/// 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 {
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")
}
}
}
},
}
+/// 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;
}
}
-// 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 {
_ => 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 {
}
}
-// 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 {
}
}
-// 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 {
ProjectionElem::Index(_) => {}
ProjectionElem::Field(..) => {
- if cx.mode == Mode::Fn {
+ if cx.mode == Mode::NonConstFn {
let base_ty = proj.base.ty(cx.body, cx.tcx).ty;
if let Some(def) = base_ty.ty_adt_def() {
+ // No promotion of union field accesses.
if def.is_union() {
return true;
}
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.body, 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");
}
}
- 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 {
assert!(op == BinOp::Eq || op == BinOp::Ne ||
op == BinOp::Le || op == BinOp::Lt ||
/// 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 {
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.body, cx.tcx).sty {
// Never promote runtime `const fn` calls of
// functions without `#[rustc_promotable]`.
}
}
+/// 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>,
// 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,
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 \
// 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 {
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.body.local_kind(local) == LocalKind::Temp {
debug!("qualify_consts: promotion candidate: local={:?}", local);
// `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);
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;
}
}
-/// 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,
.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");
}
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);
}
let base_ty = proj.base.ty(self.body, 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 {
}
},
- | Mode::Fn
+ | Mode::NonConstFn
| Mode::Static
| Mode::StaticMut
| Mode::Const
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
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(
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));
// 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(
}
_ => {
// 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
}
}
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"));
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.
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 => {
};
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) = {
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