`serde` macro lint

[Kriskras99]

Lint explanation

It would be nice to lint Serde macros for unnecessary duplication. For example:

• Specifying default on an Option (Option is automatically default)
• rename = "SomeThing" that matches the rename_all = "PascalCase" on the struct
• Suggest adding or removing rename_all = "x" if that would result in less rename = "y" lines

Example code

#[derive(Serialize, Deserialize)]
#[serde(rename_all = "PascalCase")]
pub struct BadExample {
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub default_option: Option<String>,
    #[serde(rename = "SomeThing")]
    pub some_thing: String,
}

#[derive(Serialize, Deserialize)]
#[serde(rename_all = "PascalCase")]
pub struct GoodExample {
    #[serde(skip_serializing_if = "Option::is_none")]
    pub default_option: Option<String>,
    pub some_thing: String,
}

Notes

The output/expansion of the macro is not relevent for these lints.

[xFrednet]

That's a cool suggestion, thank you for the issue! Marker is currently sadly lacking a representation for attributes (https://github.com/rust-marker/marker/issues/51), but I want to definitely support such lints.