[nimish]

toml is unstable and has no real spec + YAML has a well defined spec and is widely used?

[Ygg2]

YAML spec is also super complicated and writing parser for it is non-trivial (for comparisson YAML has ~150 EBNF rules while XML with DTD has about ~80)! I'm not comparing validation rules but they are probably about the same.

This complexity means there is already a parser for TOML, and not one for YAML. That's IMO main reason they went with TOML.

[steveklabnik]

There are actually already MULTIPLE parsers for TOML in Rust, and Cargo switched between the two yesterday.

As you say, this is a testament to TOML's simplicity.

EDIT: Furthermore, TOML is going to have a 1.0 soon: https://github.com/rust-lang/cargo/issues/46