It'd be nice to be able to set a default value without needing a function

[ilyvion]

I could be missing something, but I've got this definition:

#[derive(Debug, Deserialize)]
struct PropertyValue {
    #[serde(default = "default_property_type")]
    pub r#type: String,
    pub value: String,
}
fn default_property_type() -> String {
    "string".to_owned()
}

It feels excessive to have a whole function just for this, it'd be nice if I could do something like #[serde(default = || "string".to_owned())] or maybe #[serde(default_value = "string")].

[rustonaut]

Proc macros do support code snippets.

So instead of #[serde(default_value = "1")] you can have #[serde(default_value = 1)] or even
something like #[serde(default_value = Some("foo".into()))] or ``.

struct Foo {
    #[serde(default_value = 1)]
    num: u16,
    
    #[serde(default_value = u16::MAX)]
    num: u16,
    
    //or even
    
    #[serde(default_value = Cow::Borrowed(APPLICATION_NAME))]
    name: Cow<'static, str>,

Alternatively we could also change default to accept snippets which are functions e.g.:

struct Foo {
    #[serde(default = || "foobar".into() )]
    field: String,
    
    #[serde(default = || format!("foo-bar-{APPLICATION_NAME}")]
    field2: String,

Through there are two drawbacks:

1. AFIK in the very early stable days of rust this was not possible, I'm not sure if 1.13 is new enough we would have to test it.
2. syn has with Meta a nice helper to parse proc-macro attributes, but that helper doesn't support this. We don't have to use it as it's not reducing complexity by that much, but this means you probably would need some preparing PRs which do make sure we can parse attributes as <path> "(" <key> [ "=" <tokens> ] ")"

EDIT:

For reference: https://docs.rs/syn/1.0.103/syn/struct.Attribute.html

As you can see the syntax (ignoring the #[]/#![]) is <path> <tokens> and it's only the Meta helper which further constraints what <tokens> can be.

[kangalio]

Duplicate of #368

[ken0x0a]

@dtolnay
I couldn't find commits to close this issue in these 2 days and couldn't find attribute like default_value e.t.c.
Did you want to close this?

[oli-obk]

This issue is a duplicate of #368

You can read and subscribe to the other issue