Add regression test for #59003

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

tests/ui/lint/dead-code/issue-59003.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+// Make sure we don't have any false positives about the "struct is never constructed" lint.
+
+#![deny(dead_code)]
+
+struct Foo {
+    #[allow(dead_code)]
+    inner: u32,
+}
+
+impl From<u32> for Foo {
+    fn from(inner: u32) -> Self {
+        Self { inner }
+    }
+}
+
+fn main() {}