Auto merge of rust-lang#15515 - cardoso:flip-binexpr/lhs-binexpr, r=Veykril

Check if lhs is also a binexpr and use its rhs in flip binexpr assist

Closes rust-lang#15508

From the original PR, flip binexpr assist is not meant to preserve equivalence, so I went with the simplest solution here.

I can add some extra checks to keep equivalence, but I think they should go in different specific assists (eg. flip arith op / flip logic op / etc), otherwise this one will get out of hand pretty quickly.

Author: bors

crates/ide-assists/src/handlers/flip_binexpr.rs

@@ -19,8 +19,19 @@ use crate::{AssistContext, AssistId, AssistKind, Assists};
 // ```
 pub(crate) fn flip_binexpr(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
     let expr = ctx.find_node_at_offset::<BinExpr>()?;
-    let lhs = expr.lhs()?.syntax().clone();
     let rhs = expr.rhs()?.syntax().clone();
+    let lhs = expr.lhs()?.syntax().clone();
+
+    let lhs = if let Some(bin_expr) = BinExpr::cast(lhs.clone()) {
+        if bin_expr.op_kind() == expr.op_kind() {
+            bin_expr.rhs()?.syntax().clone()
+        } else {
+            lhs
+        }
+    } else {
+        lhs
+    };
+
     let op_range = expr.op_token()?.text_range();
     // The assist should be applied only if the cursor is on the operator
     let cursor_in_range = op_range.contains_range(ctx.selection_trimmed());
@@ -114,6 +125,24 @@ mod tests {
         )
     }
 
+    #[test]
+    fn flip_binexpr_works_for_lhs_arith() {
+        check_assist(
+            flip_binexpr,
+            r"fn f() { let res = 1 + (2 - 3) +$0 4 + 5; }",
+            r"fn f() { let res = 1 + 4 + (2 - 3) + 5; }",
+        )
+    }
+
+    #[test]
+    fn flip_binexpr_works_for_lhs_cmp() {
+        check_assist(
+            flip_binexpr,
+            r"fn f() { let res = 1 + (2 - 3) >$0 4 + 5; }",
+            r"fn f() { let res = 4 + 5 < 1 + (2 - 3); }",
+        )
+    }
+
     #[test]
     fn flip_binexpr_works_inside_match() {
         check_assist(