1 files changed, 92 insertions, 0 deletions
diff --git a/src/tools/rust-analyzer/crates/ide-assists/src/handlers/flip_comma.rs b/src/tools/rust-analyzer/crates/ide-assists/src/handlers/flip_comma.rs
new file mode 100644
index 000000000..f40f2713a
--- /dev/null
+++ b/src/tools/rust-analyzer/crates/ide-assists/src/handlers/flip_comma.rs
@@ -0,0 +1,92 @@
+use syntax::{algo::non_trivia_sibling, Direction, SyntaxKind, T};
+
+use crate::{AssistContext, AssistId, AssistKind, Assists};
+
+// Assist: flip_comma
+//
+// Flips two comma-separated items.
+//
+// ```
+// fn main() {
+//     ((1, 2),$0 (3, 4));
+// }
+// ```
+// ->
+// ```
+// fn main() {
+//     ((3, 4), (1, 2));
+// }
+// ```
+pub(crate) fn flip_comma(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
+    let comma = ctx.find_token_syntax_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;
+    }
+
+    // Don't apply a "flip" inside the macro call
+    // since macro input are just mere tokens
+    if comma.parent_ancestors().any(|it| it.kind() == SyntaxKind::MACRO_CALL) {
+        return None;
+    }
+
+    acc.add(
+        AssistId("flip_comma", AssistKind::RefactorRewrite),
+        "Flip comma",
+        comma.text_range(),
+        |edit| {
+            edit.replace(prev.text_range(), next.to_string());
+            edit.replace(next.text_range(), prev.to_string());
+        },
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::tests::{check_assist, check_assist_not_applicable, check_assist_target};
+
+    #[test]
+    fn flip_comma_works_for_function_parameters() {
+        check_assist(
+            flip_comma,
+            r#"fn foo(x: i32,$0 y: Result<(), ()>) {}"#,
+            r#"fn foo(y: Result<(), ()>, x: i32) {}"#,
+        )
+    }
+
+    #[test]
+    fn flip_comma_target() {
+        check_assist_target(flip_comma, r#"fn foo(x: i32,$0 y: Result<(), ()>) {}"#, ",")
+    }
+
+    #[test]
+    fn flip_comma_before_punct() {
+        // See https://github.com/rust-lang/rust-analyzer/issues/1619
+        // "Flip comma" assist shouldn't be applicable to the last comma in enum or struct
+        // declaration body.
+        check_assist_not_applicable(flip_comma, "pub enum Test { A,$0 }");
+        check_assist_not_applicable(flip_comma, "pub struct Test { foo: usize,$0 }");
+    }
+
+    #[test]
+    fn flip_comma_works() {
+        check_assist(
+            flip_comma,
+            r#"fn main() {((1, 2),$0 (3, 4));}"#,
+            r#"fn main() {((3, 4), (1, 2));}"#,
+        )
+    }
+
+    #[test]
+    fn flip_comma_not_applicable_for_macro_input() {
+        // "Flip comma" assist shouldn't be applicable inside the macro call
+        // See https://github.com/rust-lang/rust-analyzer/issues/7693
+        check_assist_not_applicable(flip_comma, r#"bar!(a,$0 b)"#);
+    }
+}