Tuple indexing regression

[taiki-e]

#59421 breaks some code generated by proc macros because quote converts integer to a tokenstream with a suffix.

Proc macro:

extern crate proc_macro;
use proc_macro::TokenStream;
use quote::quote;

#[proc_macro]
pub fn foo(_: TokenStream) -> TokenStream {
    let index = 0;
    TokenStream::from(quote! {
        struct Foo(());
        fn bar() {
            let x = Foo(());
            x.#index
        }
    })
}

Calling code:

foo!();

Error:

error: suffixes on a tuple index are invalid
 --> src\lib.rs:3:1
  |
3 | foo!();
  | ^^^^^^^ invalid suffix `i32`

It can be avoided by using syn::Index::from or proc_macro::Literal::*_unsuffixed.

I have already fixed my library (taiki-e/pin-project@5f5cc35), but it can happen elsewhere.

[taiki-e]

cc @estebank

[estebank]

@petrochenkov what do you think, should we accept i32 there? It feels like a bad idea and I feel the right fix is in quote, but for expidiency we could continue accepting this suffix in particular for a couple of releases.

CC @rust-lang/compiler @rust-lang/lang @dtolnay

[Centril]

I don't think there's any language design that needs this so I'm happy with a C-future-compatibility warning here for a few releases. I agree the right fix is in quote or in dependent crates.

[dtolnay]

I believe quote is behaving correctly so I won't be making any code change associated with this in quote.

[Centril]

@dtolnay Perhaps a change in documentation pointing out the right way to achieve what the user wants?

[dtolnay]

Sure, that could be good. I filed dtolnay/quote#101 to follow up. Thanks!

[estebank]

After an in person conversations with other members of the team, I feel that the right course if action is to wait until the beta crater run and depending on the extent of the in the wild breakage, backporting a beta only change that makes this a warning instead of a hard error for one release. Although it is bad that it was allowed for a while in stable, the new behavior is indeed the right behavior. If crater doesn't catch too many crates with problems we should go ahead and leave this as is