Fix BETWEEN optimization never triggering due to span comparison

The try_into_between function compared rq::Expr values including their
span field. Since the same column referenced at two source positions
has different spans, the equality check always failed, making the
BETWEEN optimization dead code.

Fix by comparing only the kind field (a_l.kind == b_l.kind) instead of
the full Expr (a_l == b_l).

Fixes #5737

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: queelius

prqlc/prqlc/src/sql/gen_expr.rs

@@ -432,7 +432,9 @@ fn try_into_between(expr: rq::Expr, ctx: &mut Context) -> Result<Option<sql_ast:
 
                     // We need for the values on each arm to be the same; e.g. x
                     // > 3 and x < 5
-                    if a_l == b_l {
+                    // Compare only `kind` (not `span`) since the same column
+                    // referenced at two source positions has different spans.
+                    if a_l.kind == b_l.kind {
                         return Ok(Some(sql_ast::Expr::Between {
                             expr: Box::new(
                                 translate_operand(a_l, true, 0, Associativity::Both, ctx)?

prqlc/prqlc/tests/integration/sql.rs

@@ -451,6 +451,22 @@ fn test_precedence_division() {
     ");
 }
 
+#[test]
+fn test_between_optimization() {
+    // Regression test: >= and <= on same column should produce BETWEEN
+    assert_snapshot!(compile(r#"
+    from t
+    filter (a >= 5 && a <= 10)
+    "#).unwrap(), @r"
+    SELECT
+      *
+    FROM
+      t
+    WHERE
+      a BETWEEN 5 AND 10
+    ");
+}
+
 #[test]
 fn test_precedence_01() {
     assert_snapshot!((compile(r###"