Deserializing a String field inside a flattende struct fails if the field contains a valid integer

[tyilo]

What version of the csv crate are you using?

1.3.0

Briefly describe the question, bug or feature request.

When deserializing to a String field using serde, csv can handle a field containing 123.
However, when doing the same to a flattened field, csv can no longer deserialize 123 to the String field and I get the error:

Err(
    Error(
        Deserialize {
            pos: Some(
                Position {
                    byte: 24,
                    line: 4,
                    record: 3,
                },
            ),
            err: DeserializeError {
                field: None,
                kind: Message(
                    "invalid type: integer `123`, expected a string",
                ),
            },
        },
    ),
)

Include a complete program demonstrating a problem.

use serde::Deserialize;

#[allow(dead_code)]
#[derive(Debug, Deserialize)]
struct Inner {
    inner_str: String,
}

#[allow(dead_code)]
#[derive(Debug, Deserialize)]
struct Row {
    str: String,

    #[serde(flatten)]
    inner: Inner,
}

fn main() {
    let source = r#"
str,inner_str
A,A
123,A
A,123
    "#
    .trim();

    let mut reader = csv::Reader::from_reader(source.as_bytes());
    for line in reader.deserialize::<Row>() {
        dbg!(&line);
    }
}

What is the observed behavior of the code above?

Output:

[src/main.rs:29] &line = Ok(
    Row {
        str: "A",
        inner: Inner {
            inner_str: "A",
        },
    },
)
[src/main.rs:29] &line = Ok(
    Row {
        str: "123",
        inner: Inner {
            inner_str: "A",
        },
    },
)
[src/main.rs:29] &line = Err(
    Error(
        Deserialize {
            pos: Some(
                Position {
                    byte: 24,
                    line: 4,
                    record: 3,
                },
            ),
            err: DeserializeError {
                field: None,
                kind: Message(
                    "invalid type: integer `123`, expected a string",
                ),
            },
        },
    ),
)

What is the expected or desired behavior of the code above?

Output:

[src/main.rs:29] &line = Ok(
    Row {
        str: "A",
        inner: Inner {
            inner_str: "A",
        },
    },
)
[src/main.rs:29] &line = Ok(
    Row {
        str: "123",
        inner: Inner {
            inner_str: "A",
        },
    },
)
[src/main.rs:29] &line = Ok(
    Row {
        str: "A",
        inner: Inner {
            inner_str: "123",
        },
    },
)

[BurntSushi]

This does indeed look like a bug. I don't know when I'll have time to look into it, so patches are welcome if you can find a fix yourself.

I'll note that the presence of serde(flatten) concerns me. IIRC, it required some hacky things to make it work and it may not have been done correctly.

[tyilo]

It seems like serde is calling deserialize_string for str, but deserialize_any for inner_str. Is this a bug in serde?

[BurntSushi]

I don't know. I don't know much about serde internals. I do know that the serde(flatten) thing is a bit of a tortured feature though. It's one of those things that folks don't have to actually use, but it affords some very nice conveniences. But it just doesn't work in all cases.

I won't have time to look into this any time soon I'm afraid.

[tyilo]

Yeah, it seems to be unsupported in serde: serde-rs/serde#1881

That's unfortunate :/