Pretty print indent

[jwpjrdev]

currently, if a user of the library wants to serialise to a pretty JSON string with an indent other than 2 spaces, this is the way to go about it:

let value = ...;

let mut buf = Vec::new();
let formatter = serde_json::ser::PrettyFormatter::with_indent(b"    ");
let mut ser = serde_json::Serializer::with_formatter(&mut buf, formatter);
value.serialize(&mut ser)?;
let pretty_json = String::from_utf8(buf)?;

not fun.

this PR adds a couple methods, culminating in serde_json::to_string_pretty_indent. it does the exact same thing as to_string_pretty but it allows you to pass an indent such as b" " down the chain to PrettyFormatter. this can be used like so:

let value = ...;

let pretty_json = serde_json::to_string_pretty_indent(&json, b"    ")?;

i have not written any tests because i poked around and saw that there were a lot of things going on. some help with that would be greatly appreciated. i already tested the additions in another project of mine to ensure that it does function.

[jwpjrdev]

i'm also curious—why must a &[u8] be provided? wouldn't a usize be better?

that way, instead of providing b" ", you could just provide 4. i continued that trend in this PR but i feel like it might be useful to add another method alongside to_string_pretty_indent that simply uses usize because that's what 99% of people will be using.

[dtolnay]

I think indenting with non-2 indentation is uncommon enough that it won't make sense to dedicate 3 whole new top-level functions to this use case. Using PrettyFormatter::with_indent / Serializer::with_formatter as you showed is what I would recommend, or you can wrap the same thing in a helper function if it's needed frequently in a particular project.

Thanks anyway for the PR!

[jwpjrdev]

thank you! i'm still rather confused about why it accepts &[u8] rather than a usize, though.

[dtolnay]

Non-space indents? b"\t"