Rollup merge of #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`.

compiler/rustc_hir_typeck/src/method/mod.rs

@@ -300,8 +300,7 @@ pub(super) fn obligation_for_method(
         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 @@ fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) ->
     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

@@ -327,7 +327,7 @@ pub struct OnUnimplementedDirective {
     pub label: Option<OnUnimplementedFormatString>,
     pub note: Option<OnUnimplementedFormatString>,
     pub parent_label: Option<OnUnimplementedFormatString>,
-    pub append_const_msg: Option<Option<Symbol>>,
+    pub append_const_msg: Option<AppendConstMessage>,
 }
 
 /// For the `#[rustc_on_unimplemented]` attribute
@@ -337,11 +337,21 @@ pub struct OnUnimplementedNote {
     pub label: Option<String>,
     pub note: Option<String>,
     pub parent_label: Option<String>,
-    /// Append a message for `~const Trait` errors. `None` means not requested and
-    /// 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..
-    pub append_const_msg: Option<Option<Symbol>>,
+    // If none, should fall back to a generic message
+    pub append_const_msg: Option<AppendConstMessage>,
+}
+
+/// Append a message for `~const Trait` errors.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub enum AppendConstMessage {
+    Default,
+    Custom(Symbol),
+}
+
+impl Default for AppendConstMessage {
+    fn default() -> Self {
+        AppendConstMessage::Default
+    }
 }
 
 impl<'tcx> OnUnimplementedDirective {
@@ -419,10 +429,10 @@ fn parse(
                 }
             } else if item.has_name(sym::append_const_msg) && append_const_msg.is_none() {
                 if let Some(msg) = item.value_str() {
-                    append_const_msg = Some(Some(msg));
+                    append_const_msg = Some(AppendConstMessage::Custom(msg));
                     continue;
                 } else if item.is_word() {
-                    append_const_msg = Some(None);
+                    append_const_msg = Some(AppendConstMessage::Default);
                     continue;
                 }
             }

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

@@ -386,7 +386,7 @@ fn probe_assoc_types_at_expr(
     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 @@ fn probe_assoc_types_at_expr(
     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,
     ) {