What causes "Expected a boolean value" in bevyengine/bevy?

Use the literal keywords `true` or `false` directly

bevyengine/bevy · error · syn::Error

Expected a boolean value

Error message

Expected a boolean value

What it means

The boolean reflect attributes (`#[reflect(from_reflect = ...)]`, `#[reflect(type_path = ...)]`) are parsed by `extract_bool` (crates/bevy_reflect/derive/src/container_attributes.rs:665), which only accepts a literal `true`/`false` (`syn::Lit::Bool`). Any other expression — a string, an integer, a path, or a constant — fails with the compile error "Expected a boolean value" pointing at the value's span.

Source

Thrown at crates/bevy_reflect/derive/src/container_attributes.rs:673

    pub fn is_opaque(&self) -> bool {
        self.is_opaque
    }
}

/// Extract a boolean value from an expression.
///
/// The mapper exists so that the caller can conditionally choose to use the given
/// value or supply their own.
fn extract_bool(
    value: &Expr,
    mut mapper: impl FnMut(&LitBool) -> LitBool,
) -> Result<LitBool, syn::Error> {
    match value {
        Expr::Lit(syn::ExprLit {
            lit: syn::Lit::Bool(lit),
            ..
        }) => Ok(mapper(lit)),
        _ => Err(syn::Error::new(value.span(), "Expected a boolean value")),
    }
}

View on GitHub (pinned to 396ca72708)

Solutions

Use the literal keywords `true` or `false` directly
If the value comes from a macro, pass the literal through as tokens (e.g. a `bool`-typed macro rule `$v:literal`) instead of computing it

Example fix

// before
#[reflect(from_reflect = "false")] // or = 1, or = SOME_CONST

// after
#[reflect(from_reflect = false)]

Defensive patterns

Strategy: validation

Validate before calling

null

Prevention

Attribute values must be literal `true`/`false` tokens — never strings, numbers, or constants
In declarative macros, pass booleans as `$v:literal` tokens rather than interpolating computed values

When it happens

Trigger: Writing `#[reflect(type_path = "false")]`, `#[reflect(from_reflect = 1)]`, `#[reflect(from_reflect = SHOULD_DERIVE)]`, or any non-literal-boolean expression as the attribute value.

Common situations: Porting old attribute syntax that used other literal kinds; macro code substituting a variable into the attribute value (proc macros see tokens, not runtime values); typoed values.

Related errors

AI-assisted analysis of bevyengine/bevy@396ca72708 (2026-08-20). Data as JSON: /api/errors/a48f1706379720ee. Report an issue: GitHub.