Release Profiles and Binary Size 🟡

# Cargo.toml — Cargo's built-in defaults (what you get if you specify nothing)

[profile.release]
opt-level = 3        # Optimization level (0=none, 1=basic, 2=good, 3=aggressive)
lto = false          # Link-time optimization OFF
codegen-units = 16   # Parallel compilation units (faster compile, less optimization)
panic = "unwind"     # Stack unwinding on panic (larger binary, catch_unwind works)
strip = "none"       # Keep all symbols and debug info
overflow-checks = false  # No integer overflow checks in release
debug = false        # No debug info in release

[profile.release]
lto = true           # Full cross-crate optimization
codegen-units = 1    # Single codegen unit — maximum optimization opportunity
panic = "abort"      # No unwinding overhead — smaller, faster
strip = true         # Remove all symbols — smaller binary

[profile.release]
# All of the above, plus:
overflow-checks = true    # Keep overflow checks even in release (safety > speed)
debug = "line-tables-only" # Minimal debug info for backtraces without full DWARF
rpath = false             # Don't embed runtime library paths
incremental = false       # Disable incremental compilation (cleaner builds)

# For size-optimized builds (embedded, WASM):
# opt-level = "z"         # Optimize for size aggressively
# strip = "symbols"       # Strip symbols but keep debug sections

# Dev builds: optimize dependencies but not your code (fast recompile)
[profile.dev.package."*"]
opt-level = 2          # Optimize all dependencies in dev mode

# Release builds: override specific crate optimization
[profile.release.package.serde_json]
opt-level = 3          # Maximum optimization for JSON parsing
codegen-units = 1

# Test profile: match release behavior for accurate integration tests
[profile.test]
opt-level = 1          # Some optimization to avoid timeout in slow tests

[profile.release]
# Option 1: Fat LTO (default when lto = true)
lto = true
# All code merged into one LLVM module → maximum optimization
# Slowest compile, smallest/fastest binary

# Option 2: Thin LTO
lto = "thin"
# Each crate stays separate but LLVM does cross-module optimization
# Faster compile than fat LTO, nearly as good optimization
# Best trade-off for most projects

# Option 3: No LTO
lto = false
# Only intra-crate optimization
# Fastest compile, larger binary

# Option 4: Off (explicit)
lto = "off"
# Same as false

[profile.release]
lto = true

# Cargo.toml — for crates using the cc crate
[build-dependencies]
cc = "1.0"

// build.rs — enable cross-language (linker-plugin) LTO
fn main() {
    // The cc crate respects CFLAGS from the environment.
    // For cross-language LTO, compile C code with:
    //   -flto=thin -O2
    cc::Build::new()
        .file("csrc/fast_parser.c")
        .flag("-flto=thin")
        .opt_level(2)
        .compile("fast_parser");
}

# Enable linker-plugin LTO (requires compatible LLD or gold linker)
RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-fuse-ld=lld" \
    cargo build --release

# Install
cargo install cargo-bloat

# Show largest functions
cargo bloat --release -n 20
# Output:
#  File  .text     Size          Crate    Name
#  2.8%   5.1%  78.5KiB  serde_json       serde_json::de::Deserializer::parse_...
#  2.1%   3.8%  58.2KiB  regex_syntax     regex_syntax::ast::parse::ParserI::p...
#  1.5%   2.7%  42.1KiB  accel_diag         accel_diag::vendor::parse_smi_output
#  ...

# Show by crate (which dependencies are biggest)
cargo bloat --release --crates
# Output:
#  File  .text     Size Crate
# 12.3%  22.1%  340KiB serde_json
#  8.7%  15.6%  240KiB regex
#  6.2%  11.1%  170KiB std
#  5.1%   9.2%  141KiB accel_diag
#  ...

# Compare two builds (before/after optimization)
cargo bloat --release --crates > before.txt
# ... make changes ...
cargo bloat --release --crates > after.txt
diff before.txt after.txt

# Install (requires nightly)
cargo install cargo-udeps

# Find unused dependencies
cargo +nightly udeps --workspace
# Output:
# unused dependencies:
# `diag_tool v0.1.0`
# └── "tempfile" (dev-dependency)
#
# `accel_diag v0.1.0`  
# └── "once_cell"    ← was needed before LazyLock, now dead

cargo install cargo-machete
cargo machete
# Faster but may have false positives (heuristic, not compilation-based)

flowchart TD
    START["Binary too large?"] --> STRIP{"strip = true?"}
    STRIP -->|"No"| DO_STRIP["Add strip = true\n-50 to -70% size"]
    STRIP -->|"Yes"| LTO{"LTO enabled?"}
    LTO -->|"No"| DO_LTO["Add lto = true\ncodegen-units = 1"]
    LTO -->|"Yes"| BLOAT["Run cargo-bloat\n--crates"]
    BLOAT --> BIG_DEP{"Large dependency?"}
    BIG_DEP -->|"Yes"| REPLACE["Replace with lighter\nalternative or disable\ndefault features"]
    BIG_DEP -->|"No"| UDEPS["cargo-udeps\nRemove unused deps"]
    UDEPS --> OPT_LEVEL{"Need smaller?"}
    OPT_LEVEL -->|"Yes"| SIZE_OPT["opt-level = 's' or 'z'"]
    
    style DO_STRIP fill:#91e5a3,color:#000
    style DO_LTO fill:#e3f2fd,color:#000
    style REPLACE fill:#ffd43b,color:#000
    style SIZE_OPT fill:#ff6b6b,color:#000

# Default release
cargo build --release
ls -lh target/release/my-binary
time cargo build --release  # Note time

# Optimized release — add to Cargo.toml:
# [profile.release]
# lto = true
# codegen-units = 1
# strip = true
# panic = "abort"

cargo clean
cargo build --release
ls -lh target/release/my-binary  # Typically 30-50% smaller
time cargo build --release       # Typically 2-3× slower to compile

cargo install cargo-bloat
cargo bloat --release --crates
# Output:
#  File  .text     Size Crate
# 12.3%  22.1%  340KiB serde_json
#  8.7%  15.6%  240KiB regex

# For regex — try regex-lite if you don't need Unicode:
# regex-lite = "0.1"  # ~10× smaller than full regex

# For serde — disable default features if you don't need std:
# serde = { version = "1", default-features = false, features = ["derive"] }

cargo bloat --release --crates  # Compare after changes