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

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] {}