From a2bcad15d5e765163b6577195efd5d63d0cbf656 Mon Sep 17 00:00:00 2001
From: A4-Tacks <wdsjxhno1001@163.com>
Date: Thu, 2 Oct 2025 13:35:35 +0800
Subject: [PATCH] Add ide-assist: flip_else_if

Flips two if-else-if branches.

Example
---
```rust
fn foo() {
    let n = if cond1 { 2 } else$0 if cond2 { 3 } else { 0 };
}
```
->
```rust
fn foo() {
    let n = if cond2 { 3 } else if cond1 { 2 } else { 0 };
}
```
---
 .../ide-assists/src/handlers/flip_else_if.rs  | 99 +++++++++++++++++++
 crates/ide-assists/src/lib.rs                 |  2 +
 crates/ide-assists/src/tests/generated.rs     | 17 ++++
 3 files changed, 118 insertions(+)
 create mode 100644 crates/ide-assists/src/handlers/flip_else_if.rs

diff --git a/crates/ide-assists/src/handlers/flip_else_if.rs b/crates/ide-assists/src/handlers/flip_else_if.rs
new file mode 100644
index 000000000000..6fb07856cb37
--- /dev/null
+++ b/crates/ide-assists/src/handlers/flip_else_if.rs
@@ -0,0 +1,99 @@
+use syntax::{AstNode, T, ast, syntax_editor::SyntaxEditor};
+
+use crate::{AssistContext, AssistId, Assists};
+
+// Assist: flip_else_if
+//
+// Flips two if-else-if branches.
+//
+// ```
+// fn foo() {
+//     let n = if cond1 { 2 } else$0 if cond2 { 3 } else { 0 };
+// }
+// ```
+// ->
+// ```
+// fn foo() {
+//     let n = if cond2 { 3 } else if cond1 { 2 } else { 0 };
+// }
+// ```
+pub(crate) fn flip_else_if(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
+    let else_kw = ctx.find_token_syntax_at_offset(T![else])?;
+    let if_expr = ast::IfExpr::cast(else_kw.parent()?)?;
+    let ast::ElseBranch::IfExpr(else_if) = if_expr.else_branch()? else { return None };
+
+    let (cond, block) = (if_expr.condition()?, if_expr.then_branch()?);
+    let (else_cond, else_block) = (else_if.condition()?, else_if.then_branch()?);
+
+    let target = else_kw.text_range();
+    acc.add(AssistId::refactor_rewrite("flip_else_if"), "Flip if else if", target, |builder| {
+        let mut edit = builder.make_editor(if_expr.syntax());
+
+        swap_node(&cond, &else_cond, &mut edit);
+        swap_node(&block, &else_block, &mut edit);
+
+        builder.add_file_edits(ctx.vfs_file_id(), edit);
+    })
+}
+
+fn swap_node(lhs: &impl AstNode, rhs: &impl AstNode, edit: &mut SyntaxEditor) {
+    edit.replace(lhs.syntax(), rhs.syntax());
+    edit.replace(rhs.syntax(), lhs.syntax());
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn not_applicable_without_else_keyword() {
+        check_assist_not_applicable(
+            flip_else_if,
+            r#"
+            fn foo() {
+                let n = if cond1 { 2 } else if$0 cond2 { 3 } else { 0 };
+            }
+            "#,
+        );
+
+        check_assist_not_applicable(
+            flip_else_if,
+            r#"
+            fn foo() {
+                let n = if$0 cond1 { 2 } else if cond2 { 3 } else { 0 };
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn without_else_branch() {
+        check_assist(
+            flip_else_if,
+            r#"
+            fn foo() {
+                if cond1 { 2 } $0else if cond2 { 3 };
+            }
+            "#,
+            r#"
+            fn foo() {
+                if cond2 { 3 } else if cond1 { 2 };
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_else_block_branch() {
+        check_assist_not_applicable(
+            flip_else_if,
+            r#"
+            fn foo() {
+                let n = if cond1 { 2 } else$0 { 3 };
+            }
+            "#,
+        );
+    }
+}
diff --git a/crates/ide-assists/src/lib.rs b/crates/ide-assists/src/lib.rs
index 4682c0473238..ecc9065150f8 100644
--- a/crates/ide-assists/src/lib.rs
+++ b/crates/ide-assists/src/lib.rs
@@ -151,6 +151,7 @@ mod handlers {
     mod fix_visibility;
     mod flip_binexpr;
     mod flip_comma;
+    mod flip_else_if;
     mod flip_or_pattern;
     mod flip_trait_bound;
     mod generate_constant;
@@ -284,6 +285,7 @@ mod handlers {
             fix_visibility::fix_visibility,
             flip_binexpr::flip_binexpr,
             flip_comma::flip_comma,
+            flip_else_if::flip_else_if,
             flip_or_pattern::flip_or_pattern,
             flip_trait_bound::flip_trait_bound,
             generate_constant::generate_constant,
diff --git a/crates/ide-assists/src/tests/generated.rs b/crates/ide-assists/src/tests/generated.rs
index e7f91ff3fbc1..ea1c789b0c60 100644
--- a/crates/ide-assists/src/tests/generated.rs
+++ b/crates/ide-assists/src/tests/generated.rs
@@ -1315,6 +1315,23 @@ fn main() {
     )
 }
 
+#[test]
+fn doctest_flip_else_if() {
+    check_doc_test(
+        "flip_else_if",
+        r#####"
+fn foo() {
+    let n = if cond1 { 2 } else$0 if cond2 { 3 } else { 0 };
+}
+"#####,
+        r#####"
+fn foo() {
+    let n = if cond2 { 3 } else if cond1 { 2 } else { 0 };
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_flip_or_pattern() {
     check_doc_test(