Function electric_columns 

pub fn electric_columns(
    lang: Lang,
    prev_nonblank: Option<&str>,
    line: &str,
    unit: IndentUnit,
) -> u16

The indent, in columns, for a line being electrically re-indented.

Deliberately lexical, not tree-driven, and that follows from IN.2’s finding rather than being a shortcut. At the instant the closer is typed, two things are true at once: the published snapshot has not caught up with the edit, and the code around the cursor is half-written — so a fresh parse would produce an ERROR node with no block structure and the engine would decline anyway. There is no version of this that the tree can answer, so asking it would cost a parse to learn nothing.

It reuses indent_columns_for_new_line with the current line passed as next: “the indent a new line here would get, given what this line starts with” is exactly the question, so no second algorithm is needed. Word closers subtract the extra level the bracket scan cannot see.