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join was written in BAML. A rest pattern copied the array, then every element went through a generic string.from call and a + that built a rope node. It is now a native that appends into one buffer.
A string element is appended as is. Any other element is rendered exactly as string.from renders it, through the override-honoring walker, one element at a time. So ToString overrides still apply at every depth and run in element order, as in the BAML loop. The separator semantics are unchanged: an empty array joins to "", one element to its own rendering. New tests pin those cases plus override classes, structural classes, a structural class holding an override, enums, maps, nested lists, and a garbage collection inside an override. The 12-hole join below joins the speedtest suite as string::join-request-line-10k.
This is the top of the stack (#5431 → #5432 → this PR), so the table also compares with canary before any of the three.
workload
canary
PR 2
this PR
vs PR 2
vs canary
12-hole request line, join
513.14
432.20
22.87
18.9× faster
22.4× faster
[a, "/", b].join("")
80.55
70.48
7.70
9.2× faster
10.5× faster
12-hole request line, template
117.77
30.27
31.02
+2%
3.8× faster
`${a}/${b}`
20.06
4.17
4.33
+4%
4.6× faster
interfaces::to-string-interpolation-10k
51.39
32.89
33.34
+1%
−35%
repro `${a}${b}` (ns/op)
1,563
100
98
−1%
15.9× faster
repro `/v1/items/${a}/tags/${b}` (ns/op)
1,703
104
104
−1%
16.5× faster
repro `n=${k}` (ns/op)
764
244
230
−6%
3.3× faster
repro a.to_string() + b.to_string() (ns/op)
826
454
432
−5%
−48%
repro loop out = `${out}${c}` (ns/op)
2,508
540
556
+3%
4.5× faster
repro a + b (ns/op, control)
114
114
113
−1%
−1%
loop, no string built (control)
2.43
2.46
2.64
+7%
+9%
a + "/" + b (control)
4.07
4.09
4.37
+7%
+7%
programs::richards-50 (control)
510.13
496.96
515.00
+4%
+1%
compute::generic-apply-inferred-1m (control)
527.90
492.42
530.13
+8%
0%
compute::fib32-recursive (control)
744.02
739.12
740.95
0%
0%
compute::pure-call-1m (control)
115.37
115.58
116.12
0%
+1%
string::trim-padded-literal-100k (control)
33.89
33.93
35.77
+5%
+6%
The controls that move run no join and no template. Against PR 2 they execute the same number of instructions (perf stat: richards-50 18.3266 vs 18.3265 billion, generic-apply 18.43 vs 18.43 billion). Only front-end stall cycles grow (richards-50 415M → 685M). That is code layout in the fat-LTO binary, not extra work. The same controls also swing ±3% between canary, PR 1 and PR 2, which don't touch them either.
ms per job (median of 5) unless marked ns/op (median of 10). AMD Ryzen 9 5950X, Linux 7.0, release builds (fat LTO, rustc 1.98.0), speedtest VM row, telemetry off, pinned to CPU 7, canary and the three stacked PRs interleaved in 5 rounds. Rows without a :: name are temporary workloads that build one string per iteration, 10,000 iterations per job.
Summary by CodeRabbit
New Features
Array joining now honors custom string representations for values, including nested values, while preserving element order and separators.
Joining supports empty and single-element arrays, primitive values, enums, classes, unions, and generic calls.
Bug Fixes
Improved consistency when custom string representations run during garbage collection.
Tests
Expanded coverage for joining behavior and added a performance workload for large batches of request-line strings.
Review was skipped as selected files did not have any reviewable changes.
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Walkthrough
Array.join now delegates to native VM code. The VM renders array elements, processes ToString overrides, and retains continuation references across garbage collection. New tests cover join behavior, and new speedtest workloads build and join request lines.
Changes: Array.join
Layer / File(s)
Summary
Native join and override processing baml_language/crates/baml_builtins2/baml_std/baml/containers.baml, baml_language/crates/bex_vm/src/package_baml/array.rs, baml_language/crates/bex_vm/src/package_baml/root.rs
Array.join delegates to native VM code. The continuation dispatches element overrides, renders values, and updates its heap references across garbage collection.
Tests cover empty and populated arrays, primitive and structured values, ToString overrides, element order, large outputs, and garbage collection during overrides.
BAML, JavaScript, and Python workloads build and join 10,000 request lines. The benchmark configuration sets the expected result, job count, and warmup.
sequenceDiagram
participant Caller
participant ArrayJoin
participant PackageBamlImpl
participant join_honoring_overrides
participant JoinContinuation
participant ToStringOverride
Caller->>ArrayJoin: values and separator
ArrayJoin->>PackageBamlImpl: native join call
PackageBamlImpl->>join_honoring_overrides: copied array and separator
join_honoring_overrides->>JoinContinuation: initialize processing
JoinContinuation->>ToStringOverride: dispatch element override
ToStringOverride-->>JoinContinuation: return rendered value
JoinContinuation-->>Caller: return joined string
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Possibly related PRs
BoundaryML/baml#5423: Updates the GC root contract used by the join continuation in package_baml/root.rs.
Merge Risk: Moderate · previous review 10210
The new join test fails on correct output. Fix its slice bound before merging so the test suite can validate the change.
Security architecture risk: Low · up to 10210
The change preserves existing rendering behavior and override authority. No introduced security concern was identified, but the new resumable execution path carries low residual risk around exceptional execution.
Retained concerns
No architecture-level concerns identified.
Security review details
Security Blast Radius
inferred — The directly established exposure is programs invoking Array.join with caller-selected elements and separators. These values influence output and applicable ToString callbacks in the current execution context. The inspected change does not establish a new cross-service, tenant, or credential boundary.
Trust Boundaries and Controls
observed — Callback selection uses the existing ToString interface identity and implementation rules, binding the resolved method to its receiver and realized type arguments. Default implementations fall back to structural rendering. This resolver is unchanged between the supplied base and head; the base join already invoked ToString through string.from.
Pre-merge checks: 4 · 1
Failed checks: 1 warning
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Status
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Resolution
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Docstring coverage is 37.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Documentation-only edits and deletions within surv…
Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed checks: 4 passed
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Check skipped - CodeRabbit’s high-level summary is enabled.
Title check
The title clearly and concisely describes the primary change: moving array joining to a native implementation.
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Check skipped because no linked issues were found for this pull request.
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Check skipped because no linked issues were found for this pull request.
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Explanation
Docstring coverage is 37.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 8 functions across 4 files. (4 skipped: 4 unsupported.)
Resolution
Write docstrings for the functions missing them to satisfy the coverage threshold.
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A rabbit joins strings in a neat little line, With commas or spaces, the pieces align. Overrides hop in, one after another, While roots stay safe as the heap shifts under. Ten thousand request lines roll by with a spring.
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
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@baml_language/crates/baml_tests/baml_src/ns_array_join/array_join.baml:
- Line 153: Update the collection test’s prefix assertion on joined to use an
end bound of 50, or derive the bound from the expected prefix string, so the
comparison excludes the following comma.
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join was BAML: a rest pattern copied the array, then each element took a
generic string.from call and a + that built a rope node. It is now a
native that appends into one buffer. A string element is appended as is;
any other element is rendered as string.from renders it, through the
override-honoring walker, one element at a time, so ToString overrides
still apply at every depth and run in element order.
Already reviewed the last commit. Use @coderabbitai full review to rerun a review of the entire changeset.
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⚠️ Please fix the size gate issues or acknowledge them by updating baselines.
Artifact
Platform
File
Gzip
Gated on
Baseline
Delta
Status
✅
baml-cli
Linux
🔒 64.1 MB
27.3 MB
file
73.1 MB
-9.0 MB (-12.3%)
OK
❌
packed-program
Linux
🔒 41.4 MB
16.9 MB
file
29.2 MB
+12.2 MB (+41.9%)
FAIL
❌
bridge_wasm
WASM
27.5 MB
🔒 8.1 MB
gzip
5.7 MB
+2.4 MB (+42.8%)
FAIL
🔒 = the size this artifact is GATED on (ceiling + delta). Binaries gate on file size (installed binary); WASM gates on gzip (download size). The other size is shown for information only.
Details & how to fix
Violations:
packed-program (Linux) file_bytes: 41.4 MB exceeds limit of 30.1 MB (exceeded by +11.3 MB, policy: max_file_bytes)
packed-program (Linux) file_delta_pct: +41.9% exceeds limit of 3.0% (exceeded by +38.9pp, policy: max_delta_pct)
bridge_wasm (WASM) gzip_bytes: 8.1 MB exceeds limit of 5.9 MB (exceeded by +2.2 MB, policy: max_gzip_bytes)
bridge_wasm (WASM) gzip_delta_pct: +42.8% exceeds limit of 3.0% (exceeded by +39.8pp, policy: max_delta_pct)
…#5436)
Fixes a flaky `btel_bcs` test that failed on [CI for
BoundaryML#5433](https://github.com/BoundaryML/baml/actions/runs/40550606011/job/122354068514)
and passed on a re-run:
```
windows_cross_batches_deduplicate_and_finish_drains_the_final_partial_file
assertion `left == right` failed (tests/publisher.rs:121)
left: [(3, 1), (1, 3), (2, 3)]
right: [(1, 3), (2, 3), (3, 1)]
```
**What raced.** Since BoundaryML#5390, delivery prepares a recording's files
concurrently (`run_worker` spawns each prepare into a `JoinSet`). So the
server can get file 3's prepare before file 1's. Five tests read the
prepares in the order the mock server got them. A sixth,
`a_plan_over_the_cas_reservation_holds_up_only_another_such_plan`,
waited for file 3's upload and then assumed file 2's had already
arrived.
**Order is not a contract, so this is a test bug, not a delivery bug.**
BCS applies a recording's files in sequence order however their prepares
and uploads arrive. See invariant 2 in `docs/design/projection.md`,
tested by `files_apply_in_sequence_order_however_their_uploads_arrive`.
The prepare handler is per file. Canary's own docs say recordings upload
"in whatever order their uploads end", and the terminal file is the one
with the highest sequence.
**Fix.** `prepare_requests` returns the prepares sorted by
`recording_file_sequence`, with a stable sort so retries keep their
order. The CAS-reservation test now waits for both small plans' uploads.
Every property the tests check still holds: dedup across windows,
candidate counts per file, the terminal file is last by sequence, and
finishing twice adds nothing.
**How I proved it.** I added a 50 ms sleep before file 1's prepare in
`delivery.rs`. On canary, five tests failed the same way CI did. With
this PR, they all passed. They also passed with every prepare's order
reversed and with random 0–30 ms jitter (40 whole-crate stress runs).
The injection is not committed.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
<!-- coderabbit-ai-generated:v1 -->
## Summary by CodeRabbit
* **Tests**
* Updated upload reservation tests to handle capture and recording
uploads being observed in either order.
* Added checks for prepared uploads arriving concurrently, confirming
they are processed by sequence while preserving arrival order for
matching sequences. These updates improve coverage of upload timing and
ordering behavior without changing the application’s user-facing
functionality.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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joinwas written in BAML. A rest pattern copied the array, then every element went through a genericstring.fromcall and a+that built a rope node. It is now a native that appends into one buffer.A string element is appended as is. Any other element is rendered exactly as
string.fromrenders it, through the override-honoring walker, one element at a time. SoToStringoverrides still apply at every depth and run in element order, as in the BAML loop. The separator semantics are unchanged: an empty array joins to"", one element to its own rendering. New tests pin those cases plus override classes, structural classes, a structural class holding an override, enums, maps, nested lists, and a garbage collection inside an override. The 12-holejoinbelow joins the speedtest suite asstring::join-request-line-10k.This is the top of the stack (#5431 → #5432 → this PR), so the table also compares with canary before any of the three.
join[a, "/", b].join("")`${a}/${b}`interfaces::to-string-interpolation-10k`${a}${b}`(ns/op)`/v1/items/${a}/tags/${b}`(ns/op)`n=${k}`(ns/op)a.to_string() + b.to_string()(ns/op)out = `${out}${c}`(ns/op)a + b(ns/op, control)a + "/" + b(control)programs::richards-50(control)compute::generic-apply-inferred-1m(control)compute::fib32-recursive(control)compute::pure-call-1m(control)string::trim-padded-literal-100k(control)The controls that move run no
joinand no template. Against PR 2 they execute the same number of instructions (perf stat:richards-5018.3266 vs 18.3265 billion,generic-apply18.43 vs 18.43 billion). Only front-end stall cycles grow (richards-50415M → 685M). That is code layout in the fat-LTO binary, not extra work. The same controls also swing ±3% between canary, PR 1 and PR 2, which don't touch them either.ms per job (median of 5) unless marked ns/op (median of 10). AMD Ryzen 9 5950X, Linux 7.0, release builds (fat LTO, rustc 1.98.0),
speedtestVM row, telemetry off, pinned to CPU 7, canary and the three stacked PRs interleaved in 5 rounds. Rows without a::name are temporary workloads that build one string per iteration, 10,000 iterations per job.Summary by CodeRabbit