355 lines
14 KiB
Rust
355 lines
14 KiB
Rust
use rustc_hir as hir;
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use rustc_hir::def_id::DefId;
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use rustc_middle::mir::{
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Body, CastKind, NullOp, Operand, Place, ProjectionElem, Rvalue, Statement, StatementKind, Terminator,
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TerminatorKind,
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};
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use rustc_middle::ty::subst::GenericArgKind;
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use rustc_middle::ty::{self, adjustment::PointerCast, Ty, TyCtxt};
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use rustc_span::symbol::sym;
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use rustc_span::Span;
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use rustc_target::spec::abi::Abi::RustIntrinsic;
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use std::borrow::Cow;
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type McfResult = Result<(), (Span, Cow<'static, str>)>;
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pub fn is_min_const_fn(tcx: TyCtxt<'tcx>, body: &'a Body<'tcx>) -> McfResult {
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let def_id = body.source.def_id();
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let mut current = def_id;
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loop {
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let predicates = tcx.predicates_of(current);
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for (predicate, _) in predicates.predicates {
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match predicate.kind().skip_binder() {
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ty::PredicateKind::RegionOutlives(_)
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| ty::PredicateKind::TypeOutlives(_)
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| ty::PredicateKind::WellFormed(_)
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| ty::PredicateKind::Projection(_)
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| ty::PredicateKind::ConstEvaluatable(..)
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| ty::PredicateKind::ConstEquate(..)
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| ty::PredicateKind::TypeWellFormedFromEnv(..) => continue,
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ty::PredicateKind::ObjectSafe(_) => panic!("object safe predicate on function: {:#?}", predicate),
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ty::PredicateKind::ClosureKind(..) => panic!("closure kind predicate on function: {:#?}", predicate),
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ty::PredicateKind::Subtype(_) => panic!("subtype predicate on function: {:#?}", predicate),
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ty::PredicateKind::Trait(pred, _) => {
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if Some(pred.def_id()) == tcx.lang_items().sized_trait() {
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continue;
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}
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match pred.self_ty().kind() {
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ty::Param(ref p) => {
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let generics = tcx.generics_of(current);
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let def = generics.type_param(p, tcx);
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let span = tcx.def_span(def.def_id);
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return Err((
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span,
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"trait bounds other than `Sized` \
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on const fn parameters are unstable"
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.into(),
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));
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},
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// other kinds of bounds are either tautologies
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// or cause errors in other passes
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_ => continue,
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}
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},
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}
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}
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match predicates.parent {
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Some(parent) => current = parent,
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None => break,
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}
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}
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for local in &body.local_decls {
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check_ty(tcx, local.ty, local.source_info.span)?;
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}
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// impl trait is gone in MIR, so check the return type manually
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check_ty(
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tcx,
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tcx.fn_sig(def_id).output().skip_binder(),
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body.local_decls.iter().next().unwrap().source_info.span,
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)?;
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for bb in body.basic_blocks() {
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check_terminator(tcx, body, bb.terminator())?;
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for stmt in &bb.statements {
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check_statement(tcx, body, def_id, stmt)?;
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}
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}
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Ok(())
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}
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fn check_ty(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span) -> McfResult {
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for arg in ty.walk() {
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let ty = match arg.unpack() {
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GenericArgKind::Type(ty) => ty,
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// No constraints on lifetimes or constants, except potentially
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// constants' types, but `walk` will get to them as well.
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GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => continue,
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};
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match ty.kind() {
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ty::Ref(_, _, hir::Mutability::Mut) => {
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return Err((span, "mutable references in const fn are unstable".into()));
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},
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ty::Opaque(..) => return Err((span, "`impl Trait` in const fn is unstable".into())),
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ty::FnPtr(..) => {
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return Err((span, "function pointers in const fn are unstable".into()));
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},
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ty::Dynamic(preds, _) => {
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for pred in preds.iter() {
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match pred.skip_binder() {
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ty::ExistentialPredicate::AutoTrait(_) | ty::ExistentialPredicate::Projection(_) => {
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return Err((
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span,
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"trait bounds other than `Sized` \
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on const fn parameters are unstable"
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.into(),
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));
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},
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ty::ExistentialPredicate::Trait(trait_ref) => {
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if Some(trait_ref.def_id) != tcx.lang_items().sized_trait() {
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return Err((
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span,
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"trait bounds other than `Sized` \
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on const fn parameters are unstable"
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.into(),
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));
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}
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},
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}
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}
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},
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_ => {},
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}
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}
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Ok(())
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}
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fn check_rvalue(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, def_id: DefId, rvalue: &Rvalue<'tcx>, span: Span) -> McfResult {
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match rvalue {
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Rvalue::ThreadLocalRef(_) => Err((span, "cannot access thread local storage in const fn".into())),
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Rvalue::Repeat(operand, _) | Rvalue::Use(operand) => check_operand(tcx, operand, span, body),
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Rvalue::Len(place) | Rvalue::Discriminant(place) | Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
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check_place(tcx, *place, span, body)
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},
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Rvalue::Cast(CastKind::Misc, operand, cast_ty) => {
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use rustc_middle::ty::cast::CastTy;
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let cast_in = CastTy::from_ty(operand.ty(body, tcx)).expect("bad input type for cast");
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let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
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match (cast_in, cast_out) {
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(CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) => {
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Err((span, "casting pointers to ints is unstable in const fn".into()))
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},
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_ => check_operand(tcx, operand, span, body),
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}
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},
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Rvalue::Cast(CastKind::Pointer(PointerCast::MutToConstPointer | PointerCast::ArrayToPointer), operand, _) => {
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check_operand(tcx, operand, span, body)
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},
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Rvalue::Cast(
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CastKind::Pointer(
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PointerCast::UnsafeFnPointer | PointerCast::ClosureFnPointer(_) | PointerCast::ReifyFnPointer,
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),
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_,
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_,
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) => Err((span, "function pointer casts are not allowed in const fn".into())),
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Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), op, cast_ty) => {
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let pointee_ty = if let Some(deref_ty) = cast_ty.builtin_deref(true) {
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deref_ty.ty
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} else {
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// We cannot allow this for now.
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return Err((span, "unsizing casts are only allowed for references right now".into()));
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};
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let unsized_ty = tcx.struct_tail_erasing_lifetimes(pointee_ty, tcx.param_env(def_id));
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if let ty::Slice(_) | ty::Str = unsized_ty.kind() {
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check_operand(tcx, op, span, body)?;
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// Casting/coercing things to slices is fine.
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Ok(())
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} else {
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// We just can't allow trait objects until we have figured out trait method calls.
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Err((span, "unsizing casts are not allowed in const fn".into()))
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}
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},
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// binops are fine on integers
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Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => {
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check_operand(tcx, lhs, span, body)?;
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check_operand(tcx, rhs, span, body)?;
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let ty = lhs.ty(body, tcx);
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if ty.is_integral() || ty.is_bool() || ty.is_char() {
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Ok(())
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} else {
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Err((
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span,
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"only int, `bool` and `char` operations are stable in const fn".into(),
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))
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}
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},
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Rvalue::NullaryOp(NullOp::SizeOf, _) => Ok(()),
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Rvalue::NullaryOp(NullOp::Box, _) => Err((span, "heap allocations are not allowed in const fn".into())),
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Rvalue::UnaryOp(_, operand) => {
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let ty = operand.ty(body, tcx);
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if ty.is_integral() || ty.is_bool() {
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check_operand(tcx, operand, span, body)
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} else {
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Err((span, "only int and `bool` operations are stable in const fn".into()))
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}
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},
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Rvalue::Aggregate(_, operands) => {
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for operand in operands {
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check_operand(tcx, operand, span, body)?;
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}
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Ok(())
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},
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}
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}
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fn check_statement(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, def_id: DefId, statement: &Statement<'tcx>) -> McfResult {
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let span = statement.source_info.span;
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match &statement.kind {
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StatementKind::Assign(box (place, rval)) => {
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check_place(tcx, *place, span, body)?;
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check_rvalue(tcx, body, def_id, rval, span)
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}
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StatementKind::FakeRead(_, place) |
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// just an assignment
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StatementKind::SetDiscriminant { place, .. } => check_place(tcx, **place, span, body),
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StatementKind::LlvmInlineAsm { .. } => Err((span, "cannot use inline assembly in const fn".into())),
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StatementKind::CopyNonOverlapping(box rustc_middle::mir::CopyNonOverlapping{
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dst, src, count,
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}) => {
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check_operand(tcx, dst, span, body)?;
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check_operand(tcx, src, span, body)?;
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check_operand(tcx, count, span, body)
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},
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// These are all NOPs
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StatementKind::StorageLive(_)
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| StatementKind::StorageDead(_)
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| StatementKind::Retag { .. }
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| StatementKind::AscribeUserType(..)
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| StatementKind::Coverage(..)
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| StatementKind::Nop => Ok(()),
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}
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}
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fn check_operand(tcx: TyCtxt<'tcx>, operand: &Operand<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
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match operand {
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Operand::Move(place) | Operand::Copy(place) => check_place(tcx, *place, span, body),
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Operand::Constant(c) => match c.check_static_ptr(tcx) {
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Some(_) => Err((span, "cannot access `static` items in const fn".into())),
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None => Ok(()),
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},
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}
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}
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fn check_place(tcx: TyCtxt<'tcx>, place: Place<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
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let mut cursor = place.projection.as_ref();
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while let [ref proj_base @ .., elem] = *cursor {
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cursor = proj_base;
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match elem {
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ProjectionElem::Field(..) => {
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let base_ty = Place::ty_from(place.local, &proj_base, body, tcx).ty;
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if let Some(def) = base_ty.ty_adt_def() {
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// No union field accesses in `const fn`
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if def.is_union() {
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return Err((span, "accessing union fields is unstable".into()));
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}
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}
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},
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ProjectionElem::ConstantIndex { .. }
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| ProjectionElem::Downcast(..)
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| ProjectionElem::Subslice { .. }
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| ProjectionElem::Deref
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| ProjectionElem::Index(_) => {},
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}
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}
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Ok(())
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}
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fn check_terminator(tcx: TyCtxt<'tcx>, body: &'a Body<'tcx>, terminator: &Terminator<'tcx>) -> McfResult {
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let span = terminator.source_info.span;
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match &terminator.kind {
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TerminatorKind::FalseEdge { .. }
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| TerminatorKind::FalseUnwind { .. }
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| TerminatorKind::Goto { .. }
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| TerminatorKind::Return
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| TerminatorKind::Resume
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| TerminatorKind::Unreachable => Ok(()),
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TerminatorKind::Drop { place, .. } => check_place(tcx, *place, span, body),
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TerminatorKind::DropAndReplace { place, value, .. } => {
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check_place(tcx, *place, span, body)?;
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check_operand(tcx, value, span, body)
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},
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TerminatorKind::SwitchInt {
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discr,
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switch_ty: _,
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targets: _,
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} => check_operand(tcx, discr, span, body),
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TerminatorKind::Abort => Err((span, "abort is not stable in const fn".into())),
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TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
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Err((span, "const fn generators are unstable".into()))
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},
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TerminatorKind::Call {
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func,
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args,
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from_hir_call: _,
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destination: _,
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cleanup: _,
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fn_span: _,
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} => {
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let fn_ty = func.ty(body, tcx);
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if let ty::FnDef(fn_def_id, _) = *fn_ty.kind() {
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if !rustc_mir::const_eval::is_min_const_fn(tcx, fn_def_id) {
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return Err((
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span,
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format!(
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"can only call other `const fn` within a `const fn`, \
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but `{:?}` is not stable as `const fn`",
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func,
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)
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.into(),
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));
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}
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// HACK: This is to "unstabilize" the `transmute` intrinsic
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// within const fns. `transmute` is allowed in all other const contexts.
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// This won't really scale to more intrinsics or functions. Let's allow const
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// transmutes in const fn before we add more hacks to this.
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if tcx.fn_sig(fn_def_id).abi() == RustIntrinsic && tcx.item_name(fn_def_id) == sym::transmute {
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return Err((
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span,
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"can only call `transmute` from const items, not `const fn`".into(),
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));
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}
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check_operand(tcx, func, span, body)?;
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for arg in args {
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check_operand(tcx, arg, span, body)?;
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}
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Ok(())
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} else {
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Err((span, "can only call other const fns within const fn".into()))
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}
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},
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TerminatorKind::Assert {
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cond,
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expected: _,
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msg: _,
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target: _,
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cleanup: _,
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} => check_operand(tcx, cond, span, body),
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TerminatorKind::InlineAsm { .. } => Err((span, "cannot use inline assembly in const fn".into())),
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}
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}
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