Fixes #2054.

This adds the `flip_trait_bound` assist which allows for the swapping of two trait bounds in a trait list that are next to each other.

crates/ra_assists/src/assists/flip_trait_bound.rs

@@ -0,0 +1,32 @@
+//! Assist for swapping traits inside of a trait bound list
+//!
+//! E.g. `A + B` => `B + A` when the cursor is placed by the `+` inside of a
+//! trait bound list
+
+use hir::db::HirDatabase;
+use ra_syntax::{algo::non_trivia_sibling, ast::TypeBoundList, Direction, T};
+
+use crate::{Assist, AssistCtx, AssistId};
+
+/// Flip trait bound assist.
+pub(crate) fn flip_trait_bound(mut ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    // Make sure we're in a `TypeBoundList`
+    ctx.node_at_offset::<TypeBoundList>()?;
+
+    // We want to replicate the behavior of `flip_binexpr` by only suggesting
+    // the assist when the cursor is on a `+`
+    let plus = ctx.token_at_offset().find(|tkn| tkn.kind() == T![+])?;
+
+    let (before, after) = (
+        non_trivia_sibling(plus.clone().into(), Direction::Prev)?,
+        non_trivia_sibling(plus.clone().into(), Direction::Next)?,
+    );
+
+    ctx.add_action(AssistId("flip_trait_bound"), "flip trait bound", |edit| {
+        edit.target(plus.text_range());
+        edit.replace(before.text_range(), after.to_string());
+        edit.replace(after.text_range(), before.to_string());
+    });
+
+    ctx.build()
+}

crates/ra_assists/src/lib.rs

@@ -97,6 +97,7 @@ mod assists {
     mod apply_demorgan;
     mod flip_comma;
     mod flip_binexpr;
+    mod flip_trait_bound;
     mod change_visibility;
     mod fill_match_arms;
     mod merge_match_arms;
@@ -123,6 +124,7 @@ mod assists {
             merge_match_arms::merge_match_arms,
             flip_comma::flip_comma,
             flip_binexpr::flip_binexpr,
+            flip_trait_bound::flip_trait_bound,
             introduce_variable::introduce_variable,
             replace_if_let_with_match::replace_if_let_with_match,
             split_import::split_import,