rust-binary-size

name	rust-binary-size
description	Guides Rust binary size reduction for ccusage. Use when changing release profiles, dependency features, native packaging size, or investigating Rust executable bloat.
paths	["rust/Cargo.toml","rust/**/*.rs","rust/**/*.toml","apps/ccusage/scripts/**"]
globs	rust/**/*.rs,rust/**/*.toml,apps/ccusage/scripts/**

Rust Binary Size

Use this skill when applying binary-size guidance to the Rust-first ccusage CLI, release profiles, native package binaries, dependency features, or size regression investigations.

Primary external reference: https://github.com/johnthagen/min-sized-rust

Baseline

Start by checking the existing release profile before adding new size settings:

sed -n '1,120p' rust/Cargo.toml

This workspace should keep stable release settings aligned with min-sized-rust unless a measured tradeoff argues otherwise:

• opt-level = "z" for workspace crates unless benchmarking shows "s" is smaller.
• lto = "fat" for maximum cross-crate dead-code elimination in release builds.
• codegen-units = 1 for better release optimization.
• panic = "abort" only for binaries where stack unwinding is not required.
• strip = "symbols" or an equivalent stable strip setting for packaged binaries.

Investigation

Prefer measurement before changing code or dependencies:

direnv exec . cargo build --manifest-path rust/Cargo.toml --release --bin ccusage
ls -lh rust/target/release/ccusage

When a size regression is not explained by the release profile, inspect dependency features and large symbols before editing:

direnv exec . cargo tree --manifest-path rust/Cargo.toml -e features -p ccusage
direnv exec . cargo bloat --manifest-path rust/Cargo.toml --release --bin ccusage --crates

If cargo bloat is unavailable, use the missing-tools skill or run it through the Nix/dev-shell path preferred by the repository. Do not add a new tool to the flake unless repeated project work needs it.

Safe Changes

Prefer stable, low-risk changes first:

• Disable unnecessary dependency default features when tests prove the disabled features are not required.
• Narrow optional dependency features instead of replacing a well-fitting crate.
• Remove unused code paths, generated assets, or format-heavy diagnostics from release-only paths only when behavior remains correct.
• Keep ccusage functionality, JSON output, table output, and packaging semantics unchanged unless the user explicitly asks for a behavior change.

Risky Changes

Treat these as opt-in only unless the user explicitly asks for an aggressive minimum-size experiment:

• Nightly-only flags such as -Zlocation-detail, -Zfmt-debug, panic=immediate-abort, or build-std.
• #![no_std], #![no_main], C entry points, or manual stdio.
• UPX or other binary packers, especially for distributed CLI artifacts.
• Dynamic Rust linking through prefer-dynamic, because deployment and ABI constraints make it unsuitable for ordinary CLI releases.

Validation

After changes, run formatting and the relevant behavioral checks:

direnv exec . just fmt
direnv exec . just test

For release-profile or packaging changes, also build the native CLI and compare the binary size with the previous measurement. Record the command and result in the PR body or review reply.