Struct Migration 

pub struct Migration {
    pub name: &'static str,
    pub description: &'static str,
    pub applies: fn(&Config, &Table) -> bool,
    pub apply: fn(&mut Config) -> Vec<String>,
}

One config change lev update knows how to make on the user’s behalf.

The mechanism exists so that a future incompatibility - a key that moved, a value whose meaning changed - is either fixed automatically or at least explained at the moment the user updates into it, rather than surfacing as a broken run days later. MIGRATIONS is empty today because no shipped version has changed a key’s name or meaning; the tests drive the machinery with a sample so the wiring is proven rather than assumed.

Fields

name: &'static str

A short stable name, shown in the report and in --json.

description: &'static str

What it changes and why, in one line.

applies: fn(&Config, &Table) -> bool

Whether this config needs it.

Gets the parsed Config and the raw document, because the two see different things: a key serde no longer reads vanishes from the parsed value entirely, and a key that is still read but now means something else is only visible there.

apply: fn(&mut Config) -> Vec<String>

Make the change, returning one line per thing it did.