allow to serialize the "build" section

[daynin]

Without this change it's impossible to setup build parameters through API like that:

use mdbook::MDBook;
use mdbook::config::Config;

let root_dir = "/path/to/book/root";

let mut cfg = Config::default();
cfg.book.title = Some("My Book".to_string());
cfg.book.authors.push("Michael-F-Bryan".to_string());

cfg.build.build_dir = PathBuf::from("output"); <---------------- we can set it, but it won't be serialized

MDBook::init(root_dir)
    .create_gitignore(true)
    .with_config(cfg)
    .build()
    .expect("Book generation failed");

[ehuss]

Thanks! A problem with this change is that mdbook init will start the book.toml file with a [build] section. Can it maybe use the same check for default as the [rust] table uses?

[daynin]

@ehuss yes, of course. I did it

[ehuss]

Sorry, forgot to ask, can you add a test for this? It looks like run_mdbook_init_with_custom_book_and_src_locations could just be updated to have an assert_eq! of the contents of book.toml, similar to how base_mdbook_init_should_create_default_content works.

[daynin]

@ehuss done

[ehuss]

Thanks!