scope: add inherits_from() to treat inherit clauses specially

src/dead_code_tests.rs

@@ -159,6 +159,12 @@ fn rec_attrset_shadowed() {
     assert_eq!(results[0].binding.name.as_str(), "dead");
 }
 
+#[test]
+fn let_inherit_in_let_inherit_alive() {
+    let results = run("let alive = true; in let inherit alive; in alive");
+    assert_eq!(0, results.len());
+}
+
 #[test]
 fn let_inherit_in_rec_attrset_alive() {
     let results = run("let alive = true; in rec { inherit alive; }");

src/scope.rs

@@ -218,4 +218,36 @@ impl Scope {
                 ),
         }
     }
+
+    /// Check the `inherit (var) ...` and `inherit vars` clauses for a
+    /// given `name`.
+    ///
+    /// Although a scope may shadow existing variable bindings, it can
+    /// `inherit` bindings from the outer scope.
+    pub fn inherits_from(&self, name: &Ident) -> bool {
+        match self {
+            Scope::LambdaPattern(_, _) | Scope::LambdaArg(_, _) =>
+                false,
+
+            Scope::LetIn(let_in) =>
+                let_in.inherits().any(|inherit|
+                    inherit.from()
+                        .map(|from|
+                             crate::usage::find_usage(name, from.node().clone())
+                        ).unwrap_or_else(||
+                             crate::usage::find_usage(name, inherit.node().clone())
+                        )
+                ),
+
+            Scope::RecAttrSet(attr_set) =>
+                attr_set.inherits().any(|inherit|
+                    inherit.from()
+                        .map(|from|
+                             crate::usage::find_usage(name, from.node().clone())
+                        ).unwrap_or_else(||
+                             crate::usage::find_usage(name, inherit.node().clone())
+                        )
+                ),
+        }
+    }
 }

src/usage.rs

@@ -13,6 +13,10 @@ use crate::scope::Scope;
 /// find out if `name` is used in `node`
 pub fn find_usage(name: &Ident, node: SyntaxNode<NixLanguage>) -> bool {
     if let Some(scope) = Scope::new(&node) {
+        if scope.inherits_from(name) {
+            return true;
+        }
+
         for binding in scope.bindings() {
             if binding.name.as_str() == name.as_str() {
                 // shadowed by a a new child scope that redefines the