From e9c80a9c256677d85398d93880eec1e54e226d2a Mon Sep 17 00:00:00 2001
From: Shoyu Vanilla <modulo641@gmail.com>
Date: Thu, 15 Feb 2024 00:59:13 +0900
Subject: [PATCH] fix: False positive diagnostic for necessary `else`

---
 crates/hir-ty/src/diagnostics/expr.rs         |  8 ++++++-
 .../src/handlers/remove_unnecessary_else.rs   | 23 +++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/crates/hir-ty/src/diagnostics/expr.rs b/crates/hir-ty/src/diagnostics/expr.rs
index 0c5d6399619..571f01dde2e 100644
--- a/crates/hir-ty/src/diagnostics/expr.rs
+++ b/crates/hir-ty/src/diagnostics/expr.rs
@@ -338,7 +338,13 @@ fn check_for_trailing_return(&mut self, body_expr: ExprId, body: &Body) {
 
     fn check_for_unnecessary_else(&mut self, id: ExprId, expr: &Expr, body: &Body) {
         if let Expr::If { condition: _, then_branch, else_branch } = expr {
-            if else_branch.is_none() {
+            if let Some(else_branch) = else_branch {
+                // If else branch has a tail, it is an "expression" that produces a value,
+                // e.g. `let a = if { ... } else { ... };` and this `else` is not unnecessary
+                if let Expr::Block { tail: Some(_), .. } = body.exprs[*else_branch] {
+                    return;
+                }
+            } else {
                 return;
             }
             if let Expr::Block { statements, tail, .. } = &body.exprs[*then_branch] {
diff --git a/crates/ide-diagnostics/src/handlers/remove_unnecessary_else.rs b/crates/ide-diagnostics/src/handlers/remove_unnecessary_else.rs
index ae8241ec2c6..813c07a505d 100644
--- a/crates/ide-diagnostics/src/handlers/remove_unnecessary_else.rs
+++ b/crates/ide-diagnostics/src/handlers/remove_unnecessary_else.rs
@@ -384,6 +384,29 @@ fn test() {
         return bar;
     }
 }
+"#,
+        );
+    }
+
+    #[test]
+    fn no_diagnostic_if_tail_exists_in_else_branch() {
+        check_diagnostics_with_needless_return_disabled(
+            r#"
+fn test1(a: bool) {
+    let _x = if a {
+        return;
+    } else {
+        1
+    };
+}
+
+fn test2(a: bool) -> i32 {
+    if a {
+        return 1;
+    } else {
+        0
+    }
+}
 "#,
         );
     }