Rollup merge of #119613 - gavinleroy:expose-obligations, r=lcnr

Expose Obligations created during type inference.

This PR is a first pass at exposing the trait obligations generated and solved for during the type-check progress. Exposing these obligations allows for rustc plugins to use the public interface for proof trees (provided by the next gen trait solver).

The changes proposed track *all* obligations during the type-check process, this is desirable to not only look at the trees of failed obligations, but also those of successfully proved obligations. This feature is placed behind an unstable compiler option `track-trait-obligations` which should be used together with the `next-solver` option. I should note that the main interface is the function `inspect_typeck` made public in `rustc_hir_typeck/src/lib.rs` which allows the caller to provide a callback granting access to the `FnCtxt`.

r? `@lcnr`

compiler/rustc_infer/src/infer/at.rs

@@ -90,6 +90,7 @@ impl<'tcx> InferCtxt<'tcx> {
             universe: self.universe.clone(),
             intercrate,
             next_trait_solver: self.next_trait_solver,
+            obligation_inspector: self.obligation_inspector.clone(),
         }
     }
 }

compiler/rustc_infer/src/infer/mod.rs

@@ -13,7 +13,9 @@ use rustc_middle::infer::unify_key::{ConstVidKey, EffectVidKey};
 use self::opaque_types::OpaqueTypeStorage;
 pub(crate) use self::undo_log::{InferCtxtUndoLogs, Snapshot, UndoLog};
 
-use crate::traits::{self, ObligationCause, PredicateObligations, TraitEngine, TraitEngineExt};
+use crate::traits::{
+    self, ObligationCause, ObligationInspector, PredicateObligations, TraitEngine, TraitEngineExt,
+};
 
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
@@ -334,6 +336,8 @@ pub struct InferCtxt<'tcx> {
     pub intercrate: bool,
 
     next_trait_solver: bool,
+
+    pub obligation_inspector: Cell<Option<ObligationInspector<'tcx>>>,
 }
 
 impl<'tcx> ty::InferCtxtLike for InferCtxt<'tcx> {
@@ -708,6 +712,7 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
             universe: Cell::new(ty::UniverseIndex::ROOT),
             intercrate,
             next_trait_solver,
+            obligation_inspector: Cell::new(None),
         }
     }
 }
@@ -1718,6 +1723,15 @@ impl<'tcx> InferCtxt<'tcx> {
             }
         }
     }
+
+    /// Attach a callback to be invoked on each root obligation evaluated in the new trait solver.
+    pub fn attach_obligation_inspector(&self, inspector: ObligationInspector<'tcx>) {
+        debug_assert!(
+            self.obligation_inspector.get().is_none(),
+            "shouldn't override a set obligation inspector"
+        );
+        self.obligation_inspector.set(Some(inspector));
+    }
 }
 
 impl<'tcx> TypeErrCtxt<'_, 'tcx> {

compiler/rustc_infer/src/traits/mod.rs

@@ -13,12 +13,15 @@ use std::hash::{Hash, Hasher};
 
 use hir::def_id::LocalDefId;
 use rustc_hir as hir;
+use rustc_middle::traits::query::NoSolution;
+use rustc_middle::traits::solve::Certainty;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::{self, Const, ToPredicate, Ty, TyCtxt};
 use rustc_span::Span;
 
 pub use self::ImplSource::*;
 pub use self::SelectionError::*;
+use crate::infer::InferCtxt;
 
 pub use self::engine::{TraitEngine, TraitEngineExt};
 pub use self::project::MismatchedProjectionTypes;
@@ -116,6 +119,11 @@ pub type PredicateObligations<'tcx> = Vec<PredicateObligation<'tcx>>;
 
 pub type Selection<'tcx> = ImplSource<'tcx, PredicateObligation<'tcx>>;
 
+/// A callback that can be provided to `inspect_typeck`. Invoked on evaluation
+/// of root obligations.
+pub type ObligationInspector<'tcx> =
+    fn(&InferCtxt<'tcx>, &PredicateObligation<'tcx>, Result<Certainty, NoSolution>);
+
 pub struct FulfillmentError<'tcx> {
     pub obligation: PredicateObligation<'tcx>,
     pub code: FulfillmentErrorCode<'tcx>,