Deserializing internally tagged enum: error line information inaccurate

[karlri]

When deserializing a internally tagged enum, everything works as expected when the input is correct, however:

If an error occurs while deserializing "inside" the internally tagged enum, the error line information points to the wrong place.

Note, simpler way to trigger bug in comments section.

Example code:

use serde_derive::*;
use serde_json::*;

#[derive(Deserialize)]
pub struct Event {
    event_id: u32,
    #[serde(flatten)]
    variant: EventVariant,
}

#[derive(Deserialize)]
#[serde(tag = "kind")]
pub enum EventVariant {
    Trigger { at: String },
    Interval { start: String, end: String },
}

fn main() {
    // at: 14 is wrong, it should be string. Expect error on line 3.
    let json_str = r#"{
        "kind": "Trigger",
        "at": 14,
        "event_id": 42
    }"#;
    let res: Result<Event> = from_str(json_str);
    match res {
        Err(e) => {
            assert_eq!(e.line(), 3)
        },
        Ok(_) => unreachable!()
    }
}

The result of running this code:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `5`,
 right: `3`', src/main.rs:29:13

Problem:
Line information is incorrect.

Actual behavior:
error line refers to end of outermost struct: line 5

Expected behavior:
error line refers to the exact line where the parse error occurred: line 3

[karlri]

Determined it has nothing to do with flatten. It is internal tagging only that triggers the issue.

Event simpler example:

use serde_derive::*;
use serde_json::*;

#[derive(Deserialize, Debug)]
#[serde(tag = "kind")]
pub enum EventVariant {
    Trigger { at: String },
    Interval { start: String, end: String },
}

fn main() {
    // at: 14 is wrong, it should be string. Expect error on line 3.
    let json_str = r#"{
        "kind": "Trigger",
        "at": 14
    }"#;
    let res: Result<EventVariant> = from_str(json_str);
    match res {
        Err(e) => {
            println!("{:?}", e);
            assert_eq!(e.line(), 3)
        },
        Ok(_) => unreachable!()
    }
}

Output:

Error("invalid type: integer `14`, expected a string", line: 0, column: 0)
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `0`,
 right: `3`', src/main.rs:21:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

[1tgr]

It looks to be due to the use of the TaggedContentVisitor to read the entire map into memory before parsing it in accordance with the "kind" field. By the time the data in memory is parsed, the reader has already advanced to the end of the map.

[dtolnay]

Line/column-numbering is format-specific functionality, so I think this ends up being a duplicate of #1183.