Handle attributes correctly in "Flip comma"

Attributes often contain path followed by a token tree (e.g. `align(2)`, and the previous code handled them as two separate items, which led to results such as `#[repr(alignC, (2))]`.

An alternative is to just make the assist unavailable in attributes, like we do in macros. But contrary to macros, attributes often have a fixed form, so this seems useful.

src/tools/rust-analyzer/crates/ide-assists/src/handlers/flip_comma.rs

@@ -1,4 +1,8 @@
-use syntax::{algo::non_trivia_sibling, Direction, SyntaxKind, T};
+use ide_db::base_db::SourceDatabase;
+use syntax::TextSize;
+use syntax::{
+    algo::non_trivia_sibling, ast, AstNode, Direction, SyntaxKind, SyntaxToken, TextRange, T,
+};
 
 use crate::{AssistContext, AssistId, AssistKind, Assists};
 
@@ -21,6 +25,8 @@ 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)?;
+    let (mut prev_text, mut next_text) = (prev.to_string(), next.to_string());
+    let (mut prev_range, mut next_range) = (prev.text_range(), next.text_range());
 
     // Don't apply a "flip" in case of a last comma
     // that typically comes before punctuation
@@ -34,17 +40,55 @@ pub(crate) fn flip_comma(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<(
         return None;
     }
 
+    if let Some(parent) = comma.parent().and_then(ast::TokenTree::cast) {
+        // An attribute. It often contains a path followed by a token tree (e.g. `align(2)`), so we have
+        // to be smarter.
+        let prev_start =
+            match comma.siblings_with_tokens(Direction::Prev).skip(1).find(|it| it.kind() == T![,])
+            {
+                Some(it) => position_after_token(it.as_token().unwrap()),
+                None => position_after_token(&parent.left_delimiter_token()?),
+            };
+        let prev_end = prev.text_range().end();
+        let next_start = next.text_range().start();
+        let next_end =
+            match comma.siblings_with_tokens(Direction::Next).skip(1).find(|it| it.kind() == T![,])
+            {
+                Some(it) => position_before_token(it.as_token().unwrap()),
+                None => position_before_token(&parent.right_delimiter_token()?),
+            };
+        prev_range = TextRange::new(prev_start, prev_end);
+        next_range = TextRange::new(next_start, next_end);
+        let file_text = ctx.db().file_text(ctx.file_id().file_id());
+        prev_text = file_text[prev_range].to_owned();
+        next_text = file_text[next_range].to_owned();
+    }
+
     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());
+            edit.replace(prev_range, next_text);
+            edit.replace(next_range, prev_text);
         },
     )
 }
 
+fn position_before_token(token: &SyntaxToken) -> TextSize {
+    match non_trivia_sibling(token.clone().into(), Direction::Prev) {
+        Some(prev_token) => prev_token.text_range().end(),
+        None => token.text_range().start(),
+    }
+}
+
+fn position_after_token(token: &SyntaxToken) -> TextSize {
+    match non_trivia_sibling(token.clone().into(), Direction::Next) {
+        Some(prev_token) => prev_token.text_range().start(),
+        None => token.text_range().end(),
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -89,4 +133,18 @@ mod tests {
         // See https://github.com/rust-lang/rust-analyzer/issues/7693
         check_assist_not_applicable(flip_comma, r#"bar!(a,$0 b)"#);
     }
+
+    #[test]
+    fn flip_comma_attribute() {
+        check_assist(
+            flip_comma,
+            r#"#[repr(align(2),$0 C)] struct Foo;"#,
+            r#"#[repr(C, align(2))] struct Foo;"#,
+        );
+        check_assist(
+            flip_comma,
+            r#"#[foo(bar, baz(1 + 1),$0 qux, other)] struct Foo;"#,
+            r#"#[foo(bar, qux, baz(1 + 1), other)] struct Foo;"#,
+        );
+    }
 }