Avoid accessing legacy TLV fields after the TLV-stream read

`legacy` fields in TLV stream reads may be used to read fields
which are later moved into some other field using either
`default_value` or `static_value` "reads". This works fine if the
field supports copy semantics, however if it does not, the
accessing of the field in `_decode_and_build` after the TLV stream
read completes but before the struct is built results in a "use
after move" error.

Instead, here, we drop the attempt to hide unused variable warnings
entirely, dropping the post-TLV-stream access to legacy variables,
allowing their use in move semantics for later fields.

Author: TheBlueMatt

lightning-macros/src/lib.rs

@@ -246,8 +246,8 @@ pub fn skip_legacy_fields(expr: TokenStream) -> TokenStream {
 /// Is expected to wrap a struct definition like
 /// ```ignore
 /// drop_legacy_field_definition!(Self {
-/// 	field1: $crate::_ignore_arg!(field1, option),
-/// 	field2: $crate::_ignore_arg!(field2, (legacy, u64, {})),
+/// 	field1: $crate::_init_tlv_based_struct_field!(field1, option),
+/// 	field2: $crate::_init_tlv_based_struct_field!(field2, (legacy, u64, {})),
 /// })
 /// ```
 /// and will drop fields defined like `field2` with a type starting with `legacy`.
@@ -277,7 +277,7 @@ pub fn drop_legacy_field_definition(expr: TokenStream) -> TokenStream {
 	for field in new_fields {
 		if let syn::Expr::Macro(syn::ExprMacro { mac, .. }) = &field.expr {
 			let macro_name = mac.path.segments.last().unwrap().ident.to_string();
-			let is_init = macro_name == "_ignore_arg";
+			let is_init = macro_name == "_init_tlv_based_struct_field";
 			// Skip `field_name` and `:`, giving us just the type's group
 			let ty_tokens = mac.tokens.clone().into_iter().skip(2).next();
 			if let Some(proc_macro2::TokenTree::Group(group)) = ty_tokens {

lightning/src/util/ser_macros.rs

@@ -894,17 +894,6 @@ macro_rules! _init_and_read_tlv_stream {
 	}
 }
 
-/// Dummy macro that drops the second argument (which is used by
-/// [`lightning_macros::drop_legacy_field_definition`] to match for legacy fields but isn't needed
-/// in the final code we want to generate).
-#[doc(hidden)]
-#[macro_export]
-macro_rules! _ignore_arg {
-	($field: ident, $fieldty: tt) => {
-		$field
-	};
-}
-
 /// Reads a TLV stream with the given fields to build a struct/enum variant of type `$thing`
 #[doc(hidden)]
 #[macro_export]
@@ -913,18 +902,8 @@ macro_rules! _decode_and_build {
 		$crate::_init_and_read_len_prefixed_tlv_fields!($stream, {
 			$(($type, $field, $fieldty)),*
 		});
-		// rustc is kinda dumb about unused variable warnings when we declare a variable via an
-		// ident in a macro and then use it in an expr also defined in the same macro call. Thus,
-		// it may generate unused variable warnings for variables that are, in fact, very much
-		// used. Instead, we just blanket ignore unused variables here as it may be useful to write
-		// field names without a _ prefix for legacy fields even if we don't care about the read
-		// value.
-		$(
-			#[allow(unused_variables)]
-			let $field = $crate::_init_tlv_based_struct_field!($field, $fieldty);
-		)*
 		::lightning_macros::drop_legacy_field_definition!($thing {
-			$($field: $crate::_ignore_arg!($field, $fieldty)),*
+			$($field: $crate::_init_tlv_based_struct_field!($field, $fieldty)),*
 		})
 	} }
 }