Assist: extract_sub_struct

struct Foo {
    $0name: String,
    id: u32$0,
    counter: u32,
}
impl Foo {
    fn clear(&mut self) {
        self.name.clear();
        self.id = 0;
    }

    fn init_id(&mut self) {
        self.id = self.counter;
        self.counter += 1;
    }
}

->

struct NewStruct {
    name: String,
    id: u32
}

impl NewStruct {
    fn clear(&mut self) {
        self.name.clear();
        self.id = 0;
    }
}

struct Foo {
    new_struct$0: NewStruct,
    counter: u32,
}
impl Foo {
    fn init_id(&mut self) {
        self.new_struct.id = self.counter;
        self.counter += 1;
    }
}

@rustbot label +A-assists

The issue names no source files, tests, or entry points. Start by locating rust-analyzer's assist implementation and related tests for extract_sub_struct; done means extracting the selected fields into a new struct, moving methods that use only those fields, and updating the remaining field accesses as shown.