Address clippy::must_use_candidate in template (#1740)

This addresses `clippy::must_use_candidate` and applies a minor
optimization: it removes a Vec realloc by simply taking the existing
one, instead of cloning and clearing it.

---------

Co-authored-by: Jubilee <[email protected]>

cargo-pgrx/src/templates/lib_rs

@@ -27,6 +27,7 @@ pub mod pg_test {{
         // perform one-off initialization when the pg_test framework starts
     }}
 
+    #[must_use]
     pub fn postgresql_conf_options() -> Vec<&'static str> {{
         // return any postgresql.conf settings that are required for your tests
         vec![]

pgrx-macros/src/rewriter.rs

@@ -10,6 +10,7 @@
 extern crate proc_macro;
 
 use quote::{format_ident, quote, quote_spanned};
+use std::mem;
 use std::ops::Deref;
 use std::str::FromStr;
 use syn::punctuated::Punctuated;
@@ -35,8 +36,7 @@ pub fn item_fn_without_rewrite(mut func: ItemFn) -> syn::Result<proc_macro2::Tok
     let input_func_name = func.sig.ident.to_string();
     let sig = func.sig.clone();
     let vis = func.vis.clone();
-    let attrs = func.attrs.clone();
-
+    let mut attrs = mem::take(&mut func.attrs);
     let generics = func.sig.generics.clone();
 
     if attrs.iter().any(|attr| attr.path().is_ident("no_mangle"))
@@ -52,7 +52,6 @@ pub fn item_fn_without_rewrite(mut func: ItemFn) -> syn::Result<proc_macro2::Tok
     // but for the inner function (the one we're wrapping) we don't need any kind of
     // abi classification
     func.sig.abi = None;
-    func.attrs.clear();
 
     // nor do we need a visibility beyond "private"
     func.vis = Visibility::Inherited;