structured configuration

Prior to rustfmt 1.0 we trimmed most of the configuration options for maintainability. I am ok to add some of them back, as long as they are orthogonal to other part of code (like imports_indent). where is not one of them, so it is unlikely that we add it back. Sorry.

If we were to add more options, I prefer those to be more structured rather than having plain namespace. E.g., something like the following:

# You can explicitly add [default] or omit it.
indent = "Block"

[fn]
indent = "Visual"

[struct.pattern]
width = 100

[struct.expr]
width = 0

and you will get:

fn foo(xxxxxxxxxxxxxxxxxxxxxxxxxx: u32,
       yyyyyyyyyyyyyyyyyyyyyyy: u32,
       zzzzzzzzzzzzzzzzz: u32)
{
    let Graphviz { label, caption, scale, width, height } = self.graphviz;
    /// ...
    let result = Ok(GraphvizGen {
        file,
        path,
        graphviz,
    });
    // ...
}

Originally posted by @topecongiro in https://github.com/rust-lang/rustfmt/issues/3316#issuecomment-477614459

Start by reviewing the issue discussion and the structured TOML examples. The proposal concerns redesigning rustfmt configuration into nested sections such as [fn], [struct.pattern], and [struct.expr], but the issue names no implementation files or tests. Done would require an agreed configuration design and corresponding formatter behavior.