diff --git a/src/librustc_mir/transform/qualify_consts.rs b/src/librustc_mir/transform/qualify_consts.rs index 69cfdbc28eb11..19bd812ec80c0 100644 --- a/src/librustc_mir/transform/qualify_consts.rs +++ b/src/librustc_mir/transform/qualify_consts.rs @@ -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 { @@ -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") } } } @@ -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; @@ -285,7 +305,11 @@ trait Qualif { } } -// Constant containing interior mutability (UnsafeCell). +/// Constant containing interior mutability (`UnsafeCell`). +/// 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 { @@ -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 { @@ -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 { @@ -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 { @@ -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.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; } @@ -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.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"); @@ -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.body, cx.tcx).sty { assert!(op == BinOp::Eq || op == BinOp::Ne || op == BinOp::Le || op == BinOp::Lt || @@ -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 { @@ -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.body, cx.tcx).sty { // Never promote runtime `const fn` calls of // functions without `#[rustc_promotable]`. @@ -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>, @@ -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, @@ -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 \ @@ -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 { @@ -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.body.local_kind(local) == LocalKind::Temp { debug!("qualify_consts: promotion candidate: local={:?}", local); @@ -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); @@ -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; @@ -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, @@ -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"); @@ -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); @@ -1005,7 +1042,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> { } 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 { @@ -1041,7 +1078,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> { } }, - | Mode::Fn + | Mode::NonConstFn | Mode::Static | Mode::StaticMut | Mode::Const @@ -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 @@ -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( @@ -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)); @@ -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( @@ -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 @@ -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")); @@ -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. @@ -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 => { @@ -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) = {