3 files changed, 46 insertions, 3 deletions

diff --git a/lib/src/lib.rs b/lib/src/lib.rs
index 93792d4..ab4a14a 100644
--- a/lib/src/lib.rs
+++ b/lib/src/lib.rs
@@ -32,7 +32,7 @@ impl Report {
         self
     }
     /// Add a diagnostic with a fix to this report
-    pub fn suggest(mut self, at: TextRange, message: String, suggestion: Suggestion) -> Self {
+    pub fn suggest<S: AsRef<str>>(mut self, at: TextRange, message: S, suggestion: Suggestion) -> Self {
         self.diagnostics.push(Diagnostic::suggest(at, message, suggestion));
         self
     }
@@ -54,8 +54,8 @@ impl Diagnostic {
         Self { at, message, suggestion: None }
     }
     /// Construct a diagnostic with a fix.
-    pub fn suggest(at: TextRange, message: String, suggestion: Suggestion) -> Self {
-        Self { at, message, suggestion: Some(suggestion) }
+    pub fn suggest<S: AsRef<str>>(at: TextRange, message: S, suggestion: Suggestion) -> Self {
+        Self { at, message: message.as_ref().into(), suggestion: Some(suggestion) }
     }
 }
 
diff --git a/lib/src/lints.rs b/lib/src/lints.rs
index 15d9063..a5fe0da 100644
--- a/lib/src/lints.rs
+++ b/lib/src/lints.rs
@@ -2,4 +2,5 @@ use crate::lint_map;
 
 lint_map! {
     bool_comparison,
+    empty_let_in,
 }
diff --git a/lib/src/lints/empty_let_in.rs b/lib/src/lints/empty_let_in.rs
new file mode 100644
index 0000000..4b074e7
--- /dev/null
+++ b/lib/src/lints/empty_let_in.rs
@@ -0,0 +1,42 @@
+use crate::{Lint, Metadata, Report, Rule, Suggestion};
+
+use if_chain::if_chain;
+use macros::lint;
+use rnix::{
+    types::{LetIn, TypedNode, 
+        EntryHolder},
+    NodeOrToken, SyntaxElement, SyntaxKind,
+};
+
+#[lint(
+    name = "empty let-in",
+    note = "Useless let-in expression",
+    code = 2,
+    match_with = SyntaxKind::NODE_LET_IN
+)]
+struct EmptyLetIn;
+
+impl Rule for EmptyLetIn {
+    fn validate(&self, node: &SyntaxElement) -> Option<Report> {
+        if_chain! {
+            if let NodeOrToken::Node(let_in_node) = node;
+            if let Some(let_in_expr) = LetIn::cast(let_in_node.clone());
+            let entries = let_in_expr.entries();
+            let inherits = let_in_expr.inherits();
+
+            if entries.count() == 0;
+            if inherits.count() == 0;
+
+            if let Some(body) = let_in_expr.body();
+            then {
+                let at = node.text_range();
+                let replacement = body;
+                let message = "This let-in expression has no entries";
+                Some(Self::report().suggest(at, message, Suggestion::new(at, replacement)))
+            } else {
+                None
+            }
+        }
+    }
+}
+