From a60429891fe9eb5290f95a52dd5e56f62d25d344 Mon Sep 17 00:00:00 2001
From: Akshay <nerdy@peppe.rs>
Date: Sat, 2 Oct 2021 10:50:47 +0530
Subject: new lint: empty-let-in

---
 Cargo.lock                    | 20 ++++++++++----------
 lib/src/lib.rs                |  6 +++---
 lib/src/lints.rs              |  1 +
 lib/src/lints/empty_let_in.rs | 42 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 56 insertions(+), 13 deletions(-)
 create mode 100644 lib/src/lints/empty_let_in.rs

diff --git a/Cargo.lock b/Cargo.lock
index f222d10..bae8674 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -23,16 +23,6 @@ version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cdb031dd78e28731d87d56cc8ffef4a8f36ca26c38fe2de700543e627f8a464a"
 
-[[package]]
-name = "bin"
-version = "0.1.0"
-dependencies = [
- "anyhow",
- "ariadne",
- "lib",
- "rnix",
-]
-
 [[package]]
 name = "cbitset"
 version = "0.2.0"
@@ -157,6 +147,16 @@ version = "0.1.18"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b203e79e90905594272c1c97c7af701533d42adaab0beb3859018e477d54a3b0"
 
+[[package]]
+name = "statix"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "ariadne",
+ "lib",
+ "rnix",
+]
+
 [[package]]
 name = "syn"
 version = "1.0.76"
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
+            }
+        }
+    }
+}
+
-- 
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