Confusing Error("trailing characters") when attempting to deserialize array into empty struct

[rshk]

This is likely related to #540, but it seems a somewhat separate case.

This took me a long time to figure out; a better error message could have definitely shortened that time.

Expected behavior (1)

Attempting to deserialize a non-empty array into an object should fail with a meaningful error.

For example, this is the error returned when attempting to deserialize a string into an object:

Error("invalid type: string \"XX\", expected struct Item", line: 3, column: 20)

Actual behavior (1)

This is the actual error being returned, which seems to indicate invalid JSON syntax instead:

: Error("trailing characters", line: 3, column: 18)

Expected behavior (2)

Attempting to deserialize an empty array into an object should fail with a meaningful error.

Actual behavior (2)

It actually just produces an empty object.

Example code to reproduce

use std::collections::HashMap;

use serde::Deserialize;
use serde_json::Value;

fn main() {
    let data = r#"
        {"items": {
            "example": [1]
        }}
    "#;

    let v: Value = serde_json::from_str(data).expect("");
    println!("{:#?}", &v);

    let v: MyObj = serde_json::from_str(data).expect("");
    println!("{:#?}", &v);
}

#[derive(Deserialize, Debug)]
pub struct MyObj {
    items: Option<HashMap<String, Item>>,
}

#[derive(Deserialize, Debug)]
struct Item {}

Which fails with this output:

Object {
    "items": Object {
        "example": Array [
            Number(1),
        ],
    },
}

thread 'main' (845267) panicked at src/main.rs:16:47:
: Error("trailing characters", line: 3, column: 18)

Even more confusingly, it seems to work perfectly fine if the array is empty, for example:

{"items": { "example": [] }}

Cargo.toml for reference:

[package]
name = "example"
version = "0.1.0"
edition = "2024"

[dependencies]
serde = { version = "1.0.228", features = ["derive"] }
serde_json = "1.0.149"

[cyruspyre]

I don't know about your use case but the reason for (1) is because deserialize_struct implementation parses either as an array or object, look here. And the reason you are getting trailing character error is because... well as you stated the struct is empty so the visitor does nothing and the deserializer comes accross the unparsed element and flags it as trailing character error. Relevant code being here.

I don't think its possible to provide meaningful error for the latter. Cause at that point the visitor has done its thing and went out of context. Although, I might be wrong in that regard.