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Show pending obligations as unsatisfied constraints in report_similar_impl_candidates #134348

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Show pending obligations as unsatisfied constraints in report_similar_impl_candidates #134348

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4 changes: 3 additions & 1 deletion compiler/rustc_infer/src/traits/engine.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,8 @@ use crate::traits::Obligation;
pub enum ScrubbedTraitError<'tcx> {
/// A real error. This goal definitely does not hold.
TrueError,
// A select error with pending obligations
Select(PredicateObligations<'tcx>),
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I understand that it's not really documented, but this is antithetical to the design of ScrubbedTraitError. That type should be very lightweight.

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Totally understand your concern. I tried this mainly as a quick hack. I did this because through my ad hoc debugging, I uncovered that select_where_possible contains the pending constraints that I needed to display within the error message. But since the returned details are erased via ScrubbedTraitError, I tried to pass on the pending obligations so that I can show it inside the error message.

I'm not sure what is the best way to workaround this, but at least with this quick hack, I'm able to use this fork of the Rust compiler to significantly simplify debugging for my projects. So regardless of which approach is taken, I probably do need to extract the pending obligations in one way or another and then surface it inside my error messages.

Btw, I have also considered alternative approaches, such as writing a compiler plugin to extract the pending obligations for my specific use cases. However, as far as I looked, I couldn't find ways to implement compiler plugins that help recover or better report compile errors. So I do hope that I can find some ways to fix this upstream in Rust, so that I don't need to instruct my users to use my custom fork of Rust compiler to use my project.

/// An ambiguity. This goal may hold if further inference is done.
Ambiguity,
/// An old-solver-style cycle error, which will fatal.
Expand All @@ -26,7 +28,7 @@ pub enum ScrubbedTraitError<'tcx> {
impl<'tcx> ScrubbedTraitError<'tcx> {
pub fn is_true_error(&self) -> bool {
match self {
ScrubbedTraitError::TrueError => true,
ScrubbedTraitError::TrueError | ScrubbedTraitError::Select(_) => true,
ScrubbedTraitError::Ambiguity | ScrubbedTraitError::Cycle(_) => false,
}
}
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17 changes: 16 additions & 1 deletion compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
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Expand Up @@ -14,6 +14,7 @@ use rustc_hir::def_id::{DefId, LOCAL_CRATE, LocalDefId};
use rustc_hir::intravisit::Visitor;
use rustc_hir::{self as hir, LangItem, Node};
use rustc_infer::infer::{InferOk, TypeTrace};
use rustc_infer::traits::ScrubbedTraitError;
use rustc_middle::traits::SignatureMismatchData;
use rustc_middle::traits::select::OverflowError;
use rustc_middle::ty::abstract_const::NotConstEvaluatable;
Expand Down Expand Up @@ -1852,7 +1853,21 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
{
terrs.push(terr);
}
if !ocx.select_where_possible().is_empty() {

let errors = ocx.select_where_possible();

if !errors.is_empty() {
for error in errors.iter() {
if let ScrubbedTraitError::Select(obligations) = error {
for obligation in obligations.iter() {
let predicate = &obligation.predicate;
err.help(format!(
"the following constraint is not satisfied: `{}`",
predicate
));
}
}
}
return false;
}
}
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9 changes: 7 additions & 2 deletions compiler/rustc_trait_selection/src/traits/fulfill.rs
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Expand Up @@ -942,8 +942,13 @@ impl<'tcx> FromSolverError<'tcx, OldSolverError<'tcx>> for FulfillmentError<'tcx
impl<'tcx> FromSolverError<'tcx, OldSolverError<'tcx>> for ScrubbedTraitError<'tcx> {
fn from_solver_error(_infcx: &InferCtxt<'tcx>, error: OldSolverError<'tcx>) -> Self {
match error.0.error {
FulfillmentErrorCode::Select(_)
| FulfillmentErrorCode::Project(_)
FulfillmentErrorCode::Select(_) => {
let pendings_obligations =
error.0.backtrace.into_iter().map(|p| p.obligation).collect();

ScrubbedTraitError::Select(pendings_obligations)
}
FulfillmentErrorCode::Project(_)
| FulfillmentErrorCode::Subtype(_, _)
| FulfillmentErrorCode::ConstEquate(_, _) => ScrubbedTraitError::TrueError,
FulfillmentErrorCode::Ambiguity { overflow: _ } => ScrubbedTraitError::Ambiguity,
Expand Down
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