Nits and formatting

compiler/rustc_infer/src/infer/canonical/query_response.rs

@@ -15,8 +15,8 @@ use crate::infer::canonical::{
 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
 use crate::infer::{DefineOpaqueTypes, InferCtxt, InferOk, InferResult};
 use crate::traits::query::NoSolution;
-use crate::traits::{FulfillmentErrorLike, TraitEngine};
 use crate::traits::{Obligation, ObligationCause, PredicateObligation};
+use crate::traits::{ScrubbedTraitError, TraitEngine};
 use rustc_data_structures::captures::Captures;
 use rustc_index::Idx;
 use rustc_index::IndexVec;
@@ -50,11 +50,11 @@ impl<'tcx> InferCtxt<'tcx> {
     /// - Finally, if any of the obligations result in a hard error,
     ///   then `Err(NoSolution)` is returned.
     #[instrument(skip(self, inference_vars, answer, fulfill_cx), level = "trace")]
-    pub fn make_canonicalized_query_response<T, E: FulfillmentErrorLike<'tcx>>(
+    pub fn make_canonicalized_query_response<T>(
         &self,
         inference_vars: CanonicalVarValues<'tcx>,
         answer: T,
-        fulfill_cx: &mut dyn TraitEngine<'tcx, E>,
+        fulfill_cx: &mut dyn TraitEngine<'tcx, ScrubbedTraitError<'tcx>>,
     ) -> Result<CanonicalQueryResponse<'tcx, T>, NoSolution>
     where
         T: Debug + TypeFoldable<TyCtxt<'tcx>>,
@@ -97,11 +97,11 @@ impl<'tcx> InferCtxt<'tcx> {
     /// Helper for `make_canonicalized_query_response` that does
     /// everything up until the final canonicalization.
     #[instrument(skip(self, fulfill_cx), level = "debug")]
-    fn make_query_response<T, E: FulfillmentErrorLike<'tcx>>(
+    fn make_query_response<T>(
         &self,
         inference_vars: CanonicalVarValues<'tcx>,
         answer: T,
-        fulfill_cx: &mut dyn TraitEngine<'tcx, E>,
+        fulfill_cx: &mut dyn TraitEngine<'tcx, ScrubbedTraitError<'tcx>>,
     ) -> Result<QueryResponse<'tcx, T>, NoSolution>
     where
         T: Debug + TypeFoldable<TyCtxt<'tcx>>,

compiler/rustc_infer/src/infer/mod.rs

@@ -12,8 +12,7 @@ pub use SubregionOrigin::*;
 pub use ValuePairs::*;
 
 use crate::traits::{
-    self, FulfillmentErrorLike, ObligationCause, ObligationInspector, PredicateObligations,
-    TraitEngine,
+    self, ObligationCause, ObligationInspector, PredicateObligations, TraitEngine,
 };
 use error_reporting::TypeErrCtxt;
 use free_regions::RegionRelations;
@@ -738,7 +737,7 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
 
 impl<'tcx, T> InferOk<'tcx, T> {
     /// Extracts `value`, registering any obligations into `fulfill_cx`.
-    pub fn into_value_registering_obligations<E: FulfillmentErrorLike<'tcx>>(
+    pub fn into_value_registering_obligations<E: 'tcx>(
         self,
         infcx: &InferCtxt<'tcx>,
         fulfill_cx: &mut dyn TraitEngine<'tcx, E>,

compiler/rustc_infer/src/traits/engine.rs

@@ -7,7 +7,32 @@ use rustc_middle::ty::{self, Ty, Upcast};
 
 use super::{ObligationCause, PredicateObligation};
 
-pub trait TraitEngine<'tcx, E: FulfillmentErrorLike<'tcx>>: 'tcx {
+/// A trait error with most of its information removed. This is the error
+/// returned by an `ObligationCtxt` by default, and suitable if you just
+/// want to see if a predicate holds, and don't particularly care about the
+/// error itself (except for if it's an ambiguity or true error).
+///
+/// use `ObligationCtxt::new_with_diagnostics` to get a `FulfillmentError`.
+#[derive(Clone, Debug)]
+pub enum ScrubbedTraitError<'tcx> {
+    /// A real error. This goal definitely does not hold.
+    TrueError,
+    /// An ambiguity. This goal may hold if further inference is done.
+    Ambiguity,
+    /// An old-solver-style cycle error, which will fatal.
+    Cycle(Vec<PredicateObligation<'tcx>>),
+}
+
+impl<'tcx> ScrubbedTraitError<'tcx> {
+    pub fn is_true_error(&self) -> bool {
+        match self {
+            ScrubbedTraitError::TrueError => true,
+            ScrubbedTraitError::Ambiguity | ScrubbedTraitError::Cycle(_) => false,
+        }
+    }
+}
+
+pub trait TraitEngine<'tcx, E: 'tcx>: 'tcx {
     /// Requires that `ty` must implement the trait with `def_id` in
     /// the given environment. This trait must not have any type
     /// parameters (except for `Self`).
@@ -73,10 +98,6 @@ pub trait TraitEngine<'tcx, E: FulfillmentErrorLike<'tcx>>: 'tcx {
     ) -> Vec<PredicateObligation<'tcx>>;
 }
 
-pub trait FulfillmentErrorLike<'tcx>: Debug + 'tcx {
-    fn is_true_error(&self) -> bool;
-}
-
-pub trait FromSolverError<'tcx, E>: FulfillmentErrorLike<'tcx> {
+pub trait FromSolverError<'tcx, E>: Debug + 'tcx {
     fn from_solver_error(infcx: &InferCtxt<'tcx>, error: E) -> Self;
 }

compiler/rustc_infer/src/traits/mod.rs

@@ -22,7 +22,7 @@ pub use self::ImplSource::*;
 pub use self::SelectionError::*;
 use crate::infer::InferCtxt;
 
-pub use self::engine::{FromSolverError, FulfillmentErrorLike, TraitEngine};
+pub use self::engine::{FromSolverError, ScrubbedTraitError, TraitEngine};
 pub use self::project::MismatchedProjectionTypes;
 pub(crate) use self::project::UndoLog;
 pub use self::project::{