Data::try_from reached unreachable!().

[Luro02]

The function Data::try_from has an unreachable!(), which will trigger, if you try to call this function with syn::Data::Union.

darling/core/src/ast/data.rs

Line 121 in c0e0cfd

syn::Data::Union(_) => unreachable!(),

Because this is definitely reachable, it should be either replaced with an Error

Error::custom("Union structs are not supported.")
    .with_span(&body);

or exposed by the data struct.

This should definitely not panic, because it makes it unnecessarily difficult to find the problem and it would be nice to get a beautiful error, instead of this:

error: proc-macro derive panicked
 --> tests/test_union.rs:3:10
  |
3 | #[derive(ShortHand)]
  |          ^^^^^^^^^
  |
  = help: message: internal error: entered unreachable code

error: aborting due to previous error

error: could not compile `shorthand`.

To learn more, run the command again with --verbose.

Implementation

/// Attempt to convert from a [`syn::Data`] instance.
///
/// # Errors
///
/// This function will error, if `body` is [`syn::Data::Union`].
pub fn try_from(body: &syn::Data) -> Result<Self> {
    match *body {
        syn::Data::Enum(ref data) => {
            let mut items = Vec::with_capacity(data.variants.len());
            let mut errors = Vec::new();
            for v_result in data.variants.iter().map(FromVariant::from_variant) {
                match v_result {
                    Ok(val) => items.push(val),
                    Err(err) => errors.push(err),
                }
            }

            if !errors.is_empty() {
                Err(Error::multiple(errors))
            } else {
                Ok(Data::Enum(items))
            }
        }
        syn::Data::Struct(ref data) => Ok(Data::Struct(Fields::try_from(&data.fields)?)),
        syn::Data::Union(u) => Err(
            Error::custom("Union structs are not supported.")
                .with_span(&u.fields)
        ),
    }
}

error: Union structs are not supported.
 --> tests/test_union.rs:5:20
  |
5 |   union ExampleUnion {
  |  ____________________^
6 | |     f1: u32,
7 | |     f2: f32,
8 | | }
  | |_^

error: aborting due to previous error

error: could not compile `shorthand`.

To learn more, run the command again with --verbose.

I am not sure, if the span is correct, or if there shouldn't be any span attached to it and instead the docs show how to attach the correct span:

fn from_derive_input(input: &DeriveInput) {
    let data = Data::try_from(&input.data).map_err(|e| e.with_span(&input.ident))
}

error: Union structs are not supported.
 --> tests/test_union.rs:5:7
  |
5 | union ExampleUnion {
  |       ^^^^^^^^^^^^

error: aborting due to previous error

error: could not compile `shorthand`.

To learn more, run the command again with --verbose.

---

There is also

darling/core/src/ast/data.rs

Line 21 in c0e0cfd

pub fn empty_from(src: &syn::Data) -> Self {

which should do the same as above, but this would be a breaking change, so it might be better to simply document, that this could panic, if called with Data::Union. I would also replace the unreachable!() with unreachable!("::darling::ast::Data::empty_from: Union structs are not supported.")

---

Another one is here (this can error?)

darling/core/src/options/mod.rs

Line 133 in c0e0cfd

Data::Union(_) => unreachable!(),

[TedDriggs]

I wonder if it's possible to get the span of the union keyword for the error here.

[TedDriggs]

Finally getting around to implementing this; it turns out that leaving the span information off completely produces the most helpful error; it's pretty clear where the union is, but it's less clear which macro is objecting to the presence of a union. Otherwise, this is very straightforward.