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//! Checking that constant values used in types can be successfully evaluated.
//!
//! For concrete constants, this is fairly simple as we can just try and evaluate it.
//!
//! When dealing with polymorphic constants, for example `std::mem::size_of::<T>() - 1`,
//! this is not as easy.
//!
//! In this case we try to build an abstract representation of this constant using
//! `thir_abstract_const` which can then be checked for structural equality with other
//! generic constants mentioned in the `caller_bounds` of the current environment.
use rustc_hir::def::DefKind;
use rustc_infer::infer::InferCtxt;
use rustc_middle::mir::interpret::ErrorHandled;
use rustc_middle::traits::ObligationCause;
use rustc_middle::ty::abstract_const::NotConstEvaluatable;
use rustc_middle::ty::{self, TyCtxt, TypeVisitable, TypeVisitableExt, TypeVisitor};
use rustc_span::Span;
use std::ops::ControlFlow;
use crate::traits::ObligationCtxt;
/// Check if a given constant can be evaluated.
#[instrument(skip(infcx), level = "debug")]
pub fn is_const_evaluatable<'tcx>(
infcx: &InferCtxt<'tcx>,
unexpanded_ct: ty::Const<'tcx>,
param_env: ty::ParamEnv<'tcx>,
span: Span,
) -> Result<(), NotConstEvaluatable> {
let tcx = infcx.tcx;
match tcx.expand_abstract_consts(unexpanded_ct).kind() {
ty::ConstKind::Unevaluated(_) | ty::ConstKind::Expr(_) => (),
ty::ConstKind::Param(_)
| ty::ConstKind::Bound(_, _)
| ty::ConstKind::Placeholder(_)
| ty::ConstKind::Value(_)
| ty::ConstKind::Error(_) => return Ok(()),
ty::ConstKind::Infer(_) => return Err(NotConstEvaluatable::MentionsInfer),
};
if tcx.features().generic_const_exprs {
let ct = tcx.expand_abstract_consts(unexpanded_ct);
let is_anon_ct = if let ty::ConstKind::Unevaluated(uv) = ct.kind() {
tcx.def_kind(uv.def) == DefKind::AnonConst
} else {
false
};
if !is_anon_ct {
if satisfied_from_param_env(tcx, infcx, ct, param_env) {
return Ok(());
}
if ct.has_non_region_infer() {
return Err(NotConstEvaluatable::MentionsInfer);
} else if ct.has_non_region_param() {
return Err(NotConstEvaluatable::MentionsParam);
}
}
match unexpanded_ct.kind() {
ty::ConstKind::Expr(_) => {
// FIXME(generic_const_exprs): we have a `ConstKind::Expr` which is fully concrete, but
// currently it is not possible to evaluate `ConstKind::Expr` so we are unable to tell if it
// is evaluatable or not. For now we just ICE until this is implemented.
Err(NotConstEvaluatable::Error(tcx.sess.delay_span_bug(
span,
"evaluating `ConstKind::Expr` is not currently supported",
)))
}
ty::ConstKind::Unevaluated(uv) => {
let concrete = infcx.const_eval_resolve(param_env, uv, Some(span));
match concrete {
Err(ErrorHandled::TooGeneric(_)) => {
Err(NotConstEvaluatable::Error(infcx.tcx.sess.delay_span_bug(
span,
"Missing value for constant, but no error reported?",
)))
}
Err(ErrorHandled::Reported(e, _)) => Err(NotConstEvaluatable::Error(e.into())),
Ok(_) => Ok(()),
}
}
_ => bug!("unexpected constkind in `is_const_evalautable: {unexpanded_ct:?}`"),
}
} else {
let uv = match unexpanded_ct.kind() {
ty::ConstKind::Unevaluated(uv) => uv,
ty::ConstKind::Expr(_) => {
bug!("`ConstKind::Expr` without `feature(generic_const_exprs)` enabled")
}
_ => bug!("unexpected constkind in `is_const_evalautable: {unexpanded_ct:?}`"),
};
// FIXME: We should only try to evaluate a given constant here if it is fully concrete
// as we don't want to allow things like `[u8; std::mem::size_of::<*mut T>()]`.
//
// We previously did not check this, so we only emit a future compat warning if
// const evaluation succeeds and the given constant is still polymorphic for now
// and hopefully soon change this to an error.
//
// See #74595 for more details about this.
let concrete = infcx.const_eval_resolve(param_env, uv, Some(span));
match concrete {
// If we're evaluating a generic foreign constant, under a nightly compiler while
// the current crate does not enable `feature(generic_const_exprs)`, abort
// compilation with a useful error.
Err(_)
if tcx.sess.is_nightly_build()
&& satisfied_from_param_env(
tcx,
infcx,
tcx.expand_abstract_consts(unexpanded_ct),
param_env,
) =>
{
tcx.sess
.struct_span_fatal(
// Slightly better span than just using `span` alone
if span == rustc_span::DUMMY_SP { tcx.def_span(uv.def) } else { span },
"failed to evaluate generic const expression",
)
.note("the crate this constant originates from uses `#![feature(generic_const_exprs)]`")
.span_suggestion_verbose(
rustc_span::DUMMY_SP,
"consider enabling this feature",
"#![feature(generic_const_exprs)]\n",
rustc_errors::Applicability::MaybeIncorrect,
)
.emit()
}
Err(ErrorHandled::TooGeneric(_)) => {
let err = if uv.has_non_region_infer() {
NotConstEvaluatable::MentionsInfer
} else if uv.has_non_region_param() {
NotConstEvaluatable::MentionsParam
} else {
let guar = infcx
.tcx
.sess
.delay_span_bug(span, "Missing value for constant, but no error reported?");
NotConstEvaluatable::Error(guar)
};
Err(err)
}
Err(ErrorHandled::Reported(e, _)) => Err(NotConstEvaluatable::Error(e.into())),
Ok(_) => Ok(()),
}
}
}
#[instrument(skip(infcx, tcx), level = "debug")]
fn satisfied_from_param_env<'tcx>(
tcx: TyCtxt<'tcx>,
infcx: &InferCtxt<'tcx>,
ct: ty::Const<'tcx>,
param_env: ty::ParamEnv<'tcx>,
) -> bool {
// Try to unify with each subtree in the AbstractConst to allow for
// `N + 1` being const evaluatable even if theres only a `ConstEvaluatable`
// predicate for `(N + 1) * 2`
struct Visitor<'a, 'tcx> {
ct: ty::Const<'tcx>,
param_env: ty::ParamEnv<'tcx>,
infcx: &'a InferCtxt<'tcx>,
single_match: Option<Result<ty::Const<'tcx>, ()>>,
}
impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for Visitor<'a, 'tcx> {
type BreakTy = ();
fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
debug!("is_const_evaluatable: candidate={:?}", c);
if self.infcx.probe(|_| {
let ocx = ObligationCtxt::new(self.infcx);
ocx.eq(&ObligationCause::dummy(), self.param_env, c.ty(), self.ct.ty()).is_ok()
&& ocx.eq(&ObligationCause::dummy(), self.param_env, c, self.ct).is_ok()
&& ocx.select_all_or_error().is_empty()
}) {
self.single_match = match self.single_match {
None => Some(Ok(c)),
Some(Ok(o)) if o == c => Some(Ok(c)),
Some(_) => Some(Err(())),
};
}
if let ty::ConstKind::Expr(e) = c.kind() {
e.visit_with(self)
} else {
// FIXME(generic_const_exprs): This doesn't recurse into `<T as Trait<U>>::ASSOC`'s args.
// This is currently unobservable as `<T as Trait<{ U + 1 }>>::ASSOC` creates an anon const
// with its own `ConstEvaluatable` bound in the param env which we will visit separately.
//
// If we start allowing directly writing `ConstKind::Expr` without an intermediate anon const
// this will be incorrect. It might be worth investigating making `predicates_of` elaborate
// all of the `ConstEvaluatable` bounds rather than having a visitor here.
ControlFlow::Continue(())
}
}
}
let mut single_match: Option<Result<ty::Const<'tcx>, ()>> = None;
for pred in param_env.caller_bounds() {
match pred.kind().skip_binder() {
ty::ClauseKind::ConstEvaluatable(ce) => {
let b_ct = tcx.expand_abstract_consts(ce);
let mut v = Visitor { ct, infcx, param_env, single_match };
let _ = b_ct.visit_with(&mut v);
single_match = v.single_match;
}
_ => {} // don't care
}
}
if let Some(Ok(c)) = single_match {
let ocx = ObligationCtxt::new(infcx);
assert!(ocx.eq(&ObligationCause::dummy(), param_env, c.ty(), ct.ty()).is_ok());
assert!(ocx.eq(&ObligationCause::dummy(), param_env, c, ct).is_ok());
assert!(ocx.select_all_or_error().is_empty());
return true;
}
debug!("is_const_evaluatable: no");
false
}