Use a fresh InferCtxt when we 'speculatively' evaluate predicates

The existing `InferCtxt` may already have in-progress projections
for some of the predicates we may (recursively evaluate). This can
cause us to incorrect produce (and cache!) `EvaluatedToRecur`, leading
us to incorrectly mark a predicate as unimplemented.

We now create a fresh `InferCtxt` for each sub-obligation that we
'speculatively' evaluate. This causes us to miss out on some
legitimate caching opportunities, but ensures that our speculative
evaluation cannot pollute any of the caches from the 'real' `InferCtxt`.
The evaluation can still update *global* caches, but our global caches
don't have any notion of 'in progress', so this is fine.

Fixes #90662

Author: Aaron1011

compiler/rustc_trait_selection/src/traits/project.rs

@@ -16,6 +16,7 @@ use super::{
     ImplSourceGeneratorData, ImplSourcePointeeData, ImplSourceUserDefinedData,
 };
 use super::{Normalized, NormalizedTy, ProjectionCacheEntry, ProjectionCacheKey};
+use rustc_infer::infer::TyCtxtInferExt;
 
 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::infer::{InferCtxt, InferOk, LateBoundRegionConversionTime};
@@ -944,8 +945,8 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
                 Normalized { value: projected_ty, obligations: projected_obligations }
             };
 
-            let mut canonical =
-                SelectionContext::with_query_mode(selcx.infcx(), TraitQueryMode::Canonical);
+            let tcx = selcx.infcx().tcx;
+
             result.obligations.drain_filter(|projected_obligation| {
                 // If any global obligations always apply, considering regions, then we don't
                 // need to include them. The `is_global` check rules out inference variables,
@@ -957,10 +958,21 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
                 // the result to `EvaluatedToOkModuloRegions`, while an
                 // `EvaluatedToOk` obligation will never change the result.
                 // See #85360 for more details
-                projected_obligation.is_global(canonical.tcx())
-                    && canonical
+                if !projected_obligation.is_global(tcx) {
+                    return false;
+                }
+
+                // We create a fresh `InferCtxt` for each predicate we speculatively evaluate,
+                // so that we won't create (and cache) any spurious projection cycles in the main
+                // `InferCtxt`
+                tcx.infer_ctxt().enter(|infcx| {
+                    let mut canonical =
+                        SelectionContext::with_query_mode(&infcx, TraitQueryMode::Canonical);
+
+                    canonical
                         .evaluate_root_obligation(projected_obligation)
                         .map_or(false, |res| res.must_apply_considering_regions())
+                })
             });
 
             if use_cache {

src/test/ui/traits/issue-90662-projection-caching.rs

@@ -0,0 +1,34 @@
+// check-pass
+
+// Regression test for issue #90662
+// Tests that projection caching does not cause a spurious error
+
+trait HasProvider<T: ?Sized> {}
+trait Provider<M> {
+    type Interface: ?Sized;
+}
+
+trait Repository {}
+trait Service {}
+
+struct DbConnection;
+impl<M> Provider<M> for DbConnection {
+    type Interface = DbConnection;
+}
+
+struct RepositoryImpl;
+impl<M: HasProvider<DbConnection>> Provider<M> for RepositoryImpl {
+    type Interface = dyn Repository;
+}
+
+struct ServiceImpl;
+impl<M: HasProvider<dyn Repository>> Provider<M> for ServiceImpl {
+    type Interface = dyn Service;
+}
+
+struct TestModule;
+impl HasProvider<<DbConnection as Provider<Self>>::Interface> for TestModule {}
+impl HasProvider<<RepositoryImpl as Provider<Self>>::Interface> for TestModule {}
+impl HasProvider<<ServiceImpl as Provider<Self>>::Interface> for TestModule {}
+
+fn main() {}