Const predicates in where clauses - #1508
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…ompatible) Complete the concrete-only half of CTFE where-clause const predicates: declaration-site evaluation of parameter-free predicates, concrete call-site discharge via first-class deferred obligations, and ADT well-formedness discharge at construction, signature, field, and body positions. All discharge runs via CTFE at the well-formedness/obligation layer, never inside the trait solver. The abstract/assumption route (proving a predicate on a still-symbolic parameter via a caller's own where-clause assumptions) is intentionally dropped at the marked seam in ty_check/mod.rs for FCO to re-add. A symbolic-argument predicate is reported as a concrete-only deferred failure rather than being proven or ICEing. Dropped scope, deferred to FCO: symbolic-parameter assumption discharge; impl-vs-trait method const-predicate compatibility; the ConstPredicateObligation call-origin metadata (re-added alongside the assumption route). Route all well-formedness diagnostic consumers (parameter, field, type-alias, impl-assoc-type, and body-expression WF) through WellFormedness::into_diag so a refuted const predicate surfaces uniformly as error 8-0085. Render const predicates in where-clause pretty-print. Add concrete-only fixtures for pretty-print and type-check; drop the tree-sitter syntax_node fixture since the tree-sitter grammar does not model const-expression predicates.
The CST formatter walked only trait-bound predicates (`WhereClause::iter()`),
so `fe fmt` silently dropped where-clause const predicates (`where N > 0`,
`where T::SIZE >= 50`, and the brace form `where { .. }`). Render both predicate
kinds via a `where_clause_predicate_doc` helper plus a `ToDoc for
WhereConstPredicate` impl, and count const predicates in the where-clause
emptiness guards so a const-predicate-only clause is not skipped.
Guard the behavior with an inline formatter test rather than a `tests/fixtures`
`.fe` file, since the tree-sitter grammar does not yet model const-expression
predicates and a fixture would trip `tree_sitter_parse_strict` (grammar support
is a separate follow-up).
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