From 9ac0da8f39680f1b09321980187bc502580b526a Mon Sep 17 00:00:00 2001
From: Maybe Waffle <waffle.lapkin@gmail.com>
Date: Sun, 13 Nov 2022 13:55:04 +0000
Subject: [PATCH] Make `unused_allocation` lint warn against `Box::new`

---
 compiler/rustc_lint/src/unused.rs | 8 ++++++--
 compiler/rustc_span/src/symbol.rs | 1 +
 library/alloc/src/boxed.rs        | 1 +
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_lint/src/unused.rs b/compiler/rustc_lint/src/unused.rs
index 3a92f5806c9..b32fc96164a 100644
--- a/compiler/rustc_lint/src/unused.rs
+++ b/compiler/rustc_lint/src/unused.rs
@@ -1349,9 +1349,8 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust
-    /// #![feature(box_syntax)]
     /// fn main() {
-    ///     let a = (box [1, 2, 3]).len();
+    ///     let a = Box::new([1, 2, 3]).len();
     /// }
     /// ```
     ///
@@ -1373,6 +1372,11 @@ impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
     fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
         match e.kind {
             hir::ExprKind::Box(_) => {}
+            hir::ExprKind::Call(path_expr, [_])
+                if let hir::ExprKind::Path(qpath) = &path_expr.kind
+                && let Some(did) = cx.qpath_res(qpath, path_expr.hir_id).opt_def_id()
+                && cx.tcx.is_diagnostic_item(sym::box_new, did)
+                => {}
             _ => return,
         }
 
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index fb579e4ff77..a76bf45ec89 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -429,6 +429,7 @@ symbols! {
         borrowck_graphviz_format,
         borrowck_graphviz_postflow,
         box_free,
+        box_new,
         box_patterns,
         box_syntax,
         bpf_target_feature,
diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index 44a37899007..241b11c3f5f 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -214,6 +214,7 @@ impl<T> Box<T> {
     #[inline(always)]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
+    #[rustc_diagnostic_item = "box_new"]
     pub fn new(x: T) -> Self {
         #[rustc_box]
         Box::new(x)