Only allow single line formatting for structure literals in `sfv2`

[eureka-cpu]

Currently the line width of an item to use the Inline formatting (single line) is only 18usize, and this almost never is a realistic number for inline formatting of small structures. Most of our tests are about 200-220usize to allow formatting of structs or enums to single line. Thoughts? @kayagokalp @mitchmindtree @sezna @mohammadfawaz @emilyaherbert

See below comments for resolved context

[mohammadfawaz]

Can you provide visual examples of what our options are? Also, what is the behaviour of rustfmt?

Thanks!

[kayagokalp]

One thing to note here is that with the current implementation (just like rustfmt), having the width of 18usize does not necessarily mean that structures that are longer than 18 characters are always going to be formatted into multiple lines. Because depending on the max_width provided by the config and DEFAULT_MAX_LINE_WIDTH it is scaled (here).

But that being said, we want the formatter to behave as good as possible without any configuration, and without any config, it is going to basically compare the current structure length with DEFAULT_STRUCTURE_VAR_WIDTH (which is 18 atm). And 18 characters can be pretty short. For example, the following code piece can easily be formatted to a single line but currently, it won't (without any config)

enum Color {
 Blue: (), 
 Red: () 
}

This example is 32 chars long, if it is formatted to a single line

enum Color { Blue: (), Red: () }

So it might be a good idea to increase DEFAULT_STRUCTURE_VAR_WIDTH

[eureka-cpu]

@kayagokalp exactly!

[eureka-cpu]

I think one of the problems here is that we are checking the entire item line width against the maximum length of what a single field should be. I'll try to address this in my expr PR as well.

[eureka-cpu]

I've found the root of the problem and the solution! We were mistakenly trying to format non-literal structs and enums as literals. So this:

pub enum Foo {
    bar: u64,
    baz: bool,
}

was becoming this:

pub enum Foo { bar: u64, baz: bool }

when really what we want is only for literals to be formatted this way:

let foo = Foo { bar: 0, baz: true };

I will fix this in expr since it is relevant to that PR