dead_code: look at trait impls even if they don't contain items

compiler/rustc_passes/src/dead.rs

@@ -369,7 +369,7 @@ fn has_allow_dead_code_or_lang_attr(
 //         - This is because lang items are always callable from elsewhere.
 //   or
 //   2) We are not sure to be live or not
-//     * Implementation of a trait method
+//     * Implementations of traits and trait methods
 struct LifeSeeder<'k, 'tcx> {
     worklist: Vec<hir::HirId>,
     krate: &'k hir::Crate<'k>,
@@ -415,6 +415,9 @@ impl<'v, 'k, 'tcx> ItemLikeVisitor<'v> for LifeSeeder<'k, 'tcx> {
                 }
             }
             hir::ItemKind::Impl { ref of_trait, items, .. } => {
+                if of_trait.is_some() {
+                    self.worklist.push(item.hir_id);
+                }
                 for impl_item_ref in items {
                     let impl_item = self.krate.impl_item(impl_item_ref.id);
                     if of_trait.is_some()

src/test/ui/const-generics/issues/issue-70225.rs

@@ -0,0 +1,21 @@
+// check-pass
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+#![deny(dead_code)]
+
+// We previously incorrectly linted `L` as unused here.
+const L: usize = 3;
+
+fn main() {
+    let p = Printer {};
+    p.print();
+}
+
+trait Print<const N: usize> {
+    fn print(&self) -> usize {
+        3
+    }
+}
+
+struct Printer {}
+impl Print<L> for Printer {}

src/test/ui/lint/dead-code/trait-impl.rs

@@ -0,0 +1,19 @@
+// check-pass
+#![deny(dead_code)]
+
+enum Foo {
+    Bar,
+}
+
+fn main() {
+    let p = [0; 0];
+    p.bar();
+}
+
+trait Bar {
+    fn bar(&self) -> usize {
+        3
+    }
+}
+
+impl Bar for [u32; Foo::Bar as usize] {}