Rollup merge of rust-lang#110563 - bryangarza:refactor-trait-selection-error-reporting, r=compiler-errors

Break up long function in trait selection error reporting + clean up nearby code

- Move blocks of code into their own functions
- Replace a few function argument types with their type aliases
- Create "AppendConstMessage" enum to replace a nested `Option`.

Author: matthiaskrgr

compiler/rustc_hir_typeck/src/method/mod.rs

@@ -300,8 +300,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         trait_def_id: DefId,
         self_ty: Ty<'tcx>,
         opt_input_types: Option<&[Ty<'tcx>]>,
-    ) -> (traits::Obligation<'tcx, ty::Predicate<'tcx>>, &'tcx ty::List<ty::subst::GenericArg<'tcx>>)
-    {
+    ) -> (traits::PredicateObligation<'tcx>, &'tcx ty::List<ty::subst::GenericArg<'tcx>>) {
         // Construct a trait-reference `self_ty : Trait<input_tys>`
         let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
             match param.kind {

compiler/rustc_infer/src/traits/mod.rs

@@ -123,7 +123,7 @@ pub struct FulfillmentError<'tcx> {
 #[derive(Clone)]
 pub enum FulfillmentErrorCode<'tcx> {
     /// Inherently impossible to fulfill; this trait is implemented if and only if it is already implemented.
-    CodeCycle(Vec<Obligation<'tcx, ty::Predicate<'tcx>>>),
+    CodeCycle(Vec<PredicateObligation<'tcx>>),
     CodeSelectionError(SelectionError<'tcx>),
     CodeProjectionError(MismatchedProjectionTypes<'tcx>),
     CodeSubtypeError(ExpectedFound<Ty<'tcx>>, TypeError<'tcx>), // always comes from a SubtypePredicate

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -591,7 +591,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
     fn evaluate_nested_obligations(
         &self,
         ty: Ty<'_>,
-        nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
+        nested: impl Iterator<Item = PredicateObligation<'tcx>>,
         computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>,
         fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
         predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,