Feature Request: Allow `const` with struct types

[dijdzv]

Is your feature request related to a problem? Please describe.

Currently, const only accepts primitive types. When defining color constants for a graphics library, I cannot use const:

struct Color {
  r : Int
  g : Int
  b : Int
  a : Double
}

const RED : Color = { r: 255, g: 0, b: 0, a: 1.0 }
// ❌ Error: Color is not a valid constant type, only immutable primitive types are allowed.

Describe the solution you'd like

Allow const with struct types when all fields are constant expressions:

const RED : Color = { r: 255, g: 0, b: 0, a: 1.0 }  // Would like this to work

Describe alternatives you've considered

Using pub let works as a workaround:

pub let red : Color = { r: 255, g: 0, b: 0, a: 1.0 }

Additional context

• Environment: moon 0.1.20251227 (nightly, feature: rupes_recta)
• I'm imagining something similar to Rust's const structs:

// rust

struct Color { r: u8, g: u8, b: u8, a: u8 }

const BLACK: Color = Color { r: 0, g: 0, b: 0, a: 255 };

fn main() {
    println!("Black: ({}, {}, {}, {})", BLACK.r, BLACK.g, BLACK.b, BLACK.a);
}

• I do think that this should be part of the core library and should be delivered to all the MoonBit developers.

[dijdzv]

Update: As I learned more about MoonBit, I found a cleaner solution for this particular case—defining colors as enum variants like Color::Red, which effectively work as constants.
This made the feature unnecessary for my case, but const struct support might still be useful in other scenarios. Feel free to close if you disagree.

[Guest0x0]

Compared to normal toplevel let, the two distinguishing features of const is:

1. const can use capitalized name
2. const can be used in pattern matching

(1) is usually not a critical issue. For (2), the semantic of const struct value is not very clear. There are two possible ways to define match with a const struct value as pattern:

1. expand the const to a struct pattern
2. use the Eq implementation of the struct

These two match semantics may be inconsistent, for example if the Eq implementation ignores some field during comparison.

[dijdzv]

Thanks for the explanation.

A stretch thought: it could be interesting if custom Eq could also be used in pattern matching when the user proves the relevant properties through moon prove / predicate. Not sure how necessary or realistic that is, though.