"Unrecognized option 'release'" when compiling with rustc in release mode

Question

I have a simple .rs file and want to compile it using rustc. I always heard about how important it is to compile in release mode, because otherwise my Rust program will be slow.

However, if I use the often quoted --release flag, it doesn't work. What is everyone talking about if the flag doesn't even exist?

$ rustc --release foo.rs
error: Unrecognized option: 'release'.

Answer 1

You made a simple mistake: The --release flag is a flag for cargo. The easiest way you can turn on optimizations with rustc is using the -O flag.

Examples:

rustc -O foo.rs
rustc -C opt-level=3 foo.rs
cargo build --release

---

A bit more detail:

You can compile your Rust program with various levels of optimization. rustc -C help says:

-C opt-level=val     -- optimize with possible levels 0-3, s, or z

To have the most control, you should compile with rustc -C opt-level=3 foo.rs (or any other level). However, this isn't always necessary. Often, you can use -O; rustc --help says:

-O                      Equivalent to -C opt-level=2

Most of the time rustc -O foo.rs is the right choice.

Cargo, on the other hand, works a bit different; or at least the --release flag does. Cargo has different profiles which dictate how cargo invokes rustc. The most important ones are dev (development- or debug-mode) and release. The --release flag switches the default profile from dev to release (duh!). This potentially changes many flags of the rustc invocation. Most importantly, it changes the values for opt-level and debuginfo:

$ rustc -C debuginfo=2 -C opt-level=0    # in `dev` profile/debug mode
$ rustc -C debuginfo=0 -C opt-level=3    # in `release` profile

You can even change the settings for each profile in your Cargo.toml. You can find more information on profiles and on the default values used in this Cargo documentation.