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Rollup merge of #81544 - JulianKnodt:sat_where, r=lcnr

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Add better diagnostic for unbounded Abst. Const

~~In the case where a generic abst. const requires a trivial where bound: `where TypeWithConst<const_fn(N)>: ,`,
instead of requiring a where bound, just check that only consts are being substituted in to skip over where check.~~

~~This is pretty sketchy, but I think it works. Presumably, if there is checking for type bounds added later, it can first check nested requirements, and see if they're satisfied by the current `ParamEnv`.~~

Changed the diagnostic to add a better example, which is more practical than what was previously proposed.

r? ```@lcnr```
This commit is contained in:
Jack Huey 2021-02-02 16:01:39 -05:00 • committed by GitHub
commit 3aed8b17a8
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7 changed files with 93 additions and 18 deletions
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30
compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
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@ -16,8 +16,7 @@ use rustc_infer::infer::InferCtxt;
use rustc_middle::mir::abstract_const::{Node, NodeId};
use rustc_middle::mir::interpret::ErrorHandled;
use rustc_middle::mir::{self, Rvalue, StatementKind, TerminatorKind};
use rustc_middle::ty::subst::Subst;
use rustc_middle::ty::subst::SubstsRef;
use rustc_middle::ty::subst::{Subst, SubstsRef};
use rustc_middle::ty::{self, TyCtxt, TypeFoldable};
use rustc_session::lint;
use rustc_span::def_id::{DefId, LocalDefId};
@ -43,10 +42,6 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
for pred in param_env.caller_bounds() {
match pred.kind().skip_binder() {
ty::PredicateKind::ConstEvaluatable(b_def, b_substs) => {
debug!(
"is_const_evaluatable: caller_bound={:?}, {:?}",
b_def, b_substs
);
if b_def == def && b_substs == substs {
debug!("is_const_evaluatable: caller_bound ~~> ok");
return Ok(());
@ -113,15 +108,24 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
}
FailureKind::MentionsParam => {
// FIXME(const_evaluatable_checked): Better error message.
infcx
.tcx
.sess
.struct_span_err(span, "unconstrained generic constant")
.span_help(
let mut err =
infcx.tcx.sess.struct_span_err(span, "unconstrained generic constant");
let const_span = tcx.def_span(def.did);
// FIXME(const_evaluatable_checked): Update this suggestion once
// explicit const evaluatable bounds are implemented.
if let Ok(snippet) = infcx.tcx.sess.source_map().span_to_snippet(const_span)
{
err.span_help(
tcx.def_span(def.did),
&format!("try adding a `where` bound using this expression: where [u8; {}]: Sized", snippet),
);
} else {
err.span_help(
const_span,
"consider adding a `where` bound for this expression",
)
.emit();
);
}
err.emit();
return Err(ErrorHandled::Reported(ErrorReported));
}
FailureKind::Concrete => {