Rollup merge of #121326 - fmease:detect-empty-leading-where-clauses-on-ty-aliases, r=compiler-errors

Detect empty leading where clauses on type aliases

1. commit: refactor the AST of type alias where clauses
   * I could no longer bear the look of `.0.1` and `.1.0`
   * Arguably moving `split` out of `TyAlias` into a substruct might not make that much sense from a semantic standpoint since it reprs an index into `TyAlias.predicates` but it's alright and it cleans up the usage sites of `TyAlias`
2. commit: fix an oversight: An empty leading where clause is still a leading where clause
   * semantically reject empty leading where clauses on lazy type aliases
     * e.g., on `#![feature(lazy_type_alias)] type X where = ();`
   * make empty leading where clauses on assoc types trigger lint `deprecated_where_clause_location`
     * e.g., `impl Trait for () { type X where = (); }`

compiler/rustc_parse/src/parser/item.rs

@@ -971,11 +971,17 @@ impl<'a> Parser<'a> {
 
         let after_where_clause = self.parse_where_clause()?;
 
-        let where_clauses = (
-            TyAliasWhereClause(before_where_clause.has_where_token, before_where_clause.span),
-            TyAliasWhereClause(after_where_clause.has_where_token, after_where_clause.span),
-        );
-        let where_predicates_split = before_where_clause.predicates.len();
+        let where_clauses = TyAliasWhereClauses {
+            before: TyAliasWhereClause {
+                has_where_token: before_where_clause.has_where_token,
+                span: before_where_clause.span,
+            },
+            after: TyAliasWhereClause {
+                has_where_token: after_where_clause.has_where_token,
+                span: after_where_clause.span,
+            },
+            split: before_where_clause.predicates.len(),
+        };
         let mut predicates = before_where_clause.predicates;
         predicates.extend(after_where_clause.predicates);
         let where_clause = WhereClause {
@@ -994,7 +1000,6 @@ impl<'a> Parser<'a> {
                 defaultness,
                 generics,
                 where_clauses,
-                where_predicates_split,
                 bounds,
                 ty,
             })),