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Introduction

mevy (pronounced “me-vee”) is a set of Rust procedural macros that make writing Bevy code less tedious. It doesn’t add plugins, resources, or systems — it just gives you better syntax for the patterns you already use.

The Problem

Bevy’s ECS and UI APIs are expressive but verbose. Consider spawning a styled panel — the same structure at three levels of verbosity:

// Raw Bevy

cmd.spawn((
    Node {
        width: Val::Px(300.0),
        height: Val::Px(200.0),
        padding: UiRect::all(Val::Px(16.0)),
        border_radius: BorderRadius::all(Val::Px(12.)),
        border: UiRect::all(Val::Px(2.0)),
        ..default()
    },
    BorderColor::all(Srgba::hex("#ee6677").unwrap().into()),
    BackgroundColor(Srgba::hex("#1a1a2e").unwrap().into()),
));

That’s a lot of boilerplate for a simple rounded rectangle. With mevy’s CSS-like mode:

// mevy — CSS-like mode
cmd.spawn(ui!((
    size: 300px 200px;
    padding: 16px;
    border_radius: 12px;
    border: 2px #ee6677;
    background: #1a1a2e;
)));

For quick UI, the slim mode cuts it even further:

// mevy — slim mode
cmd.spawn(ui!(
    w:300 h:200 p:16 round:12 
    border:2px#ee6677 bg:#1a1a2e
)?));

Three levels. Same result. And your IDE still knows what’s going on — mevy preserves the token structure so autocomplete and type inference work normally.

What mevy Gives You

A CSS-like language for Bevy UI

Write border_radius: 12px instead of BorderRadius::all(Val::Px(12.0)). mevy translates CSS concepts into Bevy components automatically. Edge selection (border: 5px 2px) and corner selection (border_radius: 5px 2px 8px) follow the same CSS conventions you already know.

Nested entity spawning without callbacks

Bevy’s .with_children() callback is the only way to build hierarchies, but it’s hard to reference children by name. mevy’s [child_name][...] syntax lets you name children and reference them anywhere - even before they’re defined.

Inline values without ceremony

Write #ff0000 for a color, 100px for a value, [>10px 5px] for a rectangle. No function calls, no type annotations, no Srgba::hex().unwrap() boilerplate.

Design Philosophy

Macros only. mevy doesn’t change how Bevy works - it just makes the code you write cleaner.

  • Token-based: Everything happens at compile time. Your IDE autocomplete, type inference, and error messages all work normally.
  • Version-aware: Feature flags handle Bevy API changes between versions. You can upgrade Bevy without changing your mevy code.
  • Composable: ui!{} and entity!{} work together. Use ui!{} inside entity!{} for nested UI, code!{} for inline values anywhere.
  • Extensible: Define your own UI components with custom fields. Reuse them across your project like CSS classes.

Supported Bevy Versions

mevy supports Bevy 0.15 through 0.18. You must pin your version in Cargo.toml:

mevy = { version = "0.3", features = ["0.18"] }