Type-checker root-cause investigation: reproduce the 16–32 GiB allocation observed on Windows in #1

[hyperpolymath]

Follow-up to #1 / #12.

PR #12 added MAX_EXPR_DEPTH = 256 in crates/my-lang/src/checker.rs so that pathological inputs now produce a clean ExpressionTooDeep diagnostic instead of OOM-aborting. That fixes the symptom.

The root cause is still unknown:

• The checker's check_expr and is_assignable_from are structurally linear in AST size on the inputs I tested on Linux.
• I could not reproduce a 16–32 GiB allocation locally on Linux even with the issue's exact str_concat / format shape, depth, and function count.
• The original report is stable-x86_64-pc-windows-msvc, rustc 1.95.0 on Windows 11 Pro for Workstations 26300, against a ~330 LOC scaffold tool.

What this issue tracks

1. Obtain a copy of the actual ~330 LOC repro that triggered the OOM (the issue describes the shape but the file itself was not attached).
2. Reproduce on Windows with the same toolchain, ideally with heaptrack / dhat / Windows ETW + -Z time-passes to localise the allocator hotspot.
3. Confirm whether the cause is in the checker, in HIR/MIR lowering (only on my run), or elsewhere (e.g. a Windows allocator interaction or a debug-info / span-table blow-up). The issue title says "type-checker" but my check is only parse + check, so it's confirmed pre-lowering.
4. If a real superlinear hotspot exists, replace the depth guard's role-of-last-resort with a targeted fix (memoisation of str_concat/format subexpressions, or a structural rewrite).

Why this matters

The depth guard prevents OOM but at the cost of rejecting deep-but-otherwise-legal programs at 256 nesting levels. If the real cost is exponential in the number of templating sites (as the original report suggests with "aggregate complexity of nested string-building constructs"), then files well under the depth limit could still hit it. We need to know.

Out of scope

• Parser-side depth guard (tracked separately).
• Memoisation refactor as a speculative fix without first having a real measurement.

[hyperpolymath]

Investigation result: root cause is not checker complexity

Delivered across #29 (linear-cost harness) and #30 (reconstructed repro + dhat profiling + Windows CI).

What was measured

A counting-allocator harness measures check() heap traffic in isolation along the two axes the report implicates:

• Breadth (number of str_concat/format sites): doubling sites doubles bytes (180k → 323k → 638k → 1208k). Per-site cost is flat/declining — no quadratic or exponential term in site count.
• Depth (single nested chain, below the fix(checker): bound expression-recursion depth to prevent OOM (#1) #12 guard): total allocation is essentially flat (~150–210 KB), dominated by fixed Checker::new() + stdlib registration. Each check_expr level only clones a fixed-size builtin signature (str_concat: (Unknown, Unknown) -> String), so there is structurally no place for super-linear growth.
• Reconstructed ~330 LOC scaffold (the original file was never attached; this is a faithful same-shape rebuild): type-checks cleanly in ~1.5 MB — roughly 20,000× below the reported 16–32 GiB.
• Windows CI: the same harness now runs on a windows-latest matrix leg and passed, with the same bounded/linear behaviour on x86_64-pc-windows-msvc.

Why this is a conclusive negative, not just "couldn't reproduce"

We measured the specific mechanism the report blamed, on the axes that would expose super-linearity, on the same OS family, and the cost is provably linear with a small constant. The hypothesised failure mode is structurally absent from check_expr / is_assignable_from and the measurements agree. This addresses issue items 3 and 4: there is no checker hotspot, so the "out of scope" call (no speculative memoisation refactor) is correct — there is nothing to memoise.

Where the 16–32 GiB likely came from (now the leading hypotheses)

Since it is neither checker algorithmic cost nor a Windows-specific checker interaction, the remaining candidates are all outside check_expr:

• Recursive-descent parser stack growth / recovery allocation on the deep input (tracked separately, Parser: add a depth guard mirroring the type-checker's MAX_EXPR_DEPTH #15 / feat(parser): add MAX_PARSE_EXPR_DEPTH guard + recover partial AST (closes #15) #21).
• Recursive Drop of the deep Box<Expr> chain (already noted in test_deeply_nested_str_concat...).
• A debug-vs-release build difference, or debug-info / span-table bloat, on the original Windows toolchain.
• A construct in the exact original file not captured by the reconstruction (only the reporter has it).

Suggestion: reframe the #12 depth-guard rationale

MAX_EXPR_DEPTH = 256 was justified as "prevents the OOM." Given the above, the guard is not load-bearing for checker memory safety. Its genuine remaining value is bounding recursion stack depth (recursive check_expr, and separately the recursive AST Drop / parser). Recommend:

1. Update the MAX_EXPR_DEPTH doc-comment in crates/my-lang/src/checker.rs to say it bounds stack recursion, not heap blow-up, so the rationale doesn't mislead future readers.
2. Reconsider the limit on its own terms — it currently rejects deep-but-legal programs at 256 nesting levels for a problem that turned out to be elsewhere; the measurements show no memory reason it must be that low.

Status of the original items

• Item 1 (exact ~330 LOC repro): cannot be obtained (owner-only); a faithful reconstruction is now an in-tree fixture.
• Item 2 (Windows heaptrack/ETW): the portable substitute (dhat per-call-site profile + Windows CI leg) is in place and runs on every change.
• Items 3 & 4: answered — not a checker hotspot.

Suggest closing this issue with the cause recorded as "not checker complexity; confirmed Linux + Windows; permanent CI regression guard in place," and redirecting the still-open stack-recursion angle to the parser-side tracking issue (#15).

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