Compiler should reject union field definitions having default values

[zopsicle]

I tried this code:

#![feature(default_field_values)]

union U
{
    x: i32 = 1,
    y: f32 = 2.,
}

I expected to see this happen:

The compiler rejects the program. The default field values RFC does not mention unions, Default cannot be derived for unions, and there appears to be no way to use these default field values.

Instead, this happened:

The compiler accepts the program.

Meta

rustc --version --verbose:

1.89.0-nightly
(2025-06-11 e703dff8fe220b78195c)

[workingjubilee]

I wonder if we can enable deriving Default for unions if a default field value is given and that union variant selected via something like #[default]?

Nonetheless, it seems far better to simply yeet this unintended consequence and figure that sort of thing out later. ( Notably, this would require that additional #[default] extension to begin with. )