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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::ty::abstract_const::{
walk_abstract_const, AbstractConst, ConstUnifyCtxt, FailureKind, Node, NotConstEvaluatable,
};
use rustc_middle::ty::{self, TyCtxt, TypeVisitable};
use rustc_session::lint;
use rustc_span::Span;
use std::cmp;
use std::ops::ControlFlow;
/// Check if a given constant can be evaluated.
#[instrument(skip(infcx), level = "debug")]
pub fn is_const_evaluatable<'cx, 'tcx>(
infcx: &InferCtxt<'cx, 'tcx>,
uv: ty::Unevaluated<'tcx, ()>,
param_env: ty::ParamEnv<'tcx>,
span: Span,
) -> Result<(), NotConstEvaluatable> {
let tcx = infcx.tcx;
if tcx.features().generic_const_exprs {
if let Some(ct) = AbstractConst::new(tcx, uv)? {
if satisfied_from_param_env(tcx, ct, param_env)? {
return Ok(());
}
let mut failure_kind = FailureKind::Concrete;
walk_abstract_const::<!, _>(tcx, ct, |node| match node.root(tcx) {
Node::Leaf(leaf) => {
if leaf.has_infer_types_or_consts() {
failure_kind = FailureKind::MentionsInfer;
} else if leaf.has_param_types_or_consts() {
failure_kind = cmp::min(failure_kind, FailureKind::MentionsParam);
}
ControlFlow::CONTINUE
}
Node::Cast(_, _, ty) => {
if ty.has_infer_types_or_consts() {
failure_kind = FailureKind::MentionsInfer;
} else if ty.has_param_types_or_consts() {
failure_kind = cmp::min(failure_kind, FailureKind::MentionsParam);
}
ControlFlow::CONTINUE
}
Node::Binop(_, _, _) | Node::UnaryOp(_, _) | Node::FunctionCall(_, _) => {
ControlFlow::CONTINUE
}
});
match failure_kind {
FailureKind::MentionsInfer => {
return Err(NotConstEvaluatable::MentionsInfer);
}
FailureKind::MentionsParam => {
return Err(NotConstEvaluatable::MentionsParam);
}
// returned below
FailureKind::Concrete => {}
}
}
let concrete = infcx.const_eval_resolve(param_env, uv.expand(), Some(span));
match concrete {
Err(ErrorHandled::TooGeneric) => Err(if !uv.has_infer_types_or_consts() {
infcx
.tcx
.sess
.delay_span_bug(span, &format!("unexpected `TooGeneric` for {:?}", uv));
NotConstEvaluatable::MentionsParam
} else {
NotConstEvaluatable::MentionsInfer
}),
Err(ErrorHandled::Linted) => {
let reported = infcx
.tcx
.sess
.delay_span_bug(span, "constant in type had error reported as lint");
Err(NotConstEvaluatable::Error(reported))
}
Err(ErrorHandled::Reported(e)) => Err(NotConstEvaluatable::Error(e)),
Ok(_) => Ok(()),
}
} else {
// 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.expand(), Some(span));
match concrete {
// If we're evaluating a foreign constant, under a nightly compiler without generic
// const exprs, AND it would've passed if that expression had been evaluated with
// generic const exprs, then suggest using generic const exprs.
Err(_) if tcx.sess.is_nightly_build()
&& let Ok(Some(ct)) = AbstractConst::new(tcx, uv)
&& satisfied_from_param_env(tcx, ct, param_env) == Ok(true) => {
tcx.sess
.struct_span_fatal(
// Slightly better span than just using `span` alone
if span == rustc_span::DUMMY_SP { tcx.def_span(uv.def.did) } 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) => Err(if uv.has_infer_types_or_consts() {
NotConstEvaluatable::MentionsInfer
} else {
NotConstEvaluatable::MentionsParam
}),
Err(ErrorHandled::Linted) => {
let reported =
infcx.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
Err(NotConstEvaluatable::Error(reported))
}
Err(ErrorHandled::Reported(e)) => Err(NotConstEvaluatable::Error(e)),
Ok(_) => {
if uv.substs.has_param_types_or_consts() {
assert!(matches!(infcx.tcx.def_kind(uv.def.did), DefKind::AnonConst));
let mir_body = infcx.tcx.mir_for_ctfe_opt_const_arg(uv.def);
if mir_body.is_polymorphic {
let Some(local_def_id) = uv.def.did.as_local() else { return Ok(()) };
tcx.struct_span_lint_hir(
lint::builtin::CONST_EVALUATABLE_UNCHECKED,
tcx.hir().local_def_id_to_hir_id(local_def_id),
span,
|err| {
err.build("cannot use constants which depend on generic parameters in types").emit();
})
}
}
Ok(())
},
}
}
}
#[instrument(skip(tcx), level = "debug")]
fn satisfied_from_param_env<'tcx>(
tcx: TyCtxt<'tcx>,
ct: AbstractConst<'tcx>,
param_env: ty::ParamEnv<'tcx>,
) -> Result<bool, NotConstEvaluatable> {
for pred in param_env.caller_bounds() {
match pred.kind().skip_binder() {
ty::PredicateKind::ConstEvaluatable(uv) => {
if let Some(b_ct) = AbstractConst::new(tcx, uv)? {
let const_unify_ctxt = ConstUnifyCtxt { tcx, param_env };
// 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`
let result = walk_abstract_const(tcx, b_ct, |b_ct| {
match const_unify_ctxt.try_unify(ct, b_ct) {
true => ControlFlow::BREAK,
false => ControlFlow::CONTINUE,
}
});
if let ControlFlow::Break(()) = result {
debug!("is_const_evaluatable: abstract_const ~~> ok");
return Ok(true);
}
}
}
_ => {} // don't care
}
}
Ok(false)
}
|
