I'm not sure if it even makes sense for a TOML file to be "read incrementally", because of the weird feature of TOML (inherited from INI conventions) that allow tables to be defined in a piecemeal, out-of-order fashion. Here's an example that the TOML spec calls "valid, but discouraged":
[fruit.apple]
[animal]
[fruit.orange]
So the only way to know that you have all the keys in a given table is to literally read the entire file. This is one of those unfortunate things in TOML that I would honestly ignore if I were writing my own TOML parser, even if it meant I wasn't "compliant".
I don't think that's worse than having to search an arbitrary distance for a matching closing bracket. There are tasks where you can start working knowing that a given array in the data might be appended to later (similarly for objects).
It's worse than having to parse a matching bracket, because any context where you have an item defined via nested brackets is going to be a subset of this use case. But yes, that doesn't mean you couldn't do some theoretical eager processing, but it's going to be context dependent. For example, consider a Cargo.toml file, where we've processed the `features` key for a given dependency. Is it safe to begin compiling that dependency with the given set of features before we finish parsing the file? No, because there might be a `default-features=false` key that applies to this dependency later in the file. In a format where tables weren't allowed to be split, the mere act of parsing a single, self-contained dependency entry would be enough to know for certain that no such `default-features` key exists. Not all potential keys are going to require this sort of consideration, but it could be a footgun depending on the semantics of your schema.
