FAQ

Frequently asked questions about mevy.

Do I need to enable all features?

No. Specify only the Bevy version feature (e.g., features = ["0.18"]). The ui feature is enabled by default. Only enable experimental if you need the experimental helper macros.

Can I use mevy with any Bevy version?

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

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

Without a version feature, compilation will fail with a compile_error.

What happens if I forget the version feature?

You’ll get a compile_error! at the macro expansion site telling you to specify the version feature. The error message will be clear about what’s missing.

Does mevy add runtime overhead?

No. mevy is purely compile-time token manipulation. All macros expand to regular Rust code — no plugins, no resources, no systems, no runtime dependency. The generated code is identical to what you’d write by hand.

Are experimental features safe for production?

No. Experimental features are unstable and may change without notice. The core macros (entity!{}, ui!{}, code!{}) are stable and production-ready. Only use experimental features for prototyping.

Can I use the individual crates separately?

Yes:

mevy_core = { version = "0.1.1", features = ["0.18"] }
mevy_ui   = { version = "0.3.2", features = ["0.18"] }
mevy_ecs  = { version = "0.2.4", features = ["0.18"] }

use mevy_core::code;
use mevy_ui::ui;
use mevy_ecs::entity;

This is useful if you only need specific macros or want to avoid the experimental default feature on mevy_ecs.

Does mevy_ecs support Bevy 0.18?

Yes — all macros (entity!{}, ui!{}, code!{}) work with Bevy 0.18.

Can I use variables inside ui!{} built-in fields?

Currently, variables can only be used in custom fields (via $var). Built-in field aliases (like bg, w, px) cannot reference variables directly. This is a known limitation.

Is there LSP autocomplete support?

Yes. mevy is token-based, preserving the original token structure. LSP autocomplete works inside entity!{}, ui!{}, and code!{} — you get suggestions for Bevy types, methods, and components just like in regular Rust code.

How does mevy compare to raw Bevy UI?

Significantly less boilerplate:

// Raw Bevy
cmd.spawn((Node { width: Val::Px(100.0), height: Val::Px(100.0), ..default() },
    BackgroundColor(Srgba::hex("#ff0000").unwrap().into()),
    BorderRadius::all(Val::Px(6.0))));

// mevy
cmd.spawn(ui!((size: 100px 100px; background: #ff0000; border_radius: 6px;)));

When should I use ui!{} vs code!{}?

• ui!{} — when you need multiple UI components for an entity (a styled panel, button, etc.)
• code!{} — when you need a single value (a color, a Val, a UiRect)
• entity!{} — when you need to spawn or modify entities (with or without UI)

They’re designed to work together: entity!{} nests ui!{}, and code!{} works inside both. See Macros Overview for more.