Support serializing into inline tables

[vlad-ivanov-name]

This issue is reopened from toml-rs/toml-rs#265 as it's still relevant

Given the following set of structures:

#[derive(Serialize)]
struct Inner {
    value: i32,
}

#[derive(Serialize)]
struct Outer {
   value: Inner,
}

#[derive(Serialize)]
struct SomeDocument {
   value: HashMap<String, Outer>,
}

It would be good to have a way to serialize Inner values as inline tables. Currently, to_string will make everything inline, and to_string_pretty will make everything a full table. It would be good to control this per-field, perhaps with a custom field attribute like what serde does.

[epage]

serde does not provide a way for you to communicate intent. That just leaves us coming up with some heuristic which I'd like to avoid.

However, you can instead serialize into a toml_edit::Document and the the mod visit_mut API to format it however you wish. This is our generally recommended approach when people want custom formatting and so I'm planning to close this. If you have a reason for us to re-open, let us know!