rust/crates/libeditor/src/code_actions.rs

use {TextUnit, File, EditBuilder, Edit};
use libsyntax2::{
    ast::{self, AstNode},
    SyntaxKind::COMMA,
    SyntaxNodeRef,
    SyntaxRoot,
    algo::{
        Direction, siblings,
        find_leaf_at_offset, ancestors,
    },
};

pub fn flip_comma<'a>(file: &'a File, offset: TextUnit) -> Option<impl FnOnce() -> Edit + 'a> {
    let syntax = file.syntax();
    let syntax = syntax.as_ref();

    let comma = find_leaf_at_offset(syntax, offset).find(|leaf| leaf.kind() == COMMA)?;
    let left = non_trivia_sibling(comma, Direction::Backward)?;
    let right = non_trivia_sibling(comma, Direction::Forward)?;
    Some(move || {
        let mut edit = EditBuilder::new();
        edit.replace(left.range(), right.text());
        edit.replace(right.range(), left.text());
        edit.finish()
    })
}

pub fn add_derive<'a>(file: &'a File, offset: TextUnit) -> Option<impl FnOnce() -> Edit + 'a> {
    let syntax = file.syntax();
    let syntax = syntax.as_ref();
    let nominal = find_node::<ast::NominalDef<_>>(syntax, offset)?;
    Some(move || {
        let mut edit = EditBuilder::new();
        edit.insert(nominal.syntax().range().start(), "#[derive()]\n".to_string());
        edit.finish()
    })
}

fn non_trivia_sibling(node: SyntaxNodeRef, direction: Direction) -> Option<SyntaxNodeRef> {
    siblings(node, direction)
        .skip(1)
        .find(|node| !node.kind().is_trivia())
}

fn find_non_trivia_leaf(syntax: SyntaxNodeRef, offset: TextUnit) -> Option<SyntaxNodeRef> {
    find_leaf_at_offset(syntax, offset)
        .find(|leaf| !leaf.kind().is_trivia())
}

fn find_node<'a, N: AstNode<&'a SyntaxRoot>>(syntax: SyntaxNodeRef<'a>, offset: TextUnit) -> Option<N> {
    let leaf = find_non_trivia_leaf(syntax, offset)?;
    ancestors(leaf)
        .filter_map(N::cast)
        .next()
}