6 files changed, 71 insertions, 4 deletions

diff --git a/Cargo.lock b/Cargo.lock
index bae8674..8b371ea 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -64,6 +64,7 @@ dependencies = [
  "lazy_static",
  "macros",
  "rnix",
+ "rowan",
 ]
 
 [[package]]
diff --git a/lib/Cargo.toml b/lib/Cargo.toml
index e371c5d..e9e9e69 100644
--- a/lib/Cargo.toml
+++ b/lib/Cargo.toml
@@ -10,3 +10,4 @@ rnix = "0.9.0"
 if_chain = "1.0"
 macros = { path = "../macros" }
 lazy_static = "1.0"
+rowan = "0.12.5"
diff --git a/lib/src/lib.rs b/lib/src/lib.rs
index f0f26dd..4065345 100644
--- a/lib/src/lib.rs
+++ b/lib/src/lib.rs
@@ -18,7 +18,7 @@ pub struct Report {
 }
 
 impl Report {
-    /// Construct a report. Do not invoke manually, see `lint` macro
+    /// Construct a report. Do not invoke `Report::new` manually, see `lint` macro
     pub fn new(note: &'static str, code: u32) -> Self {
         Self {
             note,
@@ -27,7 +27,7 @@ impl Report {
         }
     }
     /// Add a diagnostic to this report
-    pub fn diagnostic(mut self, at: TextRange, message: String) -> Self {
+    pub fn diagnostic<S: AsRef<str>>(mut self, at: TextRange, message: S) -> Self {
         self.diagnostics.push(Diagnostic::new(at, message));
         self
     }
@@ -55,10 +55,10 @@ pub struct Diagnostic {
 
 impl Diagnostic {
     /// Construct a diagnostic.
-    pub fn new(at: TextRange, message: String) -> Self {
+    pub fn new<S: AsRef<str>>(at: TextRange, message: S) -> Self {
         Self {
             at,
-            message,
+            message: message.as_ref().into(),
             suggestion: None,
         }
     }
diff --git a/lib/src/lints.rs b/lib/src/lints.rs
index 98aa404..7da4933 100644
--- a/lib/src/lints.rs
+++ b/lib/src/lints.rs
@@ -6,4 +6,5 @@ lint_map! {
     manual_inherit,
     manual_inherit_from,
     legacy_let_syntax,
+    collapsible_let_in,
 }
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
+            }
+        }
+    }
+}
+
diff --git a/lib/src/make.rs b/lib/src/make.rs
index 2a6450c..aec32d5 100644
--- a/lib/src/make.rs
+++ b/lib/src/make.rs
@@ -75,3 +75,7 @@ pub fn select(set: &SyntaxNode, index: &SyntaxNode) -> types::Select {
 pub fn ident(text: &str) -> types::Ident {
     ast_from_text(text)
 }
+
+pub fn empty() -> types::Root {
+    ast_from_text("")
+}