diff options
Diffstat (limited to 'compiler/rustc_const_eval/src/transform/promote_consts.rs')
| -rw-r--r-- | compiler/rustc_const_eval/src/transform/promote_consts.rs | 1066 |
1 files changed, 1066 insertions, 0 deletions
diff --git a/compiler/rustc_const_eval/src/transform/promote_consts.rs b/compiler/rustc_const_eval/src/transform/promote_consts.rs new file mode 100644 index 000000000..ed4d8c95d --- /dev/null +++ b/compiler/rustc_const_eval/src/transform/promote_consts.rs @@ -0,0 +1,1066 @@ +//! A pass that promotes borrows of constant rvalues. +//! +//! The rvalues considered constant are trees of temps, +//! each with exactly one initialization, and holding +//! a constant value with no interior mutability. +//! They are placed into a new MIR constant body in +//! `promoted` and the borrow rvalue is replaced with +//! a `Literal::Promoted` using the index into `promoted` +//! of that constant MIR. +//! +//! This pass assumes that every use is dominated by an +//! initialization and can otherwise silence errors, if +//! move analysis runs after promotion on broken MIR. + +use rustc_hir as hir; +use rustc_middle::mir::traversal::ReversePostorderIter; +use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor}; +use rustc_middle::mir::*; +use rustc_middle::ty::subst::InternalSubsts; +use rustc_middle::ty::{self, List, TyCtxt, TypeVisitable}; +use rustc_span::Span; + +use rustc_index::vec::{Idx, IndexVec}; + +use std::cell::Cell; +use std::{cmp, iter, mem}; + +use crate::transform::check_consts::{qualifs, ConstCx}; + +/// A `MirPass` for promotion. +/// +/// Promotion is the extraction of promotable temps into separate MIR bodies so they can have +/// `'static` lifetime. +/// +/// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each +/// newly created `Constant`. +#[derive(Default)] +pub struct PromoteTemps<'tcx> { + pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>, +} + +impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> { + fn phase_change(&self) -> Option<MirPhase> { + Some(MirPhase::ConstsPromoted) + } + + fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { + // There's not really any point in promoting errorful MIR. + // + // This does not include MIR that failed const-checking, which we still try to promote. + if body.return_ty().references_error() { + tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors"); + return; + } + + if body.source.promoted.is_some() { + return; + } + + let mut rpo = traversal::reverse_postorder(body); + let ccx = ConstCx::new(tcx, body); + let (mut temps, all_candidates) = collect_temps_and_candidates(&ccx, &mut rpo); + + let promotable_candidates = validate_candidates(&ccx, &mut temps, &all_candidates); + + let promoted = promote_candidates(body, tcx, temps, promotable_candidates); + self.promoted_fragments.set(promoted); + } +} + +/// State of a temporary during collection and promotion. +#[derive(Copy, Clone, PartialEq, Eq, Debug)] +pub enum TempState { + /// No references to this temp. + Undefined, + /// One direct assignment and any number of direct uses. + /// A borrow of this temp is promotable if the assigned + /// value is qualified as constant. + Defined { location: Location, uses: usize, valid: Result<(), ()> }, + /// Any other combination of assignments/uses. + Unpromotable, + /// This temp was part of an rvalue which got extracted + /// during promotion and needs cleanup. + PromotedOut, +} + +impl TempState { + pub fn is_promotable(&self) -> bool { + debug!("is_promotable: self={:?}", self); + matches!(self, TempState::Defined { .. }) + } +} + +/// A "root candidate" for promotion, which will become the +/// returned value in a promoted MIR, unless it's a subset +/// of a larger candidate. +#[derive(Copy, Clone, PartialEq, Eq, Debug)] +pub struct Candidate { + location: Location, +} + +struct Collector<'a, 'tcx> { + ccx: &'a ConstCx<'a, 'tcx>, + temps: IndexVec<Local, TempState>, + candidates: Vec<Candidate>, +} + +impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> { + fn visit_local(&mut self, index: Local, context: PlaceContext, location: Location) { + debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location); + // We're only interested in temporaries and the return place + match self.ccx.body.local_kind(index) { + LocalKind::Temp | LocalKind::ReturnPointer => {} + LocalKind::Arg | LocalKind::Var => return, + } + + // Ignore drops, if the temp gets promoted, + // then it's constant and thus drop is noop. + // Non-uses are also irrelevant. + if context.is_drop() || !context.is_use() { + debug!( + "visit_local: context.is_drop={:?} context.is_use={:?}", + context.is_drop(), + context.is_use(), + ); + return; + } + + let temp = &mut self.temps[index]; + debug!("visit_local: temp={:?}", temp); + if *temp == TempState::Undefined { + match context { + PlaceContext::MutatingUse(MutatingUseContext::Store) + | PlaceContext::MutatingUse(MutatingUseContext::Call) => { + *temp = TempState::Defined { location, uses: 0, valid: Err(()) }; + return; + } + _ => { /* mark as unpromotable below */ } + } + } else if let TempState::Defined { ref mut uses, .. } = *temp { + // We always allow borrows, even mutable ones, as we need + // to promote mutable borrows of some ZSTs e.g., `&mut []`. + let allowed_use = match context { + PlaceContext::MutatingUse(MutatingUseContext::Borrow) + | PlaceContext::NonMutatingUse(_) => true, + PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false, + }; + debug!("visit_local: allowed_use={:?}", allowed_use); + if allowed_use { + *uses += 1; + return; + } + /* mark as unpromotable below */ + } + *temp = TempState::Unpromotable; + } + + fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) { + self.super_rvalue(rvalue, location); + + match *rvalue { + Rvalue::Ref(..) => { + self.candidates.push(Candidate { location }); + } + _ => {} + } + } +} + +pub fn collect_temps_and_candidates<'tcx>( + ccx: &ConstCx<'_, 'tcx>, + rpo: &mut ReversePostorderIter<'_, 'tcx>, +) -> (IndexVec<Local, TempState>, Vec<Candidate>) { + let mut collector = Collector { + temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls), + candidates: vec![], + ccx, + }; + for (bb, data) in rpo { + collector.visit_basic_block_data(bb, data); + } + (collector.temps, collector.candidates) +} + +/// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable. +/// +/// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion. +struct Validator<'a, 'tcx> { + ccx: &'a ConstCx<'a, 'tcx>, + temps: &'a mut IndexVec<Local, TempState>, +} + +impl<'a, 'tcx> std::ops::Deref for Validator<'a, 'tcx> { + type Target = ConstCx<'a, 'tcx>; + + fn deref(&self) -> &Self::Target { + &self.ccx + } +} + +struct Unpromotable; + +impl<'tcx> Validator<'_, 'tcx> { + fn validate_candidate(&mut self, candidate: Candidate) -> Result<(), Unpromotable> { + let loc = candidate.location; + let statement = &self.body[loc.block].statements[loc.statement_index]; + match &statement.kind { + StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => { + // We can only promote interior borrows of promotable temps (non-temps + // don't get promoted anyway). + self.validate_local(place.local)?; + + // The reference operation itself must be promotable. + // (Needs to come after `validate_local` to avoid ICEs.) + self.validate_ref(*kind, place)?; + + // We do not check all the projections (they do not get promoted anyway), + // but we do stay away from promoting anything involving a dereference. + if place.projection.contains(&ProjectionElem::Deref) { + return Err(Unpromotable); + } + + // We cannot promote things that need dropping, since the promoted value + // would not get dropped. + if self.qualif_local::<qualifs::NeedsDrop>(place.local) { + return Err(Unpromotable); + } + + Ok(()) + } + _ => bug!(), + } + } + + // FIXME(eddyb) maybe cache this? + fn qualif_local<Q: qualifs::Qualif>(&mut self, local: Local) -> bool { + if let TempState::Defined { location: loc, .. } = self.temps[local] { + let num_stmts = self.body[loc.block].statements.len(); + + if loc.statement_index < num_stmts { + let statement = &self.body[loc.block].statements[loc.statement_index]; + match &statement.kind { + StatementKind::Assign(box (_, rhs)) => qualifs::in_rvalue::<Q, _>( + &self.ccx, + &mut |l| self.qualif_local::<Q>(l), + rhs, + ), + _ => { + span_bug!( + statement.source_info.span, + "{:?} is not an assignment", + statement + ); + } + } + } else { + let terminator = self.body[loc.block].terminator(); + match &terminator.kind { + TerminatorKind::Call { .. } => { + let return_ty = self.body.local_decls[local].ty; + Q::in_any_value_of_ty(&self.ccx, return_ty) + } + kind => { + span_bug!(terminator.source_info.span, "{:?} not promotable", kind); + } + } + } + } else { + let span = self.body.local_decls[local].source_info.span; + span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local); + } + } + + fn validate_local(&mut self, local: Local) -> Result<(), Unpromotable> { + if let TempState::Defined { location: loc, uses, valid } = self.temps[local] { + valid.or_else(|_| { + let ok = { + let block = &self.body[loc.block]; + let num_stmts = block.statements.len(); + + if loc.statement_index < num_stmts { + let statement = &block.statements[loc.statement_index]; + match &statement.kind { + StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs), + _ => { + span_bug!( + statement.source_info.span, + "{:?} is not an assignment", + statement + ); + } + } + } else { + let terminator = block.terminator(); + match &terminator.kind { + TerminatorKind::Call { func, args, .. } => { + self.validate_call(func, args) + } + TerminatorKind::Yield { .. } => Err(Unpromotable), + kind => { + span_bug!(terminator.source_info.span, "{:?} not promotable", kind); + } + } + } + }; + self.temps[local] = match ok { + Ok(()) => TempState::Defined { location: loc, uses, valid: Ok(()) }, + Err(_) => TempState::Unpromotable, + }; + ok + }) + } else { + Err(Unpromotable) + } + } + + fn validate_place(&mut self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> { + match place.last_projection() { + None => self.validate_local(place.local), + Some((place_base, elem)) => { + // Validate topmost projection, then recurse. + match elem { + ProjectionElem::Deref => { + let mut promotable = false; + // We need to make sure this is a `Deref` of a local with no further projections. + // Discussion can be found at + // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247 + if let Some(local) = place_base.as_local() { + // This is a special treatment for cases like *&STATIC where STATIC is a + // global static variable. + // This pattern is generated only when global static variables are directly + // accessed and is qualified for promotion safely. + if let TempState::Defined { location, .. } = self.temps[local] { + let def_stmt = self.body[location.block] + .statements + .get(location.statement_index); + if let Some(Statement { + kind: + StatementKind::Assign(box ( + _, + Rvalue::Use(Operand::Constant(c)), + )), + .. + }) = def_stmt + { + if let Some(did) = c.check_static_ptr(self.tcx) { + // Evaluating a promoted may not read statics except if it got + // promoted from a static (this is a CTFE check). So we + // can only promote static accesses inside statics. + if let Some(hir::ConstContext::Static(..)) = self.const_kind + { + if !self.tcx.is_thread_local_static(did) { + promotable = true; + } + } + } + } + } + } + if !promotable { + return Err(Unpromotable); + } + } + ProjectionElem::Downcast(..) => { + return Err(Unpromotable); + } + + ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {} + + ProjectionElem::Index(local) => { + let mut promotable = false; + // Only accept if we can predict the index and are indexing an array. + let val = + if let TempState::Defined { location: loc, .. } = self.temps[local] { + let block = &self.body[loc.block]; + if loc.statement_index < block.statements.len() { + let statement = &block.statements[loc.statement_index]; + match &statement.kind { + StatementKind::Assign(box ( + _, + Rvalue::Use(Operand::Constant(c)), + )) => c.literal.try_eval_usize(self.tcx, self.param_env), + _ => None, + } + } else { + None + } + } else { + None + }; + if let Some(idx) = val { + // Determine the type of the thing we are indexing. + let ty = place_base.ty(self.body, self.tcx).ty; + match ty.kind() { + ty::Array(_, len) => { + // It's an array; determine its length. + if let Some(len) = len.try_eval_usize(self.tcx, self.param_env) + { + // If the index is in-bounds, go ahead. + if idx < len { + promotable = true; + } + } + } + _ => {} + } + } + if !promotable { + return Err(Unpromotable); + } + + self.validate_local(local)?; + } + + ProjectionElem::Field(..) => { + let base_ty = place_base.ty(self.body, self.tcx).ty; + if base_ty.is_union() { + // No promotion of union field accesses. + return Err(Unpromotable); + } + } + } + + self.validate_place(place_base) + } + } + } + + fn validate_operand(&mut self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> { + match operand { + Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()), + + // The qualifs for a constant (e.g. `HasMutInterior`) are checked in + // `validate_rvalue` upon access. + Operand::Constant(c) => { + if let Some(def_id) = c.check_static_ptr(self.tcx) { + // Only allow statics (not consts) to refer to other statics. + // FIXME(eddyb) does this matter at all for promotion? + // FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`, + // and in `const` this cannot occur anyway. The only concern is that we might + // promote even `let x = &STATIC` which would be useless, but this applies to + // promotion inside statics as well. + let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_))); + if !is_static { + return Err(Unpromotable); + } + + let is_thread_local = self.tcx.is_thread_local_static(def_id); + if is_thread_local { + return Err(Unpromotable); + } + } + + Ok(()) + } + } + } + + fn validate_ref(&mut self, kind: BorrowKind, place: &Place<'tcx>) -> Result<(), Unpromotable> { + match kind { + // Reject these borrow types just to be safe. + // FIXME(RalfJung): could we allow them? Should we? No point in it until we have a usecase. + BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable), + + BorrowKind::Shared => { + let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local); + if has_mut_interior { + return Err(Unpromotable); + } + } + + BorrowKind::Mut { .. } => { + let ty = place.ty(self.body, self.tcx).ty; + + // In theory, any zero-sized value could be borrowed + // mutably without consequences. However, only &mut [] + // is allowed right now. + if let ty::Array(_, len) = ty.kind() { + match len.try_eval_usize(self.tcx, self.param_env) { + Some(0) => {} + _ => return Err(Unpromotable), + } + } else { + return Err(Unpromotable); + } + } + } + + Ok(()) + } + + fn validate_rvalue(&mut self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> { + match rvalue { + Rvalue::Use(operand) | Rvalue::Repeat(operand, _) => { + self.validate_operand(operand)?; + } + Rvalue::CopyForDeref(place) => { + let op = &Operand::Copy(*place); + self.validate_operand(op)? + } + + Rvalue::Discriminant(place) | Rvalue::Len(place) => { + self.validate_place(place.as_ref())? + } + + Rvalue::ThreadLocalRef(_) => return Err(Unpromotable), + + // ptr-to-int casts are not possible in consts and thus not promotable + Rvalue::Cast(CastKind::PointerExposeAddress, _, _) => return Err(Unpromotable), + + // all other casts including int-to-ptr casts are fine, they just use the integer value + // at pointer type. + Rvalue::Cast(_, operand, _) => { + self.validate_operand(operand)?; + } + + Rvalue::NullaryOp(op, _) => match op { + NullOp::SizeOf => {} + NullOp::AlignOf => {} + }, + + Rvalue::ShallowInitBox(_, _) => return Err(Unpromotable), + + Rvalue::UnaryOp(op, operand) => { + match op { + // These operations can never fail. + UnOp::Neg | UnOp::Not => {} + } + + self.validate_operand(operand)?; + } + + Rvalue::BinaryOp(op, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(op, box (lhs, rhs)) => { + let op = *op; + let lhs_ty = lhs.ty(self.body, self.tcx); + + if let ty::RawPtr(_) | ty::FnPtr(..) = lhs_ty.kind() { + // Raw and fn pointer operations are not allowed inside consts and thus not promotable. + assert!(matches!( + op, + BinOp::Eq + | BinOp::Ne + | BinOp::Le + | BinOp::Lt + | BinOp::Ge + | BinOp::Gt + | BinOp::Offset + )); + return Err(Unpromotable); + } + + match op { + BinOp::Div | BinOp::Rem => { + if lhs_ty.is_integral() { + // Integer division: the RHS must be a non-zero const. + let const_val = match rhs { + Operand::Constant(c) => { + c.literal.try_eval_bits(self.tcx, self.param_env, lhs_ty) + } + _ => None, + }; + match const_val { + Some(x) if x != 0 => {} // okay + _ => return Err(Unpromotable), // value not known or 0 -- not okay + } + } + } + // The remaining operations can never fail. + BinOp::Eq + | BinOp::Ne + | BinOp::Le + | BinOp::Lt + | BinOp::Ge + | BinOp::Gt + | BinOp::Offset + | BinOp::Add + | BinOp::Sub + | BinOp::Mul + | BinOp::BitXor + | BinOp::BitAnd + | BinOp::BitOr + | BinOp::Shl + | BinOp::Shr => {} + } + + self.validate_operand(lhs)?; + self.validate_operand(rhs)?; + } + + Rvalue::AddressOf(_, place) => { + // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is + // no problem, only using it is. + if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection() + { + let base_ty = place_base.ty(self.body, self.tcx).ty; + if let ty::Ref(..) = base_ty.kind() { + return self.validate_place(place_base); + } + } + return Err(Unpromotable); + } + + Rvalue::Ref(_, kind, place) => { + // Special-case reborrows to be more like a copy of the reference. + let mut place_simplified = place.as_ref(); + if let Some((place_base, ProjectionElem::Deref)) = + place_simplified.last_projection() + { + let base_ty = place_base.ty(self.body, self.tcx).ty; + if let ty::Ref(..) = base_ty.kind() { + place_simplified = place_base; + } + } + + self.validate_place(place_simplified)?; + + // Check that the reference is fine (using the original place!). + // (Needs to come after `validate_place` to avoid ICEs.) + self.validate_ref(*kind, place)?; + } + + Rvalue::Aggregate(_, operands) => { + for o in operands { + self.validate_operand(o)?; + } + } + } + + Ok(()) + } + + fn validate_call( + &mut self, + callee: &Operand<'tcx>, + args: &[Operand<'tcx>], + ) -> Result<(), Unpromotable> { + let fn_ty = callee.ty(self.body, self.tcx); + + // Inside const/static items, we promote all (eligible) function calls. + // Everywhere else, we require `#[rustc_promotable]` on the callee. + let promote_all_const_fn = matches!( + self.const_kind, + Some(hir::ConstContext::Static(_) | hir::ConstContext::Const) + ); + if !promote_all_const_fn { + if let ty::FnDef(def_id, _) = *fn_ty.kind() { + // Never promote runtime `const fn` calls of + // functions without `#[rustc_promotable]`. + if !self.tcx.is_promotable_const_fn(def_id) { + return Err(Unpromotable); + } + } + } + + let is_const_fn = match *fn_ty.kind() { + ty::FnDef(def_id, _) => self.tcx.is_const_fn_raw(def_id), + _ => false, + }; + if !is_const_fn { + return Err(Unpromotable); + } + + self.validate_operand(callee)?; + for arg in args { + self.validate_operand(arg)?; + } + + Ok(()) + } +} + +// FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`. +pub fn validate_candidates( + ccx: &ConstCx<'_, '_>, + temps: &mut IndexVec<Local, TempState>, + candidates: &[Candidate], +) -> Vec<Candidate> { + let mut validator = Validator { ccx, temps }; + + candidates + .iter() + .copied() + .filter(|&candidate| validator.validate_candidate(candidate).is_ok()) + .collect() +} + +struct Promoter<'a, 'tcx> { + tcx: TyCtxt<'tcx>, + source: &'a mut Body<'tcx>, + promoted: Body<'tcx>, + temps: &'a mut IndexVec<Local, TempState>, + extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>, + + /// If true, all nested temps are also kept in the + /// source MIR, not moved to the promoted MIR. + keep_original: bool, +} + +impl<'a, 'tcx> Promoter<'a, 'tcx> { + fn new_block(&mut self) -> BasicBlock { + let span = self.promoted.span; + self.promoted.basic_blocks_mut().push(BasicBlockData { + statements: vec![], + terminator: Some(Terminator { + source_info: SourceInfo::outermost(span), + kind: TerminatorKind::Return, + }), + is_cleanup: false, + }) + } + + fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) { + let last = self.promoted.basic_blocks().last().unwrap(); + let data = &mut self.promoted[last]; + data.statements.push(Statement { + source_info: SourceInfo::outermost(span), + kind: StatementKind::Assign(Box::new((Place::from(dest), rvalue))), + }); + } + + fn is_temp_kind(&self, local: Local) -> bool { + self.source.local_kind(local) == LocalKind::Temp + } + + /// Copies the initialization of this temp to the + /// promoted MIR, recursing through temps. + fn promote_temp(&mut self, temp: Local) -> Local { + let old_keep_original = self.keep_original; + let loc = match self.temps[temp] { + TempState::Defined { location, uses, .. } if uses > 0 => { + if uses > 1 { + self.keep_original = true; + } + location + } + state => { + span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state); + } + }; + if !self.keep_original { + self.temps[temp] = TempState::PromotedOut; + } + + let num_stmts = self.source[loc.block].statements.len(); + let new_temp = self.promoted.local_decls.push(LocalDecl::new( + self.source.local_decls[temp].ty, + self.source.local_decls[temp].source_info.span, + )); + + debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original); + + // First, take the Rvalue or Call out of the source MIR, + // or duplicate it, depending on keep_original. + if loc.statement_index < num_stmts { + let (mut rvalue, source_info) = { + let statement = &mut self.source[loc.block].statements[loc.statement_index]; + let StatementKind::Assign(box (_, ref mut rhs)) = statement.kind else { + span_bug!( + statement.source_info.span, + "{:?} is not an assignment", + statement + ); + }; + + ( + if self.keep_original { + rhs.clone() + } else { + let unit = Rvalue::Use(Operand::Constant(Box::new(Constant { + span: statement.source_info.span, + user_ty: None, + literal: ConstantKind::zero_sized(self.tcx.types.unit), + }))); + mem::replace(rhs, unit) + }, + statement.source_info, + ) + }; + + self.visit_rvalue(&mut rvalue, loc); + self.assign(new_temp, rvalue, source_info.span); + } else { + let terminator = if self.keep_original { + self.source[loc.block].terminator().clone() + } else { + let terminator = self.source[loc.block].terminator_mut(); + let target = match terminator.kind { + TerminatorKind::Call { target: Some(target), .. } => target, + ref kind => { + span_bug!(terminator.source_info.span, "{:?} not promotable", kind); + } + }; + Terminator { + source_info: terminator.source_info, + kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }), + } + }; + + match terminator.kind { + TerminatorKind::Call { mut func, mut args, from_hir_call, fn_span, .. } => { + self.visit_operand(&mut func, loc); + for arg in &mut args { + self.visit_operand(arg, loc); + } + + let last = self.promoted.basic_blocks().last().unwrap(); + let new_target = self.new_block(); + + *self.promoted[last].terminator_mut() = Terminator { + kind: TerminatorKind::Call { + func, + args, + cleanup: None, + destination: Place::from(new_temp), + target: Some(new_target), + from_hir_call, + fn_span, + }, + source_info: SourceInfo::outermost(terminator.source_info.span), + ..terminator + }; + } + ref kind => { + span_bug!(terminator.source_info.span, "{:?} not promotable", kind); + } + }; + }; + + self.keep_original = old_keep_original; + new_temp + } + + fn promote_candidate(mut self, candidate: Candidate, next_promoted_id: usize) -> Body<'tcx> { + let def = self.source.source.with_opt_param(); + let mut rvalue = { + let promoted = &mut self.promoted; + let promoted_id = Promoted::new(next_promoted_id); + let tcx = self.tcx; + let mut promoted_operand = |ty, span| { + promoted.span = span; + promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span); + let _const = tcx.mk_const(ty::ConstS { + ty, + kind: ty::ConstKind::Unevaluated(ty::Unevaluated { + def, + substs: InternalSubsts::for_item(tcx, def.did, |param, _| { + if let ty::GenericParamDefKind::Lifetime = param.kind { + tcx.lifetimes.re_erased.into() + } else { + tcx.mk_param_from_def(param) + } + }), + promoted: Some(promoted_id), + }), + }); + + Operand::Constant(Box::new(Constant { + span, + user_ty: None, + literal: ConstantKind::from_const(_const, tcx), + })) + }; + let blocks = self.source.basic_blocks.as_mut(); + let local_decls = &mut self.source.local_decls; + let loc = candidate.location; + let statement = &mut blocks[loc.block].statements[loc.statement_index]; + match statement.kind { + StatementKind::Assign(box ( + _, + Rvalue::Ref(ref mut region, borrow_kind, ref mut place), + )) => { + // Use the underlying local for this (necessarily interior) borrow. + let ty = local_decls[place.local].ty; + let span = statement.source_info.span; + + let ref_ty = tcx.mk_ref( + tcx.lifetimes.re_erased, + ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() }, + ); + + *region = tcx.lifetimes.re_erased; + + let mut projection = vec![PlaceElem::Deref]; + projection.extend(place.projection); + place.projection = tcx.intern_place_elems(&projection); + + // Create a temp to hold the promoted reference. + // This is because `*r` requires `r` to be a local, + // otherwise we would use the `promoted` directly. + let mut promoted_ref = LocalDecl::new(ref_ty, span); + promoted_ref.source_info = statement.source_info; + let promoted_ref = local_decls.push(promoted_ref); + assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref); + + let promoted_ref_statement = Statement { + source_info: statement.source_info, + kind: StatementKind::Assign(Box::new(( + Place::from(promoted_ref), + Rvalue::Use(promoted_operand(ref_ty, span)), + ))), + }; + self.extra_statements.push((loc, promoted_ref_statement)); + + Rvalue::Ref( + tcx.lifetimes.re_erased, + borrow_kind, + Place { + local: mem::replace(&mut place.local, promoted_ref), + projection: List::empty(), + }, + ) + } + _ => bug!(), + } + }; + + assert_eq!(self.new_block(), START_BLOCK); + self.visit_rvalue( + &mut rvalue, + Location { block: BasicBlock::new(0), statement_index: usize::MAX }, + ); + + let span = self.promoted.span; + self.assign(RETURN_PLACE, rvalue, span); + self.promoted + } +} + +/// Replaces all temporaries with their promoted counterparts. +impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> { + fn tcx(&self) -> TyCtxt<'tcx> { + self.tcx + } + + fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) { + if self.is_temp_kind(*local) { + *local = self.promote_temp(*local); + } + } +} + +pub fn promote_candidates<'tcx>( + body: &mut Body<'tcx>, + tcx: TyCtxt<'tcx>, + mut temps: IndexVec<Local, TempState>, + candidates: Vec<Candidate>, +) -> IndexVec<Promoted, Body<'tcx>> { + // Visit candidates in reverse, in case they're nested. + debug!("promote_candidates({:?})", candidates); + + let mut promotions = IndexVec::new(); + + let mut extra_statements = vec![]; + for candidate in candidates.into_iter().rev() { + let Location { block, statement_index } = candidate.location; + if let StatementKind::Assign(box (place, _)) = &body[block].statements[statement_index].kind + { + if let Some(local) = place.as_local() { + if temps[local] == TempState::PromotedOut { + // Already promoted. + continue; + } + } + } + + // Declare return place local so that `mir::Body::new` doesn't complain. + let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect(); + + let mut scope = body.source_scopes[body.source_info(candidate.location).scope].clone(); + scope.parent_scope = None; + + let promoted = Body::new( + body.source, // `promoted` gets filled in below + IndexVec::new(), + IndexVec::from_elem_n(scope, 1), + initial_locals, + IndexVec::new(), + 0, + vec![], + body.span, + body.generator_kind(), + body.tainted_by_errors, + ); + + let promoter = Promoter { + promoted, + tcx, + source: body, + temps: &mut temps, + extra_statements: &mut extra_statements, + keep_original: false, + }; + + let mut promoted = promoter.promote_candidate(candidate, promotions.len()); + promoted.source.promoted = Some(promotions.next_index()); + promotions.push(promoted); + } + + // Insert each of `extra_statements` before its indicated location, which + // has to be done in reverse location order, to not invalidate the rest. + extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc)); + for (loc, statement) in extra_statements { + body[loc.block].statements.insert(loc.statement_index, statement); + } + + // Eliminate assignments to, and drops of promoted temps. + let promoted = |index: Local| temps[index] == TempState::PromotedOut; + for block in body.basic_blocks_mut() { + block.statements.retain(|statement| match &statement.kind { + StatementKind::Assign(box (place, _)) => { + if let Some(index) = place.as_local() { + !promoted(index) + } else { + true + } + } + StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => { + !promoted(*index) + } + _ => true, + }); + let terminator = block.terminator_mut(); + if let TerminatorKind::Drop { place, target, .. } = &terminator.kind { + if let Some(index) = place.as_local() { + if promoted(index) { + terminator.kind = TerminatorKind::Goto { target: *target }; + } + } + } + } + + promotions +} + +/// This function returns `true` if the function being called in the array +/// repeat expression is a `const` function. +pub fn is_const_fn_in_array_repeat_expression<'tcx>( + ccx: &ConstCx<'_, 'tcx>, + place: &Place<'tcx>, + body: &Body<'tcx>, +) -> bool { + match place.as_local() { + // rule out cases such as: `let my_var = some_fn(); [my_var; N]` + Some(local) if body.local_decls[local].is_user_variable() => return false, + None => return false, + _ => {} + } + + for block in body.basic_blocks() { + if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) = + &block.terminator + { + if let Operand::Constant(box Constant { literal, .. }) = func { + if let ty::FnDef(def_id, _) = *literal.ty().kind() { + if destination == place { + if ccx.tcx.is_const_fn(def_id) { + return true; + } + } + } + } + } + } + + false +} |