It can't refer to the `#[default]` string itself, because the parsed string doesn't necessarily map one-to-one with the string written in the source. For example, I can write

```rust
#[default="{\n\t42i\n}"]
```

and this will actually parse as

```rust
{
    42i
}
```

Which is to say, I can't map any span pointing into that string back into the original string, because I don't have any way of making the transformation.

I could theoretically transform any span into pointing at the *entire* string, though. But I don't know how to do that. I could presumably transform all spans in the expression back to that string, but what about errors raised by the parsing itself, e.g.

```rust
#[default="["]
```

This (presumably) triggers the error before I even get access to the resulting `Expr`.