Move has_self field to hir::AssocKind::Fn.

`hir::AssocItem` currently has a boolean `fn_has_self_parameter` field,
which is misplaced, because it's only relevant for associated fns, not
for associated consts or types. This commit moves it (and renames it) to
the `AssocKind::Fn` variant, where it belongs.

This requires introducing a new C-style enum, `AssocTag`, which is like
`AssocKind` but without the fields. This is because `AssocKind` values
are passed to various functions like `find_by_ident_and_kind` to
indicate what kind of associated item should be searched for, and having
to specify `has_self` isn't relevant there.

New methods:
- Predicates `AssocItem::is_fn` and `AssocItem::is_method`.
- `AssocItem::as_tag` which converts `AssocItem::kind` to `AssocTag`.

Removed `find_by_name_and_kinds`, which is unused.

`AssocItem::descr` can now distinguish between methods and associated
functions, which slightly improves some error messages.

compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs

@@ -301,7 +301,7 @@ pub fn dyn_compatibility_violations_for_assoc_item(
         ty::AssocKind::Const => {
             vec![DynCompatibilityViolation::AssocConst(item.name, item.ident(tcx).span)]
         }
-        ty::AssocKind::Fn => virtual_call_violations_for_method(tcx, trait_def_id, item)
+        ty::AssocKind::Fn { .. } => virtual_call_violations_for_method(tcx, trait_def_id, item)
             .into_iter()
             .map(|v| {
                 let node = tcx.hir_get_if_local(item.def_id);
@@ -344,7 +344,7 @@ fn virtual_call_violations_for_method<'tcx>(
     let sig = tcx.fn_sig(method.def_id).instantiate_identity();
 
     // The method's first parameter must be named `self`
-    if !method.fn_has_self_parameter {
+    if !method.is_method() {
         let sugg = if let Some(hir::Node::TraitItem(hir::TraitItem {
             generics,
             kind: hir::TraitItemKind::Fn(sig, _),

compiler/rustc_trait_selection/src/traits/vtable.rs

@@ -197,10 +197,8 @@ fn own_existential_vtable_entries_iter(
     tcx: TyCtxt<'_>,
     trait_def_id: DefId,
 ) -> impl Iterator<Item = DefId> {
-    let trait_methods = tcx
-        .associated_items(trait_def_id)
-        .in_definition_order()
-        .filter(|item| item.kind == ty::AssocKind::Fn);
+    let trait_methods =
+        tcx.associated_items(trait_def_id).in_definition_order().filter(|item| item.is_fn());
 
     // Now list each method's DefId (for within its trait).
     let own_entries = trait_methods.filter_map(move |&trait_method| {