test(codegen): RED pin for owned-param leak in tail-recursion (#63)

An owned heap ADT param threaded through a tail-recursive fn whose
base-case arm neither forwards nor consumes it is never RC-dec'd, so
it leaks one slab per recursion step. Series-free minimal repro
(a plain `Box` ADT, no Series/RawBuf), so the pin guards the GENERAL
memory-model invariant rather than the Series surface that first
surfaced it (examples/series_sma.ail, live=8).

The pin builds the fixture with --alloc=rc, runs it under
AILANG_RC_STATS=1, and asserts live == 0 (allocs == frees). RED today:
`allocs=6 frees=4 live=2`. The fixture passes the replacement ctor
directly into the tail call (not let-bound) to isolate leg 1 — the
owned-param drop omission — from the second, distinct leg the
debugger found (a `let`-wrapped tail call also defeats the
scrutinee-husk shallow-dec gate; that leg is what pushes box_ctrl to
live=4 and series_sma to live=8, and gets its own pin once leg 1 is
green).

Cause (debugger, for the GREEN side): the fn-return Own-param dec in
crates/ailang-codegen/src/lib.rs (~1444) is gated behind
`!block_terminated`, so it fires only on the fall-through ret path;
crates/ailang-codegen/src/match_lower.rs (~805) drops only pattern
binders, never the fn's owned params. A tail-call-terminated match arm
thus has no drop site for an owned param with consume_count==0 that is
not forwarded into the tail call.

refs #63

crates/ail/tests/tail_recur_owned_param_no_leak_pin.rs

@@ -0,0 +1,124 @@
+//! RED-pin for the general owned-parameter drop-soundness leak across a
+//! (tail-)recursive fn whose base-case arm does not consume that param
+//! (bug #63 — the open tail of the Series milestone #61).
+//!
+//! Property protected: under `--alloc=rc`, when an owned heap ADT value
+//! is passed as an `(own ...)` parameter to a tail-recursive fn, and a
+//! recursive arm neither forwards that param into the tail call nor
+//! otherwise consumes it (here `run` rebuilds a fresh `Box` from the
+//! head element and passes THAT, dropping the incoming `b` on the
+//! floor), the superseded owned param MUST be RC-dec'd before the
+//! `musttail call`. The runtime RC stats line at exit must report
+//! `live == 0` (equivalently `allocs == frees`) even though stdout is
+//! already correct.
+//!
+//! This is deliberately Series/RawBuf-FREE: a plain user ADT (`Box`)
+//! leaks identically, so the pin guards the GENERAL memory-model drop
+//! path, not a Series-specific surface. A Series-based pin would let a
+//! Series-only patch pass while the real bug survives.
+//!
+//! Root cause: codegen never emits an owned-param drop on a tail-call-
+//! terminated match arm. The fn-return Own-param dec
+//! (`crates/ailang-codegen/src/lib.rs` ~line 1444) is gated behind
+//! `if !self.block_terminated` (~line 1437) and so only runs on the
+//! fall-through `ret` (base-case join) path; the per-arm drop in
+//! `crates/ailang-codegen/src/match_lower.rs` (~line 805) iterates
+//! only `arm_pushed_binders` (pattern binders), never the fn's owned
+//! params. The pre-tail-call shallow-dec at match_lower.rs ~line 721
+//! drops the SCRUTINEE husk only. So an owned param with
+//! `consume_count == 0` that is not forwarded into the tail call has
+//! NO drop site on the recursive arm and leaks one slab per recursion.
+//!
+//! As of HEAD this test fails: stderr reports
+//! `ailang_rc_stats: allocs=6 frees=4 live=2` — the incoming `Box`
+//! param leaks once per `ICons` element (list `[1,2]` → 2 leaks).
+//!
+//! Scope note: this fixture passes `(term-ctor Box B v)` DIRECTLY as
+//! the tail-call arg (so the scrutinee-husk shallow-dec DOES fire and
+//! `arg_input` does not leak), isolating ONLY the owned-param leg. A
+//! sibling shape that wraps the tail call in a `let`
+//! (`(let b_new (term-ctor Box B v) (tail-app run b_new rest))`)
+//! exhibits a SECOND, distinct leak: the `let`-wrapped tail call makes
+//! `arm.body` an `MTerm::Let` rather than `MTerm::App { tail: true }`,
+//! so `arm_body_is_tail_call` is false and the scrutinee-husk
+//! shallow-dec is skipped too — pushing the canonical `box_ctrl` repro
+//! and `examples/series_sma.ail` to `live=4`/`live=8`. That second leg
+//! is filed as a concern on this diagnosis; the minimal correct fix to
+//! the owned-param leg should be made first.
+
+use std::path::Path;
+use std::process::Command;
+
+fn ail_bin() -> &'static str {
+    env!("CARGO_BIN_EXE_ail")
+}
+
+#[test]
+fn alloc_rc_owned_param_not_forwarded_in_tail_recursion_does_not_leak() {
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+    let workspace = Path::new(manifest_dir).parent().unwrap().parent().unwrap();
+    let src = workspace
+        .join("examples")
+        .join("tail_recur_owned_param_no_leak_pin.ail");
+    let tmp = std::env::temp_dir().join(format!(
+        "ailang_tail_recur_owned_param_no_leak_pin_{}",
+        std::process::id()
+    ));
+    std::fs::create_dir_all(&tmp).unwrap();
+    let out = tmp.join("bin");
+    let status = Command::new(ail_bin())
+        .args(["build", src.to_str().unwrap(), "--alloc=rc", "-o"])
+        .arg(&out)
+        .status()
+        .expect("ail build failed to run");
+    assert!(
+        status.success(),
+        "ail build --alloc=rc failed for tail_recur_owned_param_no_leak_pin.ail"
+    );
+    let output = Command::new(&out)
+        .env("AILANG_RC_STATS", "1")
+        .output()
+        .expect("execute binary");
+    assert!(
+        output.status.success(),
+        "binary exited non-zero: status {:?}",
+        output.status
+    );
+    let stderr = String::from_utf8(output.stderr).expect("stderr utf8");
+    let stats_line = stderr
+        .lines()
+        .find(|l| l.starts_with("ailang_rc_stats:"))
+        .unwrap_or_else(|| {
+            panic!("missing ailang_rc_stats line in stderr; stderr was:\n{stderr}")
+        });
+    let mut allocs: Option<u64> = None;
+    let mut frees: Option<u64> = None;
+    let mut live: Option<i64> = None;
+    for tok in stats_line.split_whitespace() {
+        if let Some(v) = tok.strip_prefix("allocs=") {
+            allocs = v.parse().ok();
+        } else if let Some(v) = tok.strip_prefix("frees=") {
+            frees = v.parse().ok();
+        } else if let Some(v) = tok.strip_prefix("live=") {
+            live = v.parse().ok();
+        }
+    }
+    let allocs = allocs.expect("missing allocs= field");
+    let frees = frees.expect("missing frees= field");
+    let live = live.expect("missing live= field");
+    assert_eq!(
+        live, 0,
+        "an owned (own (con Box)) param that a tail-recursive arm \
+         neither forwards into the tail call nor consumes leaks {live} \
+         slab(s) (allocs={allocs} frees={frees}); codegen emits no \
+         owned-param drop on a tail-call-terminated match arm — the \
+         fn-return Own-param dec in crates/ailang-codegen/src/lib.rs \
+         (~line 1444) is gated behind `!block_terminated`, and \
+         match_lower.rs (~line 805) drops only pattern binders, never \
+         the fn's owned params."
+    );
+    assert_eq!(
+        allocs, frees,
+        "alloc/free mismatch (allocs={allocs} frees={frees} live={live})"
+    );
+}