Closes the carve-out shared by 18d.4 Iter A and 18g.1's pre-tail-
call seam: a let-binder whose value is Term::Var aliasing a non-
Own fn-param now inherits the param's mode in current_param_modes
for the duration of the let body. Without this, (let a t (match
a ...)) where t is Implicit / Borrow defeated scrutinee_is_owned
(default for non-fn-param scrutinees was 'owned'), and the arm-
close drop fired on pattern-binders whose underlying memory
belongs to the caller — refcount underflow / SIGSEGV on the next
recursion.
Implementation: Term::Let lowering checks if value is Term::Var
referencing a name in current_param_modes; if so, inserts that
mode under the let-binder's name for the body and restores on
let-close (push/pop pattern, symmetric with locals).
Red:
alloc_rc_let_alias_of_implicit_param_does_not_dec_borrowed_children
on examples/rc_let_alias_implicit_param. Pre-fix: SIGSEGV / RC
underflow on the second iteration of loop. Post-fix: matches
alloc=gc ('0').
The Borrow-ret-mode case is already covered by the language-design
rule (typechecker rejects 'consume-while-borrowed' on the
borrow-aliasing shape); the Implicit-mode case is what the carve-
out actually surfaces. Both are now closed at codegen-time.
Two of the three known debts from the 18g tidy now resolved
(stat-of-N earlier in the session, let-alias here). The remaining
debt — tag-conditional partial-drop helper for the dynamic-tag
carve-outs in lower_match and emit_inlined_partial_drop — is the
substantively larger piece and stays queued.
