1 files changed, 78 insertions, 0 deletions

diff --git a/lib/src/lints/bool_comparison.rs b/lib/src/lints/bool_comparison.rs
new file mode 100644
index 0000000..4476b31
--- /dev/null
+++ b/lib/src/lints/bool_comparison.rs
@@ -0,0 +1,78 @@
+use crate::{Diagnostic, Lint, Metadata, Rule};
+
+use if_chain::if_chain;
+use macros::lint;
+use rnix::{
+    types::{BinOp, BinOpKind, Ident, TokenWrapper, TypedNode},
+    NodeOrToken, SyntaxElement, SyntaxKind, SyntaxNode,
+};
+
+#[lint(
+    name = "bool_comparison",
+    note = "Unnecessary comparison with boolean",
+    match_with = "SyntaxKind::NODE_BIN_OP"
+)]
+struct BoolComparison;
+
+impl Rule for BoolComparison {
+    fn validate(&self, node: &SyntaxElement) -> Option<Diagnostic> {
+        if_chain! {
+            if let NodeOrToken::Node(bin_op_node) = node;
+            if let Some(bin_expr) = BinOp::cast(bin_op_node.clone());
+            if let Some(lhs) = bin_expr.lhs();
+            if let Some(rhs) = bin_expr.rhs();
+
+            if let BinOpKind::Equal | BinOpKind::NotEqual = bin_expr.operator();
+            let (non_bool_side, bool_side) = if is_boolean_ident(&lhs) {
+                (rhs, lhs)
+            } else if is_boolean_ident(&rhs) {
+                (lhs, rhs)
+            } else {
+                return None
+            };
+            then {
+                let at = node.text_range();
+                let message = format!(
+                    "Comparing `{}` with boolean literal `{}`",
+                    non_bool_side,
+                    bool_side
+                );
+                dbg!(Some(Diagnostic::new(at, message)))
+            } else {
+                None
+            }
+        }
+    }
+}
+
+// not entirely accurate, underhanded nix programmers might write `true = false`
+fn is_boolean_ident(node: &SyntaxNode) -> bool {
+    if let Some(ident_expr) = Ident::cast(node.clone()) {
+        ident_expr.as_str() == "true" || ident_expr.as_str() == "false"
+    } else {
+        false
+    }
+}
+
+// #[cfg(test)]
+// mod tests {
+//     use super::*;
+//     use rnix::{parser, WalkEvent};
+//
+//     #[test]
+//     fn trivial() {
+//         let src = r#"
+//         a == true
+//         "#;
+//         let parsed = rnix::parse(src).as_result().ok().unwrap();
+//         let _ = parsed
+//             .node()
+//             .preorder_with_tokens()
+//             .filter_map(|event| match event {
+//                 WalkEvent::Enter(t) => Some(t),
+//                 _ => None,
+//             })
+//             .map(|node| BoolComparison.validate(&node))
+//             .collect::<Vec<_>>();
+//     }
+// }