syn::ItemFn.to_token_stream hidden/guarded? - rust

Question

I thought I'd try to write a basic function decorator that logs the input and output of functions as they are called. I have many functions that have roughly the same signature, and I'd like to log the io of common part. That is, think of adding a decorator like this
#[my_decorator]
foo(<unknown initial args>, final_arg: T) -> R {BODY}
would effectively turn foo into
foo(<unknown initial args>, final_arg: T) -> R {
let result = {BODY};
debug!("final_arg is {:?}" final_arg)
debug!("return value is {:?}" result)
result
}
I spent today searching around for best approaches to this and it appears that the best way would be to define a #[proc_macro_attribute]
I am at the following sanity-checking stub (which the documentation here would suggest is valid)
use proc_macro::TokenStream;
use syn;
use syn::ItemFn;
#[proc_macro_attribute]
pub fn log_io(_attr: TokenStream, item: TokenStream) -> TokenStream {
let item_fn: ItemFn = syn::parse_macro_input!(item);
item_fn.to_token_stream()
}
Which IIUC should be the No-Op decorator. But the compiler gives
error[E0599]: no method named `to_token_stream` found for struct `ItemFn` in the current scope
--> src/lib.rs:8:13
|
8 | item_fn.to_token_stream()
| ^^^^^^^^^^^^^^^ method not found in `ItemFn`
|
::: /Users/arthurtilley/.cargo/registry/src/github.com-1ecc6299db9ec823/quote-1.0.21/src/to_tokens.rs:59:8
|
59 | fn to_token_stream(&self) -> TokenStream {
| --------------- the method is available for `ItemFn` here
|
= help: items from traits can only be used if the trait is in scope
help: one of the expressions' fields has a method of the same name
|
8 | item_fn.sig.to_token_stream()
| ++++
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
|
1 | use syn::__private::ToTokens;
|
Adding the suspicious looking use syn::__private::ToTokens; the compiler then gives
error[E0308]: mismatched types
--> src/lib.rs:9:5
|
7 | pub fn log_io(_attr: TokenStream, item: TokenStream) -> TokenStream {
| ----------- expected `proc_macro::TokenStream` because of return type
8 | let item_fn: ItemFn = syn::parse_macro_input!(item);
9 | item_fn.to_token_stream()
| ^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `proc_macro::TokenStream`, found struct `TokenStream2`
None of this is mentioned in the documentation https://docs.rs/syn/latest/syn/struct.ItemFn.html so I'm a little stumped.
EDIT: #Shelvacu I'm using
syn = {version ="*" ("1.0.99"), features = ["full", "printing", "extra-traits"]}
proc-macro2 = "*" ("1.0.43")

Answer 1

If you click on TokenStream in fn to_token_stream you'll notice that it returns proc_macro2::TokenStream which is different from proc_macro::TokenStream that your function is defined to return. You can read more about this in the proc-macro2 docs. The way to convert proc_macro2::TokenStream to proc_macro::TokenStream as recommended in the docs is:
proc_macro::TokenStream::from(output)
In your case, this should work:
#[proc_macro_attribute]
pub fn log_io(_attr: TokenStream, item: TokenStream) -> TokenStream {
let item_fn: ItemFn = syn::parse_macro_input!(item);
proc_macro::TokenStream::from(item_fn.to_token_stream());
}
Also, you should import ToTokens trait from quote::ToTokens (from the quote crate) as the syn re-export is marked private.