Rust Cargo release build is not the fastest

By default, cargo build will build a debug binary at ./target/debug/filename, which is helpful for development/debug but without many binary optimizations, thus, runs slow.

When build with cargo build --release, it will build an optimized binary, which is ready for production:

--release                    Build artifacts in release mode, with optimizations

But it still not the fastest it can be.

Cargo profiles

Cargo has 4 builtin profiles: dev, release, test, and bench https://doc.rust-lang.org/cargo/reference/profiles.html which pre-defined build options. User can customize them in Cargo.toml, e.g:

[profile.dev]
opt-level = 1
debug = true

LTO - Link time optimizations

https://doc.rust-lang.org/cargo/reference/profiles.html#lto

Default profile.release sets lto = false.

[profile.release]
opt-level = 3
debug = false
split-debuginfo = '...'  # Platform-specific.
debug-assertions = false
overflow-checks = false
lto = false
panic = 'unwind'
incremental = false
codegen-units = 16
rpath = false

Set lto = true or lto = "fat" will

Performs "fat" LTO which attempts to perform optimizations across all crates within the dependency graph.

which makes build much slower, but the binary often faster 10-20% compare to lto = false. User may try lto = "thin" for faster build but the output still good comparable to lto = "fat"

"thin": Performs "thin" LTO. This is similar to "fat", but takes substantially less time to run while still achieving performance gains similar to "fat".

Reference

• https://doc.rust-lang.org/cargo/reference/profiles.html#lto

Happy building!