rust/crates/ra_assists/src/flip_comma.rs

use hir::db::HirDatabase;
use ra_syntax::{algo::non_trivia_sibling, Direction, T};

use crate::{Assist, AssistCtx, AssistId};

pub(crate) fn flip_comma(mut ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
    let comma = ctx.token_at_offset().find(|leaf| leaf.kind() == T![,])?;
    let prev = non_trivia_sibling(comma.clone().into(), Direction::Prev)?;
    let next = non_trivia_sibling(comma.clone().into(), Direction::Next)?;

    // Don't apply a "flip" in case of a last comma
    // that typically comes before punctuation
    if next.kind().is_punct() {
        return None;
    }

    ctx.add_action(AssistId("flip_comma"), "flip comma", |edit| {
        edit.target(comma.text_range());
        edit.replace(prev.text_range(), next.to_string());
        edit.replace(next.text_range(), prev.to_string());
    });

    ctx.build()
}

#[cfg(test)]
mod tests {
    use super::*;

    use crate::helpers::{check_assist, check_assist_target};

    #[test]
    fn flip_comma_works_for_function_parameters() {
        check_assist(
            flip_comma,
            "fn foo(x: i32,<|> y: Result<(), ()>) {}",
            "fn foo(y: Result<(), ()>,<|> x: i32) {}",
        )
    }

    #[test]
    fn flip_comma_target() {
        check_assist_target(flip_comma, "fn foo(x: i32,<|> y: Result<(), ()>) {}", ",")
    }

    #[test]
    #[should_panic]
    fn flip_comma_before_punct() {
        // See https://github.com/rust-analyzer/rust-analyzer/issues/1619
        // "Flip comma" assist shouldn't be applicable to the last comma in enum or struct
        // declaration body.
        check_assist_target(flip_comma, 
                            "pub enum Test { \
                                A,<|> \
                            }", ",");

        
        check_assist_target(flip_comma, 
                            "pub struct Test { \
                                foo: usize,<|> \
                            }", ",");
    }
}