Brummel
2e0006029b
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.
80 lines
2.4 KiB
Plaintext
80 lines
2.4 KiB
Plaintext
; Iter 18g tidy follow-up RED-test fixture: a let-binder whose
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; value is a Term::Var referencing a non-Own fn-param must
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; inherit the param's mode for codegen's drop-emission gates.
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;
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; The carve-out documented in 18d.4's Iter A fix: the gate
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; checks `scrutinee_is_owned` against `current_param_modes`,
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; but only fn-params are recorded there. A let-binder aliasing
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; an Implicit-mode fn-param passes the gate (its lookup
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; misses, default = owned), so the arm-close drop fires on
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; memory the caller still references.
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;
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; Pattern: `pin_aliased` takes `t` as Implicit-mode (default,
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; no annotation); aliases it via `(let a t ...)`; matches `a`.
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; In the TNode arm the pattern-bindings `l` / `r` would be
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; dec'd at arm close because `a` looks owned to the gate —
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; corrupting the caller's tree.
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;
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; Pre-fix under --alloc=rc: refcount underflow at the second
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; iteration of `loop`, where the recursive call into
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; `pin_aliased(t)` re-loads cells the previous iteration
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; freed.
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; Post-fix: clean exit, output `0`. Stats are not the focus
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; here (the caller's tree leaks under the Implicit-mode
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; back-compat lane); the test asserts only correctness.
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(module rc_let_alias_implicit_param
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(data Tree
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(ctor TLeaf)
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(ctor TNode (con Int) (con Tree) (con Tree)))
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(fn build
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(type
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(fn-type
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(params (con Int))
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(ret (own (con Tree)))))
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(params d)
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(body
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(if (app == d 0)
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(term-ctor Tree TLeaf)
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(term-ctor Tree TNode 1
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(app build (app - d 1))
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(app build (app - d 1))))))
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; Pin via let-alias: the alias `a` is a Var-binding on the
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; Implicit-mode (default) param `t`. Codegen's gate must
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; recognise that `a` inherits `t`'s mode and skip the arm-
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; close drop that would otherwise fragment the caller's
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; tree.
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(fn pin_aliased
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(type
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(fn-type
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(params (con Tree))
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(ret (con Int))))
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(params t)
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(body
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(let a t
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(match a
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(case (pat-ctor TLeaf) 0)
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(case (pat-ctor TNode v l r) 1)))))
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(fn loop
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(type
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(fn-type
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(params (con Int) (con Tree))
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(ret (con Int))))
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(params n t)
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(body
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(if (app == n 0)
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0
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(let _v (app pin_aliased t)
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(app loop (app - n 1) t)))))
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(fn main
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(type (fn-type (params) (ret (con Unit)) (effects IO)))
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(params)
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(body
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(let t (app build 2)
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(do io/print_int (app loop 3 t))))))
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