1 //! Checking that constant values used in types can be successfully evaluated.
3 //! For concrete constants, this is fairly simple as we can just try and evaluate it.
5 //! When dealing with polymorphic constants, for example `std::mem::size_of::<T>() - 1`,
6 //! this is not as easy.
8 //! In this case we try to build an abstract representation of this constant using
9 //! `thir_abstract_const` which can then be checked for structural equality with other
10 //! generic constants mentioned in the `caller_bounds` of the current environment.
11 use rustc_data_structures::intern::Interned;
12 use rustc_errors::ErrorGuaranteed;
13 use rustc_hir::def::DefKind;
14 use rustc_index::vec::IndexVec;
15 use rustc_infer::infer::InferCtxt;
16 use rustc_middle::mir;
17 use rustc_middle::mir::interpret::{
18 ConstValue, ErrorHandled, LitToConstError, LitToConstInput, Scalar,
20 use rustc_middle::thir;
21 use rustc_middle::thir::abstract_const::{self, Node, NodeId, NotConstEvaluatable};
22 use rustc_middle::ty::subst::{Subst, SubstsRef};
23 use rustc_middle::ty::{self, DelaySpanBugEmitted, TyCtxt, TypeFoldable};
24 use rustc_session::lint;
25 use rustc_span::def_id::LocalDefId;
30 use std::ops::ControlFlow;
32 /// Check if a given constant can be evaluated.
33 #[instrument(skip(infcx), level = "debug")]
34 pub fn is_const_evaluatable<'cx, 'tcx>(
35 infcx: &InferCtxt<'cx, 'tcx>,
36 uv: ty::Unevaluated<'tcx, ()>,
37 param_env: ty::ParamEnv<'tcx>,
39 ) -> Result<(), NotConstEvaluatable> {
42 if tcx.features().generic_const_exprs {
43 match AbstractConst::new(tcx, uv)? {
44 // We are looking at a generic abstract constant.
46 if satisfied_from_param_env(tcx, ct, param_env)? {
50 // We were unable to unify the abstract constant with
51 // a constant found in the caller bounds, there are
52 // now three possible cases here.
53 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
55 /// The abstract const still references an inference
56 /// variable, in this case we return `TooGeneric`.
58 /// The abstract const references a generic parameter,
59 /// this means that we emit an error here.
61 /// The substs are concrete enough that we can simply
62 /// try and evaluate the given constant.
65 let mut failure_kind = FailureKind::Concrete;
66 walk_abstract_const::<!, _>(tcx, ct, |node| match node.root(tcx) {
68 if leaf.has_infer_types_or_consts() {
69 failure_kind = FailureKind::MentionsInfer;
70 } else if leaf.has_param_types_or_consts() {
71 failure_kind = cmp::min(failure_kind, FailureKind::MentionsParam);
76 Node::Cast(_, _, ty) => {
77 if ty.has_infer_types_or_consts() {
78 failure_kind = FailureKind::MentionsInfer;
79 } else if ty.has_param_types_or_consts() {
80 failure_kind = cmp::min(failure_kind, FailureKind::MentionsParam);
85 Node::Binop(_, _, _) | Node::UnaryOp(_, _) | Node::FunctionCall(_, _) => {
91 FailureKind::MentionsInfer => {
92 return Err(NotConstEvaluatable::MentionsInfer);
94 FailureKind::MentionsParam => {
95 return Err(NotConstEvaluatable::MentionsParam);
97 FailureKind::Concrete => {
98 // Dealt with below by the same code which handles this
99 // without the feature gate.
104 // If we are dealing with a concrete constant, we can
105 // reuse the old code path and try to evaluate
111 let future_compat_lint = || {
112 if let Some(local_def_id) = uv.def.did.as_local() {
113 infcx.tcx.struct_span_lint_hir(
114 lint::builtin::CONST_EVALUATABLE_UNCHECKED,
115 infcx.tcx.hir().local_def_id_to_hir_id(local_def_id),
118 err.build("cannot use constants which depend on generic parameters in types")
125 // FIXME: We should only try to evaluate a given constant here if it is fully concrete
126 // as we don't want to allow things like `[u8; std::mem::size_of::<*mut T>()]`.
128 // We previously did not check this, so we only emit a future compat warning if
129 // const evaluation succeeds and the given constant is still polymorphic for now
130 // and hopefully soon change this to an error.
132 // See #74595 for more details about this.
133 let concrete = infcx.const_eval_resolve(param_env, uv.expand(), Some(span));
135 if concrete.is_ok() && uv.substs.has_param_types_or_consts() {
136 match infcx.tcx.def_kind(uv.def.did) {
137 DefKind::AnonConst | DefKind::InlineConst => {
138 let mir_body = infcx.tcx.mir_for_ctfe_opt_const_arg(uv.def);
140 if mir_body.is_polymorphic {
141 future_compat_lint();
144 _ => future_compat_lint(),
148 // If we're evaluating a foreign constant, under a nightly compiler without generic
149 // const exprs, AND it would've passed if that expression had been evaluated with
150 // generic const exprs, then suggest using generic const exprs.
152 && tcx.sess.is_nightly_build()
153 && !uv.def.did.is_local()
154 && !tcx.features().generic_const_exprs
155 && let Ok(Some(ct)) = AbstractConst::new(tcx, uv)
156 && satisfied_from_param_env(tcx, ct, param_env) == Ok(true)
160 // Slightly better span than just using `span` alone
161 if span == rustc_span::DUMMY_SP { tcx.def_span(uv.def.did) } else { span },
162 "failed to evaluate generic const expression",
164 .note("the crate this constant originates from uses `#![feature(generic_const_exprs)]`")
165 .span_suggestion_verbose(
166 rustc_span::DUMMY_SP,
167 "consider enabling this feature",
168 "#![feature(generic_const_exprs)]\n".to_string(),
169 rustc_errors::Applicability::MaybeIncorrect,
172 rustc_errors::FatalError.raise();
175 debug!(?concrete, "is_const_evaluatable");
177 Err(ErrorHandled::TooGeneric) => Err(match uv.has_infer_types_or_consts() {
178 true => NotConstEvaluatable::MentionsInfer,
179 false => NotConstEvaluatable::MentionsParam,
181 Err(ErrorHandled::Linted) => {
183 infcx.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
184 Err(NotConstEvaluatable::Error(reported))
186 Err(ErrorHandled::Reported(e)) => Err(NotConstEvaluatable::Error(e)),
191 #[instrument(skip(tcx), level = "debug")]
192 fn satisfied_from_param_env<'tcx>(
194 ct: AbstractConst<'tcx>,
195 param_env: ty::ParamEnv<'tcx>,
196 ) -> Result<bool, NotConstEvaluatable> {
197 for pred in param_env.caller_bounds() {
198 match pred.kind().skip_binder() {
199 ty::PredicateKind::ConstEvaluatable(uv) => {
200 if let Some(b_ct) = AbstractConst::new(tcx, uv)? {
201 let const_unify_ctxt = ConstUnifyCtxt { tcx, param_env };
203 // Try to unify with each subtree in the AbstractConst to allow for
204 // `N + 1` being const evaluatable even if theres only a `ConstEvaluatable`
205 // predicate for `(N + 1) * 2`
206 let result = walk_abstract_const(tcx, b_ct, |b_ct| {
207 match const_unify_ctxt.try_unify(ct, b_ct) {
208 true => ControlFlow::BREAK,
209 false => ControlFlow::CONTINUE,
213 if let ControlFlow::Break(()) = result {
214 debug!("is_const_evaluatable: abstract_const ~~> ok");
219 _ => {} // don't care
226 /// A tree representing an anonymous constant.
228 /// This is only able to represent a subset of `MIR`,
229 /// and should not leak any information about desugarings.
230 #[derive(Debug, Clone, Copy)]
231 pub struct AbstractConst<'tcx> {
232 // FIXME: Consider adding something like `IndexSlice`
233 // and use this here.
234 inner: &'tcx [Node<'tcx>],
235 substs: SubstsRef<'tcx>,
238 impl<'tcx> AbstractConst<'tcx> {
241 uv: ty::Unevaluated<'tcx, ()>,
242 ) -> Result<Option<AbstractConst<'tcx>>, ErrorGuaranteed> {
243 let inner = tcx.thir_abstract_const_opt_const_arg(uv.def)?;
244 debug!("AbstractConst::new({:?}) = {:?}", uv, inner);
245 Ok(inner.map(|inner| AbstractConst { inner, substs: uv.substs }))
251 ) -> Result<Option<AbstractConst<'tcx>>, ErrorGuaranteed> {
253 ty::ConstKind::Unevaluated(uv) => AbstractConst::new(tcx, uv.shrink()),
254 ty::ConstKind::Error(DelaySpanBugEmitted { reported, .. }) => Err(reported),
260 pub fn subtree(self, node: NodeId) -> AbstractConst<'tcx> {
261 AbstractConst { inner: &self.inner[..=node.index()], substs: self.substs }
265 pub fn root(self, tcx: TyCtxt<'tcx>) -> Node<'tcx> {
266 let node = self.inner.last().copied().unwrap();
268 Node::Leaf(leaf) => Node::Leaf(leaf.subst(tcx, self.substs)),
269 Node::Cast(kind, operand, ty) => Node::Cast(kind, operand, ty.subst(tcx, self.substs)),
270 // Don't perform substitution on the following as they can't directly contain generic params
271 Node::Binop(_, _, _) | Node::UnaryOp(_, _) | Node::FunctionCall(_, _) => node,
276 struct AbstractConstBuilder<'a, 'tcx> {
278 body_id: thir::ExprId,
279 body: &'a thir::Thir<'tcx>,
280 /// The current WIP node tree.
281 nodes: IndexVec<NodeId, Node<'tcx>>,
284 impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
285 fn root_span(&self) -> Span {
286 self.body.exprs[self.body_id].span
289 fn error(&mut self, span: Span, msg: &str) -> Result<!, ErrorGuaranteed> {
293 .struct_span_err(self.root_span(), "overly complex generic constant")
294 .span_label(span, msg)
295 .help("consider moving this anonymous constant into a `const` function")
300 fn maybe_supported_error(&mut self, span: Span, msg: &str) -> Result<!, ErrorGuaranteed> {
304 .struct_span_err(self.root_span(), "overly complex generic constant")
305 .span_label(span, msg)
306 .help("consider moving this anonymous constant into a `const` function")
307 .note("this operation may be supported in the future")
313 #[instrument(skip(tcx, body, body_id), level = "debug")]
316 (body, body_id): (&'a thir::Thir<'tcx>, thir::ExprId),
317 ) -> Result<Option<AbstractConstBuilder<'a, 'tcx>>, ErrorGuaranteed> {
318 let builder = AbstractConstBuilder { tcx, body_id, body, nodes: IndexVec::new() };
320 struct IsThirPolymorphic<'a, 'tcx> {
322 thir: &'a thir::Thir<'tcx>,
325 use crate::rustc_middle::thir::visit::Visitor;
328 impl<'a, 'tcx> IsThirPolymorphic<'a, 'tcx> {
329 fn expr_is_poly(&mut self, expr: &thir::Expr<'tcx>) -> bool {
330 if expr.ty.has_param_types_or_consts() {
335 thir::ExprKind::NamedConst { substs, .. } => substs.has_param_types_or_consts(),
336 thir::ExprKind::ConstParam { .. } => true,
337 thir::ExprKind::Repeat { value, count } => {
338 self.visit_expr(&self.thir()[value]);
339 count.has_param_types_or_consts()
345 fn pat_is_poly(&mut self, pat: &thir::Pat<'tcx>) -> bool {
346 if pat.ty.has_param_types_or_consts() {
350 match pat.kind.as_ref() {
351 thir::PatKind::Constant { value } => value.has_param_types_or_consts(),
352 thir::PatKind::Range(thir::PatRange { lo, hi, .. }) => {
353 lo.has_param_types_or_consts() || hi.has_param_types_or_consts()
360 impl<'a, 'tcx> visit::Visitor<'a, 'tcx> for IsThirPolymorphic<'a, 'tcx> {
361 fn thir(&self) -> &'a thir::Thir<'tcx> {
365 #[instrument(skip(self), level = "debug")]
366 fn visit_expr(&mut self, expr: &thir::Expr<'tcx>) {
367 self.is_poly |= self.expr_is_poly(expr);
369 visit::walk_expr(self, expr)
373 #[instrument(skip(self), level = "debug")]
374 fn visit_pat(&mut self, pat: &thir::Pat<'tcx>) {
375 self.is_poly |= self.pat_is_poly(pat);
377 visit::walk_pat(self, pat);
382 let mut is_poly_vis = IsThirPolymorphic { is_poly: false, thir: body };
383 visit::walk_expr(&mut is_poly_vis, &body[body_id]);
384 debug!("AbstractConstBuilder: is_poly={}", is_poly_vis.is_poly);
385 if !is_poly_vis.is_poly {
392 /// We do not allow all binary operations in abstract consts, so filter disallowed ones.
393 fn check_binop(op: mir::BinOp) -> bool {
396 Add | Sub | Mul | Div | Rem | BitXor | BitAnd | BitOr | Shl | Shr | Eq | Lt | Le
397 | Ne | Ge | Gt => true,
402 /// While we currently allow all unary operations, we still want to explicitly guard against
403 /// future changes here.
404 fn check_unop(op: mir::UnOp) -> bool {
411 /// Builds the abstract const by walking the thir and bailing out when
412 /// encountering an unspported operation.
413 fn build(mut self) -> Result<&'tcx [Node<'tcx>], ErrorGuaranteed> {
414 debug!("Abstractconstbuilder::build: body={:?}", &*self.body);
415 self.recurse_build(self.body_id)?;
417 for n in self.nodes.iter() {
418 if let Node::Leaf(ty::Const(Interned(
419 ty::ConstS { val: ty::ConstKind::Unevaluated(ct), ty: _ },
423 // `AbstractConst`s should not contain any promoteds as they require references which
425 assert_eq!(ct.promoted, None);
429 Ok(self.tcx.arena.alloc_from_iter(self.nodes.into_iter()))
432 fn recurse_build(&mut self, node: thir::ExprId) -> Result<NodeId, ErrorGuaranteed> {
434 let node = &self.body.exprs[node];
435 Ok(match &node.kind {
436 // I dont know if handling of these 3 is correct
437 &ExprKind::Scope { value, .. } => self.recurse_build(value)?,
438 &ExprKind::PlaceTypeAscription { source, .. }
439 | &ExprKind::ValueTypeAscription { source, .. } => self.recurse_build(source)?,
440 &ExprKind::Literal { lit, neg} => {
443 match self.tcx.at(sp).lit_to_const(LitToConstInput { lit: &lit.node, ty: node.ty, neg }) {
445 Err(LitToConstError::Reported) => {
446 self.tcx.const_error(node.ty)
448 Err(LitToConstError::TypeError) => {
449 bug!("encountered type error in lit_to_const")
453 self.nodes.push(Node::Leaf(constant))
455 &ExprKind::NonHirLiteral { lit , user_ty: _} => {
456 // FIXME Construct a Valtree from this ScalarInt when introducing Valtrees
457 let const_value = ConstValue::Scalar(Scalar::Int(lit));
458 self.nodes.push(Node::Leaf(ty::Const::from_value(self.tcx, const_value, node.ty)))
460 &ExprKind::NamedConst { def_id, substs, user_ty: _ } => {
461 let uneval = ty::Unevaluated::new(ty::WithOptConstParam::unknown(def_id), substs);
463 let constant = self.tcx.mk_const(ty::ConstS {
464 val: ty::ConstKind::Unevaluated(uneval),
468 self.nodes.push(Node::Leaf(constant))
471 ExprKind::ConstParam {param, ..} => {
472 let const_param = self.tcx.mk_const(ty::ConstS {
473 val: ty::ConstKind::Param(*param),
476 self.nodes.push(Node::Leaf(const_param))
479 ExprKind::Call { fun, args, .. } => {
480 let fun = self.recurse_build(*fun)?;
482 let mut new_args = Vec::<NodeId>::with_capacity(args.len());
483 for &id in args.iter() {
484 new_args.push(self.recurse_build(id)?);
486 let new_args = self.tcx.arena.alloc_slice(&new_args);
487 self.nodes.push(Node::FunctionCall(fun, new_args))
489 &ExprKind::Binary { op, lhs, rhs } if Self::check_binop(op) => {
490 let lhs = self.recurse_build(lhs)?;
491 let rhs = self.recurse_build(rhs)?;
492 self.nodes.push(Node::Binop(op, lhs, rhs))
494 &ExprKind::Unary { op, arg } if Self::check_unop(op) => {
495 let arg = self.recurse_build(arg)?;
496 self.nodes.push(Node::UnaryOp(op, arg))
498 // This is necessary so that the following compiles:
501 // fn foo<const N: usize>(a: [(); N + 1]) {
502 // bar::<{ N + 1 }>();
505 ExprKind::Block { body: thir::Block { stmts: box [], expr: Some(e), .. } } => {
506 self.recurse_build(*e)?
508 // `ExprKind::Use` happens when a `hir::ExprKind::Cast` is a
509 // "coercion cast" i.e. using a coercion or is a no-op.
510 // This is important so that `N as usize as usize` doesnt unify with `N as usize`. (untested)
511 &ExprKind::Use { source } => {
512 let arg = self.recurse_build(source)?;
513 self.nodes.push(Node::Cast(abstract_const::CastKind::Use, arg, node.ty))
515 &ExprKind::Cast { source } => {
516 let arg = self.recurse_build(source)?;
517 self.nodes.push(Node::Cast(abstract_const::CastKind::As, arg, node.ty))
519 ExprKind::Borrow{ arg, ..} => {
520 let arg_node = &self.body.exprs[*arg];
522 // Skip reborrows for now until we allow Deref/Borrow/AddressOf
524 // FIXME(generic_const_exprs): Verify/explain why this is sound
525 if let ExprKind::Deref {arg} = arg_node.kind {
526 self.recurse_build(arg)?
528 self.maybe_supported_error(
530 "borrowing is not supported in generic constants",
534 // FIXME(generic_const_exprs): We may want to support these.
535 ExprKind::AddressOf { .. } | ExprKind::Deref {..}=> self.maybe_supported_error(
537 "dereferencing or taking the address is not supported in generic constants",
539 ExprKind::Repeat { .. } | ExprKind::Array { .. } => self.maybe_supported_error(
541 "array construction is not supported in generic constants",
543 ExprKind::Block { .. } => self.maybe_supported_error(
545 "blocks are not supported in generic constant",
547 ExprKind::NeverToAny { .. } => self.maybe_supported_error(
549 "converting nevers to any is not supported in generic constant",
551 ExprKind::Tuple { .. } => self.maybe_supported_error(
553 "tuple construction is not supported in generic constants",
555 ExprKind::Index { .. } => self.maybe_supported_error(
557 "indexing is not supported in generic constant",
559 ExprKind::Field { .. } => self.maybe_supported_error(
561 "field access is not supported in generic constant",
563 ExprKind::ConstBlock { .. } => self.maybe_supported_error(
565 "const blocks are not supported in generic constant",
567 ExprKind::Adt(_) => self.maybe_supported_error(
569 "struct/enum construction is not supported in generic constants",
571 // dont know if this is correct
572 ExprKind::Pointer { .. } =>
573 self.error(node.span, "pointer casts are not allowed in generic constants")?,
574 ExprKind::Yield { .. } =>
575 self.error(node.span, "generator control flow is not allowed in generic constants")?,
576 ExprKind::Continue { .. } | ExprKind::Break { .. } | ExprKind::Loop { .. } => self
579 "loops and loop control flow are not supported in generic constants",
581 ExprKind::Box { .. } =>
582 self.error(node.span, "allocations are not allowed in generic constants")?,
584 ExprKind::Unary { .. } => unreachable!(),
585 // we handle valid unary/binary ops above
586 ExprKind::Binary { .. } =>
587 self.error(node.span, "unsupported binary operation in generic constants")?,
588 ExprKind::LogicalOp { .. } =>
589 self.error(node.span, "unsupported operation in generic constants, short-circuiting operations would imply control flow")?,
590 ExprKind::Assign { .. } | ExprKind::AssignOp { .. } => {
591 self.error(node.span, "assignment is not supported in generic constants")?
593 ExprKind::Closure { .. } | ExprKind::Return { .. } => self.error(
595 "closures and function keywords are not supported in generic constants",
597 // let expressions imply control flow
598 ExprKind::Match { .. } | ExprKind::If { .. } | ExprKind::Let { .. } =>
599 self.error(node.span, "control flow is not supported in generic constants")?,
600 ExprKind::InlineAsm { .. } => {
601 self.error(node.span, "assembly is not supported in generic constants")?
604 // we dont permit let stmts so `VarRef` and `UpvarRef` cant happen
605 ExprKind::VarRef { .. }
606 | ExprKind::UpvarRef { .. }
607 | ExprKind::StaticRef { .. }
608 | ExprKind::ThreadLocalRef(_) => {
609 self.error(node.span, "unsupported operation in generic constant")?
615 /// Builds an abstract const, do not use this directly, but use `AbstractConst::new` instead.
616 pub(super) fn thir_abstract_const<'tcx>(
618 def: ty::WithOptConstParam<LocalDefId>,
619 ) -> Result<Option<&'tcx [thir::abstract_const::Node<'tcx>]>, ErrorGuaranteed> {
620 if tcx.features().generic_const_exprs {
621 match tcx.def_kind(def.did) {
622 // FIXME(generic_const_exprs): We currently only do this for anonymous constants,
623 // meaning that we do not look into associated constants. I(@lcnr) am not yet sure whether
624 // we want to look into them or treat them as opaque projections.
626 // Right now we do neither of that and simply always fail to unify them.
627 DefKind::AnonConst | DefKind::InlineConst => (),
628 _ => return Ok(None),
631 let body = tcx.thir_body(def)?;
633 AbstractConstBuilder::new(tcx, (&*body.0.borrow(), body.1))?
634 .map(AbstractConstBuilder::build)
641 pub(super) fn try_unify_abstract_consts<'tcx>(
643 (a, b): (ty::Unevaluated<'tcx, ()>, ty::Unevaluated<'tcx, ()>),
644 param_env: ty::ParamEnv<'tcx>,
647 if let Some(a) = AbstractConst::new(tcx, a)? {
648 if let Some(b) = AbstractConst::new(tcx, b)? {
649 let const_unify_ctxt = ConstUnifyCtxt { tcx, param_env };
650 return Ok(const_unify_ctxt.try_unify(a, b));
656 .unwrap_or_else(|_: ErrorGuaranteed| true)
657 // FIXME(generic_const_exprs): We should instead have this
658 // method return the resulting `ty::Const` and return `ConstKind::Error`
659 // on `ErrorGuaranteed`.
662 #[instrument(skip(tcx, f), level = "debug")]
663 pub fn walk_abstract_const<'tcx, R, F>(
665 ct: AbstractConst<'tcx>,
669 F: FnMut(AbstractConst<'tcx>) -> ControlFlow<R>,
671 #[instrument(skip(tcx, f), level = "debug")]
674 ct: AbstractConst<'tcx>,
675 f: &mut dyn FnMut(AbstractConst<'tcx>) -> ControlFlow<R>,
676 ) -> ControlFlow<R> {
678 let root = ct.root(tcx);
681 Node::Leaf(_) => ControlFlow::CONTINUE,
682 Node::Binop(_, l, r) => {
683 recurse(tcx, ct.subtree(l), f)?;
684 recurse(tcx, ct.subtree(r), f)
686 Node::UnaryOp(_, v) => recurse(tcx, ct.subtree(v), f),
687 Node::FunctionCall(func, args) => {
688 recurse(tcx, ct.subtree(func), f)?;
689 args.iter().try_for_each(|&arg| recurse(tcx, ct.subtree(arg), f))
691 Node::Cast(_, operand, _) => recurse(tcx, ct.subtree(operand), f),
695 recurse(tcx, ct, &mut f)
698 struct ConstUnifyCtxt<'tcx> {
700 param_env: ty::ParamEnv<'tcx>,
703 impl<'tcx> ConstUnifyCtxt<'tcx> {
704 // Substitutes generics repeatedly to allow AbstractConsts to unify where a
705 // ConstKind::Unevalated could be turned into an AbstractConst that would unify e.g.
706 // Param(N) should unify with Param(T), substs: [Unevaluated("T2", [Unevaluated("T3", [Param(N)])])]
708 #[instrument(skip(self), level = "debug")]
709 fn try_replace_substs_in_root(
711 mut abstr_const: AbstractConst<'tcx>,
712 ) -> Option<AbstractConst<'tcx>> {
713 while let Node::Leaf(ct) = abstr_const.root(self.tcx) {
714 match AbstractConst::from_const(self.tcx, ct) {
715 Ok(Some(act)) => abstr_const = act,
717 Err(_) => return None,
724 /// Tries to unify two abstract constants using structural equality.
725 #[instrument(skip(self), level = "debug")]
726 fn try_unify(&self, a: AbstractConst<'tcx>, b: AbstractConst<'tcx>) -> bool {
727 let a = if let Some(a) = self.try_replace_substs_in_root(a) {
733 let b = if let Some(b) = self.try_replace_substs_in_root(b) {
739 let a_root = a.root(self.tcx);
740 let b_root = b.root(self.tcx);
741 debug!(?a_root, ?b_root);
743 match (a_root, b_root) {
744 (Node::Leaf(a_ct), Node::Leaf(b_ct)) => {
745 let a_ct = a_ct.eval(self.tcx, self.param_env);
746 debug!("a_ct evaluated: {:?}", a_ct);
747 let b_ct = b_ct.eval(self.tcx, self.param_env);
748 debug!("b_ct evaluated: {:?}", b_ct);
750 if a_ct.ty() != b_ct.ty() {
754 match (a_ct.val(), b_ct.val()) {
755 // We can just unify errors with everything to reduce the amount of
756 // emitted errors here.
757 (ty::ConstKind::Error(_), _) | (_, ty::ConstKind::Error(_)) => true,
758 (ty::ConstKind::Param(a_param), ty::ConstKind::Param(b_param)) => {
761 (ty::ConstKind::Value(a_val), ty::ConstKind::Value(b_val)) => a_val == b_val,
762 // If we have `fn a<const N: usize>() -> [u8; N + 1]` and `fn b<const M: usize>() -> [u8; 1 + M]`
763 // we do not want to use `assert_eq!(a(), b())` to infer that `N` and `M` have to be `1`. This
764 // means that we only allow inference variables if they are equal.
765 (ty::ConstKind::Infer(a_val), ty::ConstKind::Infer(b_val)) => a_val == b_val,
766 // We expand generic anonymous constants at the start of this function, so this
767 // branch should only be taking when dealing with associated constants, at
768 // which point directly comparing them seems like the desired behavior.
770 // FIXME(generic_const_exprs): This isn't actually the case.
771 // We also take this branch for concrete anonymous constants and
772 // expand generic anonymous constants with concrete substs.
773 (ty::ConstKind::Unevaluated(a_uv), ty::ConstKind::Unevaluated(b_uv)) => {
776 // FIXME(generic_const_exprs): We may want to either actually try
777 // to evaluate `a_ct` and `b_ct` if they are are fully concrete or something like
778 // this, for now we just return false here.
782 (Node::Binop(a_op, al, ar), Node::Binop(b_op, bl, br)) if a_op == b_op => {
783 self.try_unify(a.subtree(al), b.subtree(bl))
784 && self.try_unify(a.subtree(ar), b.subtree(br))
786 (Node::UnaryOp(a_op, av), Node::UnaryOp(b_op, bv)) if a_op == b_op => {
787 self.try_unify(a.subtree(av), b.subtree(bv))
789 (Node::FunctionCall(a_f, a_args), Node::FunctionCall(b_f, b_args))
790 if a_args.len() == b_args.len() =>
792 self.try_unify(a.subtree(a_f), b.subtree(b_f))
793 && iter::zip(a_args, b_args)
794 .all(|(&an, &bn)| self.try_unify(a.subtree(an), b.subtree(bn)))
796 (Node::Cast(a_kind, a_operand, a_ty), Node::Cast(b_kind, b_operand, b_ty))
797 if (a_ty == b_ty) && (a_kind == b_kind) =>
799 self.try_unify(a.subtree(a_operand), b.subtree(b_operand))
801 // use this over `_ => false` to make adding variants to `Node` less error prone
803 | (Node::FunctionCall(..), _)
804 | (Node::UnaryOp(..), _)
805 | (Node::Binop(..), _)
806 | (Node::Leaf(..), _) => false,