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.

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/rustc_middle/src/ty/instance.rs b/compiler/rustc_middle/src/ty/instance.rs
index 07f2a602f2b..faad0a82acb 100644
--- a/compiler/rustc_middle/src/ty/instance.rs
+++ b/compiler/rustc_middle/src/ty/instance.rs
@@ -746,7 +746,7 @@ impl<'tcx> Instance<'tcx> {
         let call_once = tcx
             .associated_items(fn_once)
             .in_definition_order()
-            .find(|it| it.kind == ty::AssocKind::Fn)
+            .find(|it| it.is_fn())
             .unwrap()
             .def_id;
         let track_caller =