if !Q::in_qualifs(&qualifs) {
return false;
}

// Just in case the type is more specific than
// the definition, e.g., impl associated const
// with type parameters, take it into account.
}
}
// Otherwise use the qualifs of the type.
Q::in_any_value_of_ty(cx, constant.literal.ty)
}
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