CODEWITHSUNDEEP
jsonrustserde
the_endian
the_endian

Reputation: 2527

Why does this JSON parse fail in the specified location in Rust?

The following Rust program works fine using cargo run on cargo 1.39.0-nightly:

extern crate serde;
extern crate serde_json;
#[macro_use]
extern crate serde_derive;
use serde_json::Value as JsonValue;

#[derive(Serialize,Deserialize)]
struct JData
{
    names: Vec<String>
}
fn main() {
    let json_str = r#"

    {
        "names": ["James", "Carl", "Megan"]
    }   

    "#;

    let res = serde_json::from_str(json_str); // res is Result

    if res.is_ok()
    {
        let p: JData = res.unwrap();

        println!("{}", p.names[1]);
    }
    else
    {
        eprintln!("Sorry, couldn't parse JSON :(");
    }

}

However, if I change the line names: Vec<String> to names: Vec<u8> it fails with this error:

Sorry, couldn't parse JSON :(

Understandbly, this program should fail. But I don't understand why it reaches this line since this error is displayed as a result of let res = serde_json::from_str(json_str); which has nothing to do with the let p: JData = res.unwrap(); step of the program. In other words, the from_str() line has no dependency on the subsequent try to parse into JData struct line, yet the error indicates that the from_str() line failed, not that it could not parse into JData.

Upvotes: 2

Views: 213

Answers (1)

guest
guest

Reputation: 6698

The type of res is determined by the let p: JData at compile time, even if control never reaches that line at runtime. That type, Result<JData, …>, controls the behavior of serde_json::from_str, which causes it to fail at reading string literals in to u8s.

Upvotes: 3

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