Serialization of keys varies depending on whether the feature flag "parse" is enabled

[Zireael-N]

If this feature flag is disabled, then all keys are double quoted, otherwise they aren't (unless necessary).

Code to reproduce:

use serde::Serialize;
use std::collections::BTreeMap;
 
#[derive(Serialize)]
struct Example {
    name: String,
    extra: BTreeMap<String, u8>,
}
 
fn main() {
    let example = Example {
        name: "Example".into(),
        extra: (0..3)
            .map(|i| (format!("{}", (b'a' + i) as char), i))
            .collect(),
    };
    println!("{}", toml::to_string_pretty(&example).unwrap());
}

Output with default-features = false, features = ["display"]:

"name" = "Example"

["extra"]
"a" = 0
"b" = 1
"c" = 2

Output with default features:

name = "Example"

[extra]
a = 0
b = 1
c = 2