Reducing a contract's size

Advice & examples

This page is made for developers familiar with lower-level concepts who wish to reduce their contract size significantly, perhaps at the expense of code readability.

Some common scenarios where this approach may be helpful:

• contracts intended to be tied to one's account management
• contracts deployed using a factory
• future advancements similar to the EVM on NEAR

There have been a few items that may add unwanted bytes to a contract's size when compiled. Some of these may be more easily swapped for other approaches while others require more internal knowledge about system calls.

Small wins

Using flags

When compiling a contract make sure to pass flag -C link-arg=-s to the rust compiler:

RUSTFLAGS='-C link-arg=-s' cargo build --target wasm32-unknown-unknown --release

Here is the parameters we use for the most examples in Cargo.toml:

[profile.release]
codegen-units = 1
opt-level = "s"
lto = true
debug = false
panic = "abort"
overflow-checks = true

You may want to experiment with using opt-level = "z" instead of opt-level = "s" to see if generates a smaller binary. See more details on this in The Cargo Book Profiles section. You may also reference this Shrinking .wasm Size resource.

Removing rlib from the manifest

Ensure that your manifest (Cargo.toml) doesn't contain rlib unless it needs to. Some NEAR examples have included this:

Adds unnecessary bloat

[lib]
crate-type = ["cdylib", "rlib"]

when it could be:

tip

[lib]
crate-type = ["cdylib"]

3. When using the Rust SDK, you may override the default JSON serialization to use Borsh instead. See this page for more information and an example.
4. When using assertions or guards, avoid using the standard assert macros like assert!, assert_eq!, or assert_ne! as these may add bloat for information regarding the line number of the error. There are similar issues with unwrap, expect, and Rust's panic!() macro.

Example of a standard assertion:

Adds unnecessary bloat

assert_eq!(contract_owner, predecessor_account, "ERR_NOT_OWNER");

when it could be:

tip

if contract_owner != predecessor_account {
  env::panic(b"ERR_NOT_OWNER");
}

Example of removing expect:

Adds unnecessary bloat

let owner_id = self.owner_by_id.get(&token_id).expect("Token not found");

when it could be:

tip

fn expect_token_found<T>(option: Option<T>) -> T {
  option.unwrap_or_else(|| env::panic_str("Token not found"))
}
let owner_id = expect_token_found(self.owner_by_id.get(&token_id));  

Example of changing standard panic!():

Adds unnecessary bloat

panic!("ERR_MSG_HERE"); 

when it could be:

tip

env::panic_str("ERR_MSG_HERE");  

Ready to use script

We have prepared a simple bash script that can be used to minify .wasm contract file. You can find it here.

The current approach to minification is the following:

1. Snip (i.e. just replace with unreachable instruction) few known fat functions from the standard library (such as float formatting and panic-related) with wasm-snip.
2. Run wasm-gc to eliminate all functions reachable from the snipped functions.
3. Strip unneeded sections, such as names with wasm-strip.
4. Run binaryen wasm-opt, which cleans up the rest.

Requirements to run the script:

• install wasm-snip and wasm-gc with Cargo:

cargo install wasm-snip wasm-gc

• install binaryen and wabt on your system. For Ubuntu and other Debian based Linux distributions run:

apt install binaryen wabt

danger

Minification could be rather aggressive, so you must test the contract after minification. Standalone NEAR runtime could be helpful here.

Lower-level approach

For a no_std approach to minimal contracts, observe the following examples:

• Tiny contract
• NEAR ETH Gateway
• This YouTube video where Eugene demonstrates a fungible token in no_std mode. The code for this example lives here.
• Examples using a project called nesdie.
• Note that Aurora has found success using rjson as a lightweight JSON serialization crate. It has a smaller footprint than serde which is currently packaged with the Rust SDK. See this example of rjson in an Aurora repository, although implementation details will have to be gleaned by the reader and won't be expanded upon here. This nesdie example also uses the miniserde crate, which is another option to consider for folks who choose to avoid using the Rust SDK.

Information on system calls

Expand to see what's available from sys.rs

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See full example on GitHub