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Rollup merge of rust-lang#61492 - RalfJung:const-qualif-comments, r=e…
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…ddyb

Const qualification comments

I extracted some const-qualif knowledge from @eddyb. This is my attempt to turn that into comments.

Cc @oli-obk 	@eddyb
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@Centril
Centril committed Jun 10, 2019
2 parents cdfbf14 + 38c7f3e commit 02d8876
Showing 1 changed file with 81 additions and 43 deletions.
124 changes: 81 additions & 43 deletions src/librustc_mir/transform/qualify_consts.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -35,11 +35,25 @@ use super::promote_consts::{self, Candidate, TempState};
/// What kind of item we are in.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
enum Mode {
Const,
/// A `static` item.
Static,
/// A `static mut` item.
StaticMut,
/// A `const fn` item.
ConstFn,
Fn
/// A `const` item or an anonymous constant (e.g. in array lengths).
Const,
/// Other type of `fn`.
NonConstFn,
}

impl Mode {
/// Determine whether we have to do full const-checking because syntactically, we
/// are required to be "const".
#[inline]
fn requires_const_checking(self) -> bool {
self != Mode::NonConstFn
}
}

impl fmt::Display for Mode {
Expand All @@ -48,7 +62,7 @@ impl fmt::Display for Mode {
Mode::Const => write!(f, "constant"),
Mode::Static | Mode::StaticMut => write!(f, "static"),
Mode::ConstFn => write!(f, "constant function"),
Mode::Fn => write!(f, "function")
Mode::NonConstFn => write!(f, "function")
}
}
}
Expand Down Expand Up @@ -135,6 +149,12 @@ enum ValueSource<'a, 'tcx> {
},
}

/// 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;

Expand Down Expand Up @@ -285,7 +305,11 @@ trait Qualif {
}
}

// Constant containing interior mutability (UnsafeCell).
/// 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 {
Expand Down Expand Up @@ -314,10 +338,10 @@ impl Qualif for HasMutInterior {
_ => return true,
}
} else if let ty::Array(_, len) = ty.sty {
// FIXME(eddyb) the `cx.mode == Mode::Fn` condition
// FIXME(eddyb) the `cx.mode == Mode::NonConstFn` condition
// seems unnecessary, given that this is merely a ZST.
match len.assert_usize(cx.tcx) {
Some(0) if cx.mode == Mode::Fn => {},
Some(0) if cx.mode == Mode::NonConstFn => {},
_ => return true,
}
} else {
Expand All @@ -343,7 +367,10 @@ impl Qualif for HasMutInterior {
}
}

// Constant containing an ADT that implements Drop.
/// 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 {
Expand All @@ -366,8 +393,12 @@ impl Qualif for NeedsDrop {
}
}

// Not promotable at all - non-`const fn` calls, asm!,
// pointer comparisons, ptr-to-int casts, etc.
/// Not promotable at all - non-`const fn` calls, `asm!`,
/// pointer comparisons, ptr-to-int casts, etc.
/// Inside a const context all constness rules apply, so promotion simply has to follow the regular
/// constant rules (modulo interior mutability or `Drop` rules which are handled `HasMutInterior`
/// and `NeedsDrop` respectively). Basically this duplicates the checks that the const-checking
/// visitor enforces by emitting errors when working in const context.
struct IsNotPromotable;

impl Qualif for IsNotPromotable {
Expand Down Expand Up @@ -398,9 +429,10 @@ impl Qualif for IsNotPromotable {
ProjectionElem::Index(_) => {}

ProjectionElem::Field(..) => {
if cx.mode == Mode::Fn {
if cx.mode == Mode::NonConstFn {
let base_ty = proj.base.ty(cx.mir, cx.tcx).ty;
if let Some(def) = base_ty.ty_adt_def() {
// No promotion of union field accesses.
if def.is_union() {
return true;
}
Expand All @@ -414,7 +446,7 @@ impl Qualif for IsNotPromotable {

fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
match *rvalue {
Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if cx.mode == Mode::Fn => {
Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if cx.mode == Mode::NonConstFn => {
let operand_ty = operand.ty(cx.mir, cx.tcx);
let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
Expand All @@ -428,7 +460,7 @@ impl Qualif for IsNotPromotable {
}
}

Rvalue::BinaryOp(op, ref lhs, _) if cx.mode == Mode::Fn => {
Rvalue::BinaryOp(op, ref lhs, _) if cx.mode == Mode::NonConstFn => {
if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(cx.mir, cx.tcx).sty {
assert!(op == BinOp::Eq || op == BinOp::Ne ||
op == BinOp::Le || op == BinOp::Lt ||
Expand Down Expand Up @@ -511,12 +543,9 @@ impl Qualif for IsNotPromotable {

/// Refers to temporaries which cannot be promoted *implicitly*.
/// Explicit promotion happens e.g. for constant arguments declared via `rustc_args_required_const`.
/// Inside a const context all constness rules
/// apply, so implicit promotion simply has to follow the regular constant rules (modulo interior
/// mutability or `Drop` rules which are handled `HasMutInterior` and `NeedsDrop` respectively).
/// Implicit promotion inside regular functions does not happen if `const fn` calls are involved,
/// as the call may be perfectly alright at runtime, but fail at compile time e.g. due to addresses
/// being compared inside the function.
/// Implicit promotion has almost the same rules, except that disallows `const fn` except for
/// those marked `#[rustc_promotable]`. This is to avoid changing a legitimate run-time operation
/// into a failing compile-time operation e.g. due to addresses being compared inside the function.
struct IsNotImplicitlyPromotable;

impl Qualif for IsNotImplicitlyPromotable {
Expand All @@ -528,7 +557,7 @@ impl Qualif for IsNotImplicitlyPromotable {
args: &[Operand<'tcx>],
_return_ty: Ty<'tcx>,
) -> bool {
if cx.mode == Mode::Fn {
if cx.mode == Mode::NonConstFn {
if let ty::FnDef(def_id, _) = callee.ty(cx.mir, cx.tcx).sty {
// Never promote runtime `const fn` calls of
// functions without `#[rustc_promotable]`.
Expand Down Expand Up @@ -589,6 +618,11 @@ impl ConstCx<'_, 'tcx> {
}
}

/// Checks MIR for being admissible as a compile-time constant, using `ConstCx`
/// for value qualifications, and accumulates writes of
/// rvalue/call results to locals, in `local_qualif`.
/// It also records candidates for promotion in `promotion_candidates`,
/// both in functions and const/static items.
struct Checker<'a, 'tcx> {
cx: ConstCx<'a, 'tcx>,

Expand Down Expand Up @@ -672,7 +706,7 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
// slightly pointless (even with feature-gating).
fn not_const(&mut self) {
unleash_miri!(self);
if self.mode != Mode::Fn {
if self.mode.requires_const_checking() {
let mut err = struct_span_err!(
self.tcx.sess,
self.span,
Expand Down Expand Up @@ -707,7 +741,7 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
qualifs[HasMutInterior] = false;
qualifs[IsNotPromotable] = true;

if self.mode != Mode::Fn {
if self.mode.requires_const_checking() {
if let BorrowKind::Mut { .. } = kind {
let mut err = struct_span_err!(self.tcx.sess, self.span, E0017,
"references in {}s may only refer \
Expand Down Expand Up @@ -737,7 +771,7 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {

// We might have a candidate for promotion.
let candidate = Candidate::Ref(location);
// We can only promote interior borrows of promotable temps.
// Start by traversing to the "base", with non-deref projections removed.
let mut place = place;
while let Place::Projection(ref proj) = *place {
if proj.elem == ProjectionElem::Deref {
Expand All @@ -746,6 +780,10 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
place = &proj.base;
}
debug!("qualify_consts: promotion candidate: place={:?}", place);
// We can only promote interior borrows of promotable temps (non-temps
// don't get promoted anyway).
// (If we bailed out of the loop due to a `Deref` above, we will definitely
// not enter the conditional here.)
if let Place::Base(PlaceBase::Local(local)) = *place {
if self.mir.local_kind(local) == LocalKind::Temp {
debug!("qualify_consts: promotion candidate: local={:?}", local);
Expand All @@ -756,6 +794,10 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
// `HasMutInterior`, from a type that does, e.g.:
// `let _: &'static _ = &(Cell::new(1), 2).1;`
let mut local_qualifs = self.qualifs_in_local(local);
// Any qualifications, except HasMutInterior (see above), disqualify
// from promotion.
// This is, in particular, the "implicit promotion" version of
// the check making sure that we don't run drop glue during const-eval.
local_qualifs[HasMutInterior] = false;
if !local_qualifs.0.iter().any(|&qualif| qualif) {
debug!("qualify_consts: promotion candidate: {:?}", candidate);
Expand Down Expand Up @@ -803,7 +845,7 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
debug!("store to {:?} {:?}", kind, index);

// Only handle promotable temps in non-const functions.
if self.mode == Mode::Fn {
if self.mode == Mode::NonConstFn {
if kind != LocalKind::Temp ||
!self.temp_promotion_state[index].is_promotable() {
return;
Expand Down Expand Up @@ -920,11 +962,6 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
}
}

/// Checks MIR for const-correctness, using `ConstCx`
/// for value qualifications, and accumulates writes of
/// rvalue/call results to locals, in `local_qualif`.
/// For functions (constant or not), it also records
/// candidates for promotion in `promotion_candidates`.
impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
fn visit_place_base(
&mut self,
Expand All @@ -943,7 +980,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
.get_attrs(*def_id)
.iter()
.any(|attr| attr.check_name(sym::thread_local)) {
if self.mode != Mode::Fn {
if self.mode.requires_const_checking() {
span_err!(self.tcx.sess, self.span, E0625,
"thread-local statics cannot be \
accessed at compile-time");
Expand All @@ -967,7 +1004,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
}
unleash_miri!(self);

if self.mode != Mode::Fn {
if self.mode.requires_const_checking() {
let mut err = struct_span_err!(self.tcx.sess, self.span, E0013,
"{}s cannot refer to statics, use \
a constant instead", self.mode);
Expand Down Expand Up @@ -1005,7 +1042,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
}
let base_ty = proj.base.ty(self.mir, self.tcx).ty;
match self.mode {
Mode::Fn => {},
Mode::NonConstFn => {},
_ => {
if let ty::RawPtr(_) = base_ty.sty {
if !self.tcx.features().const_raw_ptr_deref {
Expand Down Expand Up @@ -1041,7 +1078,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
}
},

| Mode::Fn
| Mode::NonConstFn
| Mode::Static
| Mode::StaticMut
| Mode::Const
Expand Down Expand Up @@ -1131,7 +1168,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
match (cast_in, cast_out) {
(CastTy::Ptr(_), CastTy::Int(_)) |
(CastTy::FnPtr, CastTy::Int(_)) if self.mode != Mode::Fn => {
(CastTy::FnPtr, CastTy::Int(_)) if self.mode != Mode::NonConstFn => {
unleash_miri!(self);
if !self.tcx.features().const_raw_ptr_to_usize_cast {
// in const fn and constants require the feature gate
Expand All @@ -1158,7 +1195,9 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
op == BinOp::Offset);

unleash_miri!(self);
if self.mode != Mode::Fn && !self.tcx.features().const_compare_raw_pointers {
if self.mode.requires_const_checking() &&
!self.tcx.features().const_compare_raw_pointers
{
// require the feature gate inside constants and const fn
// FIXME: make it unsafe to use these operations
emit_feature_err(
Expand All @@ -1174,7 +1213,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {

Rvalue::NullaryOp(NullOp::Box, _) => {
unleash_miri!(self);
if self.mode != Mode::Fn {
if self.mode.requires_const_checking() {
let mut err = struct_span_err!(self.tcx.sess, self.span, E0010,
"allocations are not allowed in {}s", self.mode);
err.span_label(self.span, format!("allocation not allowed in {}s", self.mode));
Expand Down Expand Up @@ -1219,8 +1258,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
// special intrinsic that can be called diretly without an intrinsic
// feature gate needs a language feature gate
"transmute" => {
// never promote transmute calls
if self.mode != Mode::Fn {
if self.mode.requires_const_checking() {
// const eval transmute calls only with the feature gate
if !self.tcx.features().const_transmute {
emit_feature_err(
Expand All @@ -1243,7 +1281,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
}
_ => {
// In normal functions no calls are feature-gated.
if self.mode != Mode::Fn {
if self.mode.requires_const_checking() {
let unleash_miri = self
.tcx
.sess
Expand Down Expand Up @@ -1302,7 +1340,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
}
}
ty::FnPtr(_) => {
if self.mode != Mode::Fn {
if self.mode.requires_const_checking() {
let mut err = self.tcx.sess.struct_span_err(
self.span,
&format!("function pointers are not allowed in const fn"));
Expand Down Expand Up @@ -1361,7 +1399,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
self.super_terminator_kind(kind, location);

// Deny *any* live drops anywhere other than functions.
if self.mode != Mode::Fn {
if self.mode.requires_const_checking() {
unleash_miri!(self);
// HACK(eddyb): emulate a bit of dataflow analysis,
// conservatively, that drop elaboration will do.
Expand Down Expand Up @@ -1472,12 +1510,12 @@ impl MirPass for QualifyAndPromoteConstants {
let id = tcx.hir().as_local_hir_id(def_id).unwrap();
let mut const_promoted_temps = None;
let mode = match tcx.hir().body_owner_kind_by_hir_id(id) {
hir::BodyOwnerKind::Closure => Mode::Fn,
hir::BodyOwnerKind::Closure => Mode::NonConstFn,
hir::BodyOwnerKind::Fn => {
if tcx.is_const_fn(def_id) {
Mode::ConstFn
} else {
Mode::Fn
Mode::NonConstFn
}
}
hir::BodyOwnerKind::Const => {
Expand All @@ -1489,7 +1527,7 @@ impl MirPass for QualifyAndPromoteConstants {
};

debug!("run_pass: mode={:?}", mode);
if mode == Mode::Fn || mode == Mode::ConstFn {
if mode == Mode::NonConstFn || mode == Mode::ConstFn {
// This is ugly because Checker holds onto mir,
// which can't be mutated until its scope ends.
let (temps, candidates) = {
Expand Down

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