Find used variables that start with an underscore

Fixes Github issue #90

example.nix

@@ -9,10 +9,12 @@ let
   used4 = { t = used_inherit; };
   shadowed = 42;
   _unused = unused: false;
+  _used = 23;
 in {
   x = { unusedArg2, x ? args.y, ... }@args: used1 + x;
   inherit used2;
   "${used3}" = true;
   y = used4.t;
   z = let shadowed = 23; in shadowed;
+  inherit _used;
 }

src/dead_code.rs

@@ -15,11 +15,17 @@ pub struct DeadCode {
     pub scope: Scope,
     /// The [`Binding`] that is found to be unused
     pub binding: Binding,
+    /// Used or unused?
+    unused: bool,
 }
 
 impl fmt::Display for DeadCode {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        write!(fmt, "Unused {}: {}", self.scope, self.binding.name)
+        if self.unused {
+            write!(fmt, "Unused {}: {}", self.scope, self.binding.name)
+        } else {
+            write!(fmt, "Used {}: {}", self.scope, self.binding.name)
+        }
     }
 }
 
@@ -32,6 +38,8 @@ pub struct Settings {
     pub no_lambda_pattern_names: bool,
     /// Ignore all `Binding` that start with `_`
     pub no_underscore: bool,
+    /// Warn on used binding that starts with `_`
+    pub warn_used_underscore: bool,
 }
 
 impl Settings {
@@ -71,25 +79,31 @@ impl Settings {
                         continue;
                     }
 
-                    if binding.is_mortal()
-                    && scope.bodies().all(|body|
-                        // remove this binding's own node
-                        body == binding.decl_node
-                        // excluding already unused results
-                        || dead.contains(&body)
-                        || is_dead_inherit(dead, &body)
-                        // or not used anywhere
-                        || ! usage::find(&binding.name, &body)
-                    )
-                    && ! binding.has_pragma_skip() {
-                        dead.insert(binding.decl_node.clone());
-                        results.insert(
-                            binding.decl_node.clone(),
-                            DeadCode {
-                                scope: scope.clone(),
-                                binding,
-                            },
+                    if binding.is_mortal() && ! binding.has_pragma_skip() {
+                        let unused = scope.bodies().all(|body|
+                            // remove this binding's own node
+                            body == binding.decl_node
+                            // excluding already unused results
+                            || dead.contains(&body)
+                            || is_dead_inherit(dead, &body)
+                            // or not used anywhere
+                            || ! usage::find(&binding.name, &body)
                         );
+                        if unused || (
+                            self.warn_used_underscore &&
+                            binding.name.syntax().text().char_at(0.into()) == Some('_') &&
+                            ! unused
+                        ) {
+                            dead.insert(binding.decl_node.clone());
+                            results.insert(
+                                binding.decl_node.clone(),
+                                DeadCode {
+                                    scope: scope.clone(),
+                                    binding,
+                                    unused,
+                                },
+                            );
+                        }
                     }
                 }
             }

src/dead_code_tests.rs

@@ -3,16 +3,20 @@
 use rowan::ast::AstNode;
 use crate::dead_code::{DeadCode, Settings};
 
-fn run(content: &str) -> Vec<DeadCode> {
+fn run_settings(content: &str, settings: Settings) -> Vec<DeadCode> {
     let ast = rnix::Root::parse(content);
     assert_eq!(0, ast.errors().len());
 
-    Settings {
+    settings.find_dead_code(&ast.syntax())
+}
+
+fn run(content: &str) -> Vec<DeadCode> {
+    run_settings(content, Settings {
         no_lambda_arg: false,
         no_lambda_pattern_names: false,
         no_underscore: false,
-    }
-    .find_dead_code(&ast.syntax())
+        warn_used_underscore: false,
+    })
 }
 
 #[test]
@@ -482,3 +486,17 @@ fn let_args_string_splice() {
     ");
     assert_eq!(0, results.len());
 }
+
+#[test]
+fn used_underscore_let() {
+    let results = run_settings("
+      let _x = 23;
+      in _x
+    ", Settings {
+        no_lambda_arg: false,
+        no_lambda_pattern_names: false,
+        no_underscore: false,
+        warn_used_underscore: true,
+    });
+    assert_eq!(1, results.len());
+}

src/edit_tests.rs

@@ -10,6 +10,7 @@ fn run(content: &str) -> (String, bool) {
         no_lambda_arg: false,
         no_lambda_pattern_names: false,
         no_underscore: false,
+        warn_used_underscore: false,
     }
     .find_dead_code(&ast.syntax());
     crate::edit::edit_dead_code(content, results.into_iter())

src/lib.rs

@@ -14,6 +14,7 @@
 //!     no_lambda_arg: false,
 //!     no_lambda_pattern_names: false,
 //!     no_underscore: false,
+//!     warn_used_underscore: false,
 //! }.find_dead_code(&ast.syntax());
 //!
 //! for dead_code in &results {

src/main.rs

@@ -47,6 +47,13 @@ fn main() {
                        (Lambda arguments starting with _ are not checked anyway.)"
                 ),
         )
+        .arg(
+            Arg::new("WARN_USED_UNDERSCORE")
+                .action(ArgAction::SetTrue)
+                .short('W')
+                .long("warn-used-underscore")
+                .help("Warn if bindings are referenced that start with '_'"),
+        )
         .arg(
             Arg::new("QUIET")
                 .action(ArgAction::SetTrue)
@@ -112,6 +119,7 @@ fn main() {
         no_lambda_arg: matches.get_flag("NO_LAMBDA_ARG"),
         no_lambda_pattern_names: matches.get_flag("NO_LAMBDA_PATTERN_NAMES"),
         no_underscore: matches.get_flag("NO_UNDERSCORE"),
+        warn_used_underscore: matches.get_flag("WARN_USED_UNDERSCORE"),
     };
     let quiet = matches.get_flag("QUIET");
     let edit = matches.get_flag("EDIT");