Break up long function in trait selection error reporting

- Move blocks of code into their own functions
- Replace a few function argument types with their type aliases

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>>,

compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs

@@ -341,6 +341,7 @@ pub struct OnUnimplementedNote {
     /// should fallback to a generic message, `Some(None)` suggests using the default
     /// appended message, `Some(Some(s))` suggests use the `s` message instead of the
     /// default one..
+    /// FIXME(bryangarza): Change this to an enum with the 3 variants described above.
     pub append_const_msg: Option<Option<Symbol>>,
 }
 

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -386,7 +386,7 @@ pub trait TypeErrCtxtExt<'tcx> {
     fn maybe_suggest_convert_to_slice(
         &self,
         err: &mut Diagnostic,
-        trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
+        trait_ref: ty::PolyTraitRef<'tcx>,
         candidate_impls: &[ImplCandidate<'tcx>],
         span: Span,
     );
@@ -3848,7 +3848,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
     fn maybe_suggest_convert_to_slice(
         &self,
         err: &mut Diagnostic,
-        trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
+        trait_ref: ty::PolyTraitRef<'tcx>,
         candidate_impls: &[ImplCandidate<'tcx>],
         span: Span,
     ) {