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Auto merge of #61492 - RalfJung:const-qualif-comments, r=eddyb

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Const qualification comments

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

Cc @oli-obk 	@eddyb
This commit is contained in:
bors 2019-06-11 05:24:41 +00:00
commit 9d9c7ad323
1 changed files with 81 additions and 43 deletions
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124
src/librustc_mir/transform/qualify_consts.rs
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@ -35,11 +35,25 @@
/// What kind of item we are in. /// What kind of item we are in.
#[derive(Copy, Clone, Debug, PartialEq, Eq)] #[derive(Copy, Clone, Debug, PartialEq, Eq)]
enum Mode { enum Mode {
Const, /// A `static` item.
Static, Static,
/// A `static mut` item.
StaticMut, StaticMut,
/// A `const fn` item.
ConstFn, 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 { impl fmt::Display for Mode {
@ -48,7 +62,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
Mode::Const => write!(f, "constant"), Mode::Const => write!(f, "constant"),
Mode::Static | Mode::StaticMut => write!(f, "static"), Mode::Static | Mode::StaticMut => write!(f, "static"),
Mode::ConstFn => write!(f, "constant function"), Mode::ConstFn => write!(f, "constant function"),
Mode::Fn => write!(f, "function") Mode::NonConstFn => write!(f, "function")
} }
} }
} }
@ -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 { trait Qualif {
const IDX: usize; const IDX: usize;
@ -285,7 +305,11 @@ fn in_value(cx: &ConstCx<'_, 'tcx>, source: ValueSource<'_, 'tcx>) -> bool {
} }
} }
// 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; struct HasMutInterior;
impl Qualif for HasMutInterior { impl Qualif for HasMutInterior {
@ -314,10 +338,10 @@ fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
_ => return true, _ => return true,
} }
} else if let ty::Array(_, len) = ty.sty { } 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. // seems unnecessary, given that this is merely a ZST.
match len.assert_usize(cx.tcx) { match len.assert_usize(cx.tcx) {
Some(0) if cx.mode == Mode::Fn => {}, Some(0) if cx.mode == Mode::NonConstFn => {},
_ => return true, _ => return true,
} }
} else { } else {
@ -343,7 +367,10 @@ fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
} }
} }
// 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; struct NeedsDrop;
impl Qualif for NeedsDrop { impl Qualif for NeedsDrop {
@ -366,8 +393,12 @@ fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
} }
} }
// Not promotable at all - non-`const fn` calls, asm!, /// Not promotable at all - non-`const fn` calls, `asm!`,
// pointer comparisons, ptr-to-int casts, etc. /// 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; struct IsNotPromotable;
impl Qualif for IsNotPromotable { impl Qualif for IsNotPromotable {
@ -398,9 +429,10 @@ fn in_projection(cx: &ConstCx<'_, 'tcx>, proj: &Projection<'tcx>) -> bool {
ProjectionElem::Index(_) => {} ProjectionElem::Index(_) => {}
ProjectionElem::Field(..) => { ProjectionElem::Field(..) => {
if cx.mode == Mode::Fn { if cx.mode == Mode::NonConstFn {
let base_ty = proj.base.ty(cx.body, cx.tcx).ty; let base_ty = proj.base.ty(cx.body, cx.tcx).ty;
if let Some(def) = base_ty.ty_adt_def() { if let Some(def) = base_ty.ty_adt_def() {
// No promotion of union field accesses.
if def.is_union() { if def.is_union() {
return true; return true;
} }
@ -414,7 +446,7 @@ fn in_projection(cx: &ConstCx<'_, 'tcx>, proj: &Projection<'tcx>) -> bool {
fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool { fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
match *rvalue { 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.body, cx.tcx); let operand_ty = operand.ty(cx.body, cx.tcx);
let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast"); 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"); let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
@ -428,7 +460,7 @@ fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
} }
} }
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.body, cx.tcx).sty { if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(cx.body, cx.tcx).sty {
assert!(op == BinOp::Eq || op == BinOp::Ne || assert!(op == BinOp::Eq || op == BinOp::Ne ||
op == BinOp::Le || op == BinOp::Lt || op == BinOp::Le || op == BinOp::Lt ||
@ -511,12 +543,9 @@ fn in_call(
/// Refers to temporaries which cannot be promoted *implicitly*. /// Refers to temporaries which cannot be promoted *implicitly*.
/// Explicit promotion happens e.g. for constant arguments declared via `rustc_args_required_const`. /// Explicit promotion happens e.g. for constant arguments declared via `rustc_args_required_const`.
/// Inside a const context all constness rules /// Implicit promotion has almost the same rules, except that disallows `const fn` except for
/// apply, so implicit promotion simply has to follow the regular constant rules (modulo interior /// those marked `#[rustc_promotable]`. This is to avoid changing a legitimate run-time operation
/// mutability or `Drop` rules which are handled `HasMutInterior` and `NeedsDrop` respectively). /// into a failing compile-time operation e.g. due to addresses being compared inside the function.
/// 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.
struct IsNotImplicitlyPromotable; struct IsNotImplicitlyPromotable;
impl Qualif for IsNotImplicitlyPromotable { impl Qualif for IsNotImplicitlyPromotable {
@ -528,7 +557,7 @@ fn in_call(
args: &[Operand<'tcx>], args: &[Operand<'tcx>],
_return_ty: Ty<'tcx>, _return_ty: Ty<'tcx>,
) -> bool { ) -> bool {
if cx.mode == Mode::Fn { if cx.mode == Mode::NonConstFn {
if let ty::FnDef(def_id, _) = callee.ty(cx.body, cx.tcx).sty { if let ty::FnDef(def_id, _) = callee.ty(cx.body, cx.tcx).sty {
// Never promote runtime `const fn` calls of // Never promote runtime `const fn` calls of
// functions without `#[rustc_promotable]`. // functions without `#[rustc_promotable]`.
@ -589,6 +618,11 @@ fn qualifs_in_value(&self, source: ValueSource<'_, 'tcx>) -> PerQualif<bool> {
} }
} }
/// 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> { struct Checker<'a, 'tcx> {
cx: ConstCx<'a, 'tcx>, cx: ConstCx<'a, 'tcx>,
@ -672,7 +706,7 @@ fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>,
// slightly pointless (even with feature-gating). // slightly pointless (even with feature-gating).
fn not_const(&mut self) { fn not_const(&mut self) {
unleash_miri!(self); unleash_miri!(self);
if self.mode != Mode::Fn { if self.mode.requires_const_checking() {
let mut err = struct_span_err!( let mut err = struct_span_err!(
self.tcx.sess, self.tcx.sess,
self.span, self.span,
@ -707,7 +741,7 @@ fn assign(&mut self, dest: &Place<'tcx>, source: ValueSource<'_, 'tcx>, location
qualifs[HasMutInterior] = false; qualifs[HasMutInterior] = false;
qualifs[IsNotPromotable] = true; qualifs[IsNotPromotable] = true;
if self.mode != Mode::Fn { if self.mode.requires_const_checking() {
if let BorrowKind::Mut { .. } = kind { if let BorrowKind::Mut { .. } = kind {
let mut err = struct_span_err!(self.tcx.sess, self.span, E0017, let mut err = struct_span_err!(self.tcx.sess, self.span, E0017,
"references in {}s may only refer \ "references in {}s may only refer \
@ -737,7 +771,7 @@ fn assign(&mut self, dest: &Place<'tcx>, source: ValueSource<'_, 'tcx>, location
// We might have a candidate for promotion. // We might have a candidate for promotion.
let candidate = Candidate::Ref(location); 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; let mut place = place;
while let Place::Projection(ref proj) = *place { while let Place::Projection(ref proj) = *place {
if proj.elem == ProjectionElem::Deref { if proj.elem == ProjectionElem::Deref {
@ -746,6 +780,10 @@ fn assign(&mut self, dest: &Place<'tcx>, source: ValueSource<'_, 'tcx>, location
place = &proj.base; place = &proj.base;
} }
debug!("qualify_consts: promotion candidate: place={:?}", place); 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 let Place::Base(PlaceBase::Local(local)) = *place {
if self.body.local_kind(local) == LocalKind::Temp { if self.body.local_kind(local) == LocalKind::Temp {
debug!("qualify_consts: promotion candidate: local={:?}", local); debug!("qualify_consts: promotion candidate: local={:?}", local);
@ -756,6 +794,10 @@ fn assign(&mut self, dest: &Place<'tcx>, source: ValueSource<'_, 'tcx>, location
// `HasMutInterior`, from a type that does, e.g.: // `HasMutInterior`, from a type that does, e.g.:
// `let _: &'static _ = &(Cell::new(1), 2).1;` // `let _: &'static _ = &(Cell::new(1), 2).1;`
let mut local_qualifs = self.qualifs_in_local(local); 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; local_qualifs[HasMutInterior] = false;
if !local_qualifs.0.iter().any(|&qualif| qualif) { if !local_qualifs.0.iter().any(|&qualif| qualif) {
debug!("qualify_consts: promotion candidate: {:?}", candidate); debug!("qualify_consts: promotion candidate: {:?}", candidate);
@ -803,7 +845,7 @@ fn assign(&mut self, dest: &Place<'tcx>, source: ValueSource<'_, 'tcx>, location
debug!("store to {:?} {:?}", kind, index); debug!("store to {:?} {:?}", kind, index);
// Only handle promotable temps in non-const functions. // Only handle promotable temps in non-const functions.
if self.mode == Mode::Fn { if self.mode == Mode::NonConstFn {
if kind != LocalKind::Temp || if kind != LocalKind::Temp ||
!self.temp_promotion_state[index].is_promotable() { !self.temp_promotion_state[index].is_promotable() {
return; return;
@ -920,11 +962,6 @@ fn check_const(&mut self) -> (u8, &'tcx BitSet<Local>) {
} }
} }
/// 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> { impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
fn visit_place_base( fn visit_place_base(
&mut self, &mut self,
@ -943,7 +980,7 @@ fn visit_place_base(
.get_attrs(*def_id) .get_attrs(*def_id)
.iter() .iter()
.any(|attr| attr.check_name(sym::thread_local)) { .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, span_err!(self.tcx.sess, self.span, E0625,
"thread-local statics cannot be \ "thread-local statics cannot be \
accessed at compile-time"); accessed at compile-time");
@ -967,7 +1004,7 @@ fn visit_place_base(
} }
unleash_miri!(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, E0013, let mut err = struct_span_err!(self.tcx.sess, self.span, E0013,
"{}s cannot refer to statics, use \ "{}s cannot refer to statics, use \
a constant instead", self.mode); a constant instead", self.mode);
@ -1005,7 +1042,7 @@ fn visit_projection(
} }
let base_ty = proj.base.ty(self.body, self.tcx).ty; let base_ty = proj.base.ty(self.body, self.tcx).ty;
match self.mode { match self.mode {
Mode::Fn => {}, Mode::NonConstFn => {},
_ => { _ => {
if let ty::RawPtr(_) = base_ty.sty { if let ty::RawPtr(_) = base_ty.sty {
if !self.tcx.features().const_raw_ptr_deref { if !self.tcx.features().const_raw_ptr_deref {
@ -1041,7 +1078,7 @@ fn visit_projection(
} }
}, },
| Mode::Fn | Mode::NonConstFn
| Mode::Static | Mode::Static
| Mode::StaticMut | Mode::StaticMut
| Mode::Const | Mode::Const
@ -1131,7 +1168,7 @@ fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast"); let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
match (cast_in, cast_out) { match (cast_in, cast_out) {
(CastTy::Ptr(_), CastTy::Int(_)) | (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); unleash_miri!(self);
if !self.tcx.features().const_raw_ptr_to_usize_cast { if !self.tcx.features().const_raw_ptr_to_usize_cast {
// in const fn and constants require the feature gate // in const fn and constants require the feature gate
@ -1158,7 +1195,9 @@ fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
op == BinOp::Offset); op == BinOp::Offset);
unleash_miri!(self); 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 // require the feature gate inside constants and const fn
// FIXME: make it unsafe to use these operations // FIXME: make it unsafe to use these operations
emit_feature_err( emit_feature_err(
@ -1174,7 +1213,7 @@ fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
Rvalue::NullaryOp(NullOp::Box, _) => { Rvalue::NullaryOp(NullOp::Box, _) => {
unleash_miri!(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, E0010, let mut err = struct_span_err!(self.tcx.sess, self.span, E0010,
"allocations are not allowed in {}s", self.mode); "allocations are not allowed in {}s", self.mode);
err.span_label(self.span, format!("allocation not allowed in {}s", self.mode)); err.span_label(self.span, format!("allocation not allowed in {}s", self.mode));
@ -1219,8 +1258,7 @@ fn visit_terminator_kind(&mut self,
// special intrinsic that can be called diretly without an intrinsic // special intrinsic that can be called diretly without an intrinsic
// feature gate needs a language feature gate // feature gate needs a language feature gate
"transmute" => { "transmute" => {
// never promote transmute calls if self.mode.requires_const_checking() {
if self.mode != Mode::Fn {
// const eval transmute calls only with the feature gate // const eval transmute calls only with the feature gate
if !self.tcx.features().const_transmute { if !self.tcx.features().const_transmute {
emit_feature_err( emit_feature_err(
@ -1243,7 +1281,7 @@ fn visit_terminator_kind(&mut self,
} }
_ => { _ => {
// In normal functions no calls are feature-gated. // In normal functions no calls are feature-gated.
if self.mode != Mode::Fn { if self.mode.requires_const_checking() {
let unleash_miri = self let unleash_miri = self
.tcx .tcx
.sess .sess
@ -1302,7 +1340,7 @@ fn visit_terminator_kind(&mut self,
} }
} }
ty::FnPtr(_) => { ty::FnPtr(_) => {
if self.mode != Mode::Fn { if self.mode.requires_const_checking() {
let mut err = self.tcx.sess.struct_span_err( let mut err = self.tcx.sess.struct_span_err(
self.span, self.span,
&format!("function pointers are not allowed in const fn")); &format!("function pointers are not allowed in const fn"));
@ -1361,7 +1399,7 @@ fn visit_terminator_kind(&mut self,
self.super_terminator_kind(kind, location); self.super_terminator_kind(kind, location);
// Deny *any* live drops anywhere other than functions. // Deny *any* live drops anywhere other than functions.
if self.mode != Mode::Fn { if self.mode.requires_const_checking() {
unleash_miri!(self); unleash_miri!(self);
// HACK(eddyb): emulate a bit of dataflow analysis, // HACK(eddyb): emulate a bit of dataflow analysis,
// conservatively, that drop elaboration will do. // conservatively, that drop elaboration will do.
@ -1472,12 +1510,12 @@ fn run_pass<'a, 'tcx>(&self,
let id = tcx.hir().as_local_hir_id(def_id).unwrap(); let id = tcx.hir().as_local_hir_id(def_id).unwrap();
let mut const_promoted_temps = None; let mut const_promoted_temps = None;
let mode = match tcx.hir().body_owner_kind_by_hir_id(id) { 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 => { hir::BodyOwnerKind::Fn => {
if tcx.is_const_fn(def_id) { if tcx.is_const_fn(def_id) {
Mode::ConstFn Mode::ConstFn
} else { } else {
Mode::Fn Mode::NonConstFn
} }
} }
hir::BodyOwnerKind::Const => { hir::BodyOwnerKind::Const => {
@ -1489,7 +1527,7 @@ fn run_pass<'a, 'tcx>(&self,
}; };
debug!("run_pass: mode={:?}", mode); 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, // This is ugly because Checker holds onto mir,
// which can't be mutated until its scope ends. // which can't be mutated until its scope ends.
let (temps, candidates) = { let (temps, candidates) = {