Add proc macro for unused assignments and corresponding test

Author: chenyukang

tests/ui/liveness/auxiliary/aux_issue_147648.rs

@@ -0,0 +1,23 @@
+#![feature(proc_macro_quote)]
+
+extern crate proc_macro;
+use proc_macro::*;
+
+#[proc_macro_derive(UnusedAssign)]
+pub fn generate(ts: TokenStream) -> TokenStream {
+    let mut ts = ts.into_iter();
+    let _pub = ts.next();
+    let _struct = ts.next();
+    let name = ts.next().unwrap();
+    let TokenTree::Group(fields) = ts.next().unwrap() else { panic!() };
+    let mut fields = fields.stream().into_iter();
+    let field = fields.next().unwrap();
+
+    quote! {
+        impl Drop for $name {
+            fn drop(&mut self) {
+                let Self { $field } = self;
+            }
+        }
+    }
+}

tests/ui/liveness/unused-assignments-from-macro-147648.rs

@@ -0,0 +1,18 @@
+//@ check-pass
+//@ proc-macro: aux_issue_147648.rs
+
+#![deny(unused_assignments)]
+#![allow(dead_code)]
+#![allow(unused_variables)]
+
+extern crate aux_issue_147648;
+use aux_issue_147648::UnusedAssign;
+
+#[derive(UnusedAssign)]
+pub struct MyError {
+    source_code: (),
+}
+
+fn main() {
+    let _error = MyError { source_code: () };
+}