Function lex_json 

pub fn lex_json(source: &str) -> String

Lexes source and answers a colour for every part of it.

{"ok":bool,"spans":[{"at":int,"len":int,"kind":string}]}

The spans tile: the first begins at zero, each begins where the last ended, and together they cover the source, so a page renders the whole editor by walking the list and slicing its own text. at and len count UTF-16 code units, which is what a JavaScript string is indexed in.

kind is one of keyword, type, string, number, comment and plain. highlight::Kind says what falls into each and which two are not decided by the lexer. Its seventh, slot, is a disassembly’s and never appears here.

ok is whether the source lexed without complaint. It is false whenever the reader is part-way through typing a string literal, which is most of the time one is being typed, and the spans are still a tiling of what was sent: highlight says what a broken source is coloured as and why the answer is not “nothing”.

This is the front end’s first stage and none of the rest, so it costs a pass over the text and nothing else. That is what makes it the thing to call on every keystroke, where compile_json is not.