1 files changed, 60 insertions, 0 deletions

diff --git a/lib/src/lints/collapsible_let_in.rs b/lib/src/lints/collapsible_let_in.rs
new file mode 100644
index 0000000..c2cdf9b
--- /dev/null
+++ b/lib/src/lints/collapsible_let_in.rs
@@ -0,0 +1,60 @@
+use crate::{make, Lint, Metadata, Report, Rule, Suggestion};
+
+use if_chain::if_chain;
+use macros::lint;
+use rowan::Direction;
+use rnix::{
+    types::{LetIn, TypedNode},
+    NodeOrToken, SyntaxElement, SyntaxKind, TextRange
+};
+
+#[lint(
+    name = "collapsible let in",
+    note = "These let-in expressions are collapsible",
+    code = 6,
+    match_with = SyntaxKind::NODE_LET_IN
+)]
+struct CollapsibleLetIn;
+
+impl Rule for CollapsibleLetIn {
+    fn validate(&self, node: &SyntaxElement) -> Option<Report> {
+        if_chain! {
+            if let NodeOrToken::Node(node) = node;
+            if let Some(let_in_expr) = LetIn::cast(node.clone());
+            if let Some(body) = let_in_expr.body();
+
+            if LetIn::cast(body.clone()).is_some();
+            then {
+                let first_annotation = node.text_range();
+                let first_message = "This let-in expression contains a nested let-in expression";
+
+                let second_annotation = body.text_range();
+                let second_message = "This let-in expression is nested";
+
+                let replacement_at = {
+                    let start = body
+                        .siblings_with_tokens(Direction::Prev)
+                        .find(|elem| elem.kind() == SyntaxKind::TOKEN_IN)?
+                        .text_range()
+                        .start();
+                    let end = body
+                        .descendants_with_tokens()
+                        .find(|elem| elem.kind() == SyntaxKind::TOKEN_LET)?
+                        .text_range()
+                        .end();
+                    TextRange::new(start, end)
+                };
+                let replacement = make::empty().node().clone();
+
+                Some(
+                    Self::report()
+                        .diagnostic(first_annotation, first_message)
+                        .suggest(second_annotation, second_message, Suggestion::new(replacement_at, replacement))
+                )
+            } else {
+                None
+            }
+        }
+    }
+}
+