1 files changed, 80 insertions, 0 deletions

diff --git a/crates/ra_assists/src/handlers/flip_comma.rs b/crates/ra_assists/src/handlers/flip_comma.rs
new file mode 100644
index 000000000..1dacf29f8
--- /dev/null
+++ b/crates/ra_assists/src/handlers/flip_comma.rs
@@ -0,0 +1,80 @@
+use ra_syntax::{algo::non_trivia_sibling, Direction, T};
+
+use crate::{Assist, AssistCtx, AssistId};
+
+// Assist: flip_comma
+//
+// Flips two comma-separated items.
+//
+// ```
+// fn main() {
+//     ((1, 2),<|> (3, 4));
+// }
+// ```
+// ->
+// ```
+// fn main() {
+//     ((3, 4), (1, 2));
+// }
+// ```
+pub(crate) fn flip_comma(ctx: AssistCtx) -> Option<Assist> {
+    let comma = ctx.find_token_at_offset(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_assist(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());
+    })
+}
+
+#[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,<|> \
+             }",
+            ",",
+        );
+    }
+}