#size-optimization #perseus #web-dev


A plugin for Perseus to add size optimizations in one line of code, without needing to eject

10 releases

0.1.9 Apr 20, 2022
0.1.8 Apr 20, 2022
0.1.7 Dec 21, 2021
0.1.3 Nov 29, 2021
0.1.1 Oct 30, 2021

#445 in WebAssembly

Download history 39/week @ 2024-02-26 5/week @ 2024-03-04 6/week @ 2024-03-11 105/week @ 2024-04-01

112 downloads per month
Used in 2 crates (via tribble-app)

MIT license

167 lines

Perseus Size Optimization Plugin

Note: users of v0.1.0-v0.1.3 should upgrade to v0.1.4 if they're using Rust 2021, as an upstream bug occurs if you attempt to compile your app in release mode. See here for further details.

This is a very simple plugin for Perseus that applies size optimizations automatically, which decrease the size of your final Wasm bundle significantly, meaning faster loads for users because a smaller amount of data needs to be transferred to their browsers. Because Perseus renders a page almost immediately through static generation, and then the Wasm bundle is needed to make that page interactive, applying this plugin will decrease your app's time-to-interactive and its total blocking time (a Lighthouse metric that normal Perseus apps don't do so well in on mobile).

If you're new to Perseus, check it out on it's website and on GitHub! Basically though, it's a really fast and fully-featured web framework for Rust!


In your src/lib.rs, call the following function on PerseusApp:

I'm using `define_app!`

If you're still using define_app! from v0.3.3, you should upgrade to using PerseusApp soon, but you can still use this plugin by adding the following to the bottom of the define_app! call:

plugins: Plugins::new().plugin(perseus_size_opt, SizeOpts::default())

If you have any other plugins defined, add the .plugin() call where appropriate. You'll also need to add the following imports:

use perseus_size_opt::{perseus_size_opt, SizeOpts};

Once that's done, run perseus tinker to apply the optimizations (this needs a separate command because they involve modifying the .perseus/ directory), and then you can work with your app as normal!

If you ever want to uninstall the plugin, just remove the relevant .plugin() call and re-run perseus tinker, and it'll be completely removed.


This plugin currently performs the following optimizations:

  • wee_alloc -- an alternative allocator designed for Wasm that reduces binary size at the expense of slightly slower allocations
  • lto -- reduces binary size when set to true by enabling link-time optimizations
  • opt-level -- optimizes aggressively for binary size when set to z
  • codegen-units -- makes faster and smaller code when set to lower values (but makes compile times slower in release mode)

Note that all optimizations will only apply to release builds. except for the use of wee_alloc, which will also affect development builds.


There are a few defaults available for setting size optimization levels, or you can build your own custom settings easily by constructing SizeOpts manually.

  • ::default() -- enables all optimizations
  • ::default_no_lto() -- enables all optimizations except lto = true, because that can break compilation of execution on some hosting providers, like Netlify.
  • ::only_wee_alloc() -- only uses wee_alloc, applying no other optimizations
  • ::no_wee_alloc() -- applies all optimizations other than wee_alloc

Known Bugs

Compiling in release mode never finishes

This is due to an upstream issue in Rust 2021 that leads to size optimizations of the fluent-bundle package causing an overload of LLVM (see this issue). As of v0.1.4, this plugin accounts for this and does not attempt to optimize fluent-bundle with the default() options. However, this will increase bundle size, so it's recommended that, until this upstream issue is fixed, users of this plugin remain on Rust 2018 for now and use the default_2018() function instead. This will optimize fluent-bundle appropriately while ensuring that this bug does not occur. If moving to Rust 2018 is infeasible, you'll have to put up with slightly larger bundles for now until the upstream issue is fixed (this seems to depend on the LLVM team now).


This plugin is considered quite stable due to how basic its optimizations are (the whole thing is one file), and so its stability is mostly dependent on that of Perseus. If you're happy to use Perseus, you shouldn't need to worry about using this plugin as well (in fact, it's recommended that all Perseus apps use this plugin).




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