Brummel
449df13c9c
`Subst::apply` rebuilt every `Type::Fn` with `param_modes: vec![]` and `ret_mode: Implicit`, discarding the modes the surrounding type carried. That was harmless while nothing downstream of inference read fn-type modes — but it is a latent under-specification: substitution is meant to be type-preserving, and a fn type's modes are part of its identity (an `(own T) -> (own U)` contract is not the same contract as its borrow-returning sibling). Modes are positional over `params`, and substitution remaps each param type element-wise without changing the arity, so the mode lists stay aligned — preserving them is a clone, not a re-derivation. The typed-MIR boundary (milestone 0060) makes this load-bearing: a node type synthesised by `lower_to_mir::synth_pure` ends with `subst.apply`, and the codegen drop path reads `ret_mode == Own` off a call's callee node to decide RC trackability. With the modes stripped here, an Own-returning call read back `Implicit`, nothing was trackable, and a heap-allocated binder leaked at scope close. Preserving the modes restores that signal at the source rather than re-deriving it in codegen (which is exactly the re-derivation the milestone exists to delete). Safe by construction: `unify`'s Fn arm destructures modes with `..` and compares only params/ret/effects, so carrying modes through `apply` cannot change unification; and `Type`'s custom `PartialEq` already treats `Implicit` and `Own` as interchangeable (only `Borrow` is distinct), so equality outcomes are unaffected except where preserving a `Borrow` is the more correct answer anyway. Whole `ailang-check` suite stays green. RED pin: crates/ailang-check/tests/subst_preserves_modes.rs.