Fix #3090: let binding in command leaks when error occurs (#4022)

crates/nu-command/src/commands/core_commands/if_.rs

@@ -101,17 +101,14 @@ fn if_command(args: CommandArgs) -> Result<OutputStream, ShellError> {
 
     //FIXME: should we use the scope that's brought in as well?
     let condition = evaluate_baseline_expr(cond, &context);
-    match condition {
+    let result = match condition {
         Ok(condition) => match condition.as_bool() {
             Ok(b) => {
-                let result = if b {
+                if b {
                     run_block(&then_case.block, &context, input, external_redirection)
                 } else {
                     run_block(&else_case.block, &context, input, external_redirection)
-                };
-                context.scope.exit_scope();
-
-                result
+                }
             }
             Err(e) => Ok(OutputStream::from_stream(
                 vec![UntaggedValue::Error(e).into_untagged_value()].into_iter(),
@@ -120,13 +117,16 @@ fn if_command(args: CommandArgs) -> Result<OutputStream, ShellError> {
         Err(e) => Ok(OutputStream::from_stream(
             vec![UntaggedValue::Error(e).into_untagged_value()].into_iter(),
         )),
-    }
+    };
+    context.scope.exit_scope();
+    result
 }
 
 #[cfg(test)]
 mod tests {
     use super::If;
     use super::ShellError;
+    use nu_test_support::nu;
 
     #[test]
     fn examples_work_as_expected() -> Result<(), ShellError> {
@@ -134,4 +134,21 @@ mod tests {
 
         test_examples(If {})
     }
+
+    #[test]
+    fn if_doesnt_leak_on_error() {
+        let actual = nu!(
+            ".",
+            r#"
+                def test-leak [] {
+                    let var = "hello"
+                    if 0 == "" {echo ok} {echo not}
+                }
+                test-leak
+                echo $var
+            "#
+        );
+
+        assert!(actual.err.contains("unknown variable"));
+    }
 }