The heapless feature does not select allocation-free serialization

Found in Report #9 v1 · Updated 2026-10-28 13:51 UTC

Description

With heapless enabled, human-readable serialization should use an allocation-free buffer; the serializer itself may still allocate. Instead, the following implementations can allocate while formatting:

Serialize for Ipv4Net
Serialize for Ipv6Net
Serialize for IpNet, which delegates to those implementations

The feature definition describes heapless as avoiding dynamic allocations during serialization. In the serialization implementation, the buffer path is selected by ser_as_str, while heapless alone uses collect_str.

Steps to Reproduce

Create a project with these files:

`Cargo.toml`

[package]
name = "ipnet-heapless-repro"
version = "0.1.0"
edition = "2021"
[workspace]
[features]
heapless = ["ipnet/heapless"]
legacy = ["ipnet/serde", "ipnet/ser_as_str"]
[dependencies]
ipnet = "=2.12.2"
serde = "=1.0.229"
serde_test = "=1.0.177"
allocation-counter = "=0.8.1"
heapless = "=0.9.2"

`src/main.rs`

use serde_test::{assert_ser_tokens, Configure, Token};

fn main() {
    let net: ipnet::Ipv4Net = "255.255.255.255/32".parse().unwrap();
    let info = allocation_counter::measure(|| {
        assert_ser_tokens(&net.readable(), &[Token::Str("255.255.255.255/32")]);
    });
    assert_eq!(info.count_total, 0);
}

Run each command separately:

cargo +1.91.1 run --features heapless
cargo +1.91.1 run --features legacy

serde_test checks the exact string and its serialize_str does not allocate. The example measures serialization after parsing; the assertion expects zero allocations. legacy enables serde and ser_as_str as a control.

Expected Behavior

Both commands succeed because formatting uses an allocation-free buffer with this serializer.

Actual Behavior

The heapless command fails the assertion (three allocations in the observed run); the legacy command succeeds. Allocation counts can vary with the environment.

Environment

ipnet 2.12.2, serde 1.0.229, heapless 0.9.2, Rust 1.91.1, macOS arm64.

Change history

v1 · created · 2026-10-28 13:36 UTC

{
  "title": "The heapless feature does not select allocation-free serialization",
  "body": "## Description\n\nWith `heapless` enabled, human-readable serialization should use an allocation-free buffer; the serializer itself may still allocate. Instead, the following implementations can allocate while formatting:\n\n- `Serialize` for `Ipv4Net`\n- `Serialize` for `Ipv6Net`\n- `Serialize` for `IpNet`, which delegates to those implementations\n\nThe [feature definition](https://github.com/krisprice/ipnet/blob/13c5c5873c02127679b1e8e64b538e4e36942640/Cargo.toml#L18) describes `heapless` as avoiding dynamic allocations during serialization. In the [serialization implementation](https://github.com/krisprice/ipnet/blob/13c5c5873c02127679b1e8e64b538e4e36942640/src/ipnet_serde.rs), the buffer path is selected by `ser_as_str`, while `heapless` alone uses `collect_str`.\n\n## Steps to Reproduce\n\nCreate a project with these files:\n\n### `Cargo.toml`\n\n```toml\n[package]\nname = \"ipnet-heapless-repro\"\nversion = \"0.1.0\"\nedition = \"2021\"\n[workspace]\n[features]\nheapless = [\"ipnet/heapless\"]\nlegacy = [\"ipnet/serde\", \"ipnet/ser_as_str\"]\n[dependencies]\nipnet = \"=2.12.2\"\nserde = \"=1.0.229\"\nserde_test = \"=1.0.177\"\nallocation-counter = \"=0.8.1\"\nheapless = \"=0.9.2\"\n```\n\n### `src/main.rs`\n\n```rust\nuse serde_test::{assert_ser_tokens, Configure, Token};\n\nfn main() {\n    let net: ipnet::Ipv4Net = \"255.255.255.255/32\".parse().unwrap();\n    let info = allocation_counter::measure(|| {\n        assert_ser_tokens(&net.readable(), &[Token::Str(\"255.255.255.255/32\")]);\n    });\n    assert_eq!(info.count_total, 0);\n}\n```\n\nRun each command separately:\n\n```sh\ncargo +1.91.1 run --features heapless\ncargo +1.91.1 run --features legacy\n```\n\n`serde_test` checks the exact string and its `serialize_str` does not allocate. The example measures serialization after parsing; the assertion expects zero allocations. `legacy` enables `serde` and `ser_as_str` as a control.\n\n## Expected Behavior\n\nBoth commands succeed because formatting uses an allocation-free buffer with this serializer.\n\n## Actual Behavior\n\nThe `heapless` command fails the assertion (three allocations in the observed run); the `legacy` command succeeds. Allocation counts can vary with the environment.\n\n## Environment\n\nipnet 2.12.2, serde 1.0.229, heapless 0.9.2, Rust 1.91.1, macOS arm64.",
  "validity": "reported",
  "upstream_status": "not_reported"
}

v2 · link_added · 2026-10-28 13:51 UTC

{
  "id": "dfd0e29a-c3f9-4af9-8668-dfcfe98f6288",
  "kind": "issue",
  "url": "https://github.com/krisprice/ipnet/issues/73",
  "version": null,
  "target_report_id": null,
  "target_revision_no": null,
  "edit_version": 1
}

The heapless feature does not select allocation-free serialization