From f40199bea21507c59798dc9ec8653ccf2fbf143a Mon Sep 17 00:00:00 2001
From: oxalica <oxalicc@pm.me>
Date: Sat, 4 Mar 2023 01:53:52 +0800
Subject: [PATCH] Warn unused pat parameters for modules and packages

---
 crates/ide/src/def/liveness.rs | 42 +++++++++++++++++++++++++++++++---
 docs/features.md               |  3 ++-
 2 files changed, 41 insertions(+), 4 deletions(-)

diff --git a/crates/ide/src/def/liveness.rs b/crates/ide/src/def/liveness.rs
index 6b4e935..6e53750 100644
--- a/crates/ide/src/def/liveness.rs
+++ b/crates/ide/src/def/liveness.rs
@@ -16,8 +16,9 @@
 //! - Unused `let` bindings.
 //! - Unused `with` expressions.
 //! - Unnecessary `rec` attrsets.
+//! - Unused parameters of a package.
 use super::{BindingValue, DefDatabase, Expr, ExprId, NameId, ResolveResult};
-use crate::{Diagnostic, DiagnosticKind, FileId};
+use crate::{Diagnostic, DiagnosticKind, FileId, ModuleKind};
 use la_arena::ArenaMap;
 use std::collections::HashMap;
 use std::sync::Arc;
@@ -75,6 +76,13 @@ pub(crate) fn liveness_check_query(
     let module = db.module(file_id);
     let name_res = db.name_resolution(file_id);
 
+    let must_use_params_expr = match &*db.module_kind(file_id) {
+        ModuleKind::Package { lambda_expr }
+        | ModuleKind::ConfigModule { lambda_expr }
+        | ModuleKind::Config { lambda_expr } => Some(*lambda_expr),
+        _ => None,
+    };
+
     // Unused let-bindings are eagerly collected into this.
     let mut unused_defs = Vec::new();
 
@@ -145,8 +153,26 @@ pub(crate) fn liveness_check_query(
     let mut unused_recs = Vec::new();
     for (expr, kind) in module.exprs() {
         match kind {
-            &Expr::Lambda(Some(param), Some(_), _) if visited_defs.get(param).is_none() => {
-                unused_defs.push(param);
+            Expr::Lambda(param, pat, _) => {
+                // `{ ... }@bar: ...`
+                //          ^ Unused and removable.
+                if let Some(param) = *param {
+                    if pat.is_some() && visited_defs.get(param).is_none() {
+                        unused_defs.push(param);
+                    }
+                }
+                // `{ foo, ... }[@bar]: ...`
+                //    ^ Unused and removable, only for packages and configurations.
+                if must_use_params_expr == Some(expr) {
+                    if let Some(pat) = pat {
+                        unused_defs.extend(
+                            pat.fields
+                                .iter()
+                                .filter_map(|(name, _)| *name)
+                                .filter(|name| visited_defs.get(*name).is_none()),
+                        );
+                    }
+                }
             }
             &Expr::With(..) if visited_withs.get(expr).is_none() => {
                 unused_withs.push(expr);
@@ -251,4 +277,14 @@ mod tests {
     fn with_used_by_unused_let() {
         check("with 1; let $0a = from_with; in 1");
     }
+
+    #[test]
+    fn unused_pat_param_package() {
+        check("{ stdenv, $0hello }: stdenv.mkDerivation { }");
+    }
+
+    #[test]
+    fn unused_pat_param_config() {
+        check("{ $0lib, pkgs, ... }: { foo = with pkgs; [ bar ]; }");
+    }
 }
diff --git a/docs/features.md b/docs/features.md
index cb67e85..3111f76 100644
--- a/docs/features.md
+++ b/docs/features.md
@@ -43,12 +43,13 @@ This incomplete list tracks noteble features currently implemented or planned.
 
 - [x] Diagnostics. `textDocument/publishDiagnostics`
 
-  - [x] Syntax errors. 
+  - [x] Syntax errors.
   - [x] Hard semantic errors reported as parse errors by Nix, like duplicated keys in attrsets.
   - [x] Undefiend names.
   - [x] Warnings of legacy syntax.
   - [x] Warnings of unnecessary syntax.
   - [x] Warnings of unused bindings, `with` and `rec`.
+  - [x] Warnings of unused parameters for packages and modules.
   - [ ] Client pulled diagnostics.
   - [x] Custom filter on kinds.
   - [x] Exclude files.