Using fn params in generated code

[CodeSandwich]

I know it's not a Syn issue, but I have problems with using function parameters in macro-generated code.

I'm writing a procedural macro, which injects some code on the beginning of a function. The code uses function's parameters. Up to now it used Syn 0.11 and after upgrading to 0.12 it still worked fine.

After upgrade I enabled proc-macro2's nightly feature to enable error reporting in code processed by macro. Suddenly all the tests, where injected code used function's parameters started to fail:

error[E0425]: cannot find value `x` in this scope
   --> tests/injecting.rs:641:5
    |
641 |     #[mockable]
    |     ^^^^^^^^^^^ not found in this scope

I tried boiling issue down to minimum example and came up with a primitive macro:

#[proc_macro_attribute]
pub fn print_x(_: TokenStream, token_stream: TokenStream) -> TokenStream {
    println!("BEFORE:\n{}", token_stream);
    let mut tokens: Vec<TokenTree> = token_stream.into_iter().collect();
    tokens.pop();
    tokens.extend(TokenStream::from_str("{println!(\"{}\", x)}").unwrap());
    let new_tokens = tokens.into_iter().collect();
    println!("AFTER:\n{}", new_tokens);
    new_tokens
}

And used it:

#[print_x]
pub fn args_test(x: u32) {
    assert!(x > 1)
}

It failed perfectly:

BEFORE:
pub fn args_test(x: u32) { assert!(x > 1) }
AFTER:
pub fn args_test ( x : u32 ) { println ! ( "{}" , x ) }
error[E0425]: cannot find value `x` in this scope
   --> tests/injecting.rs:662:1
    |
662 | #[print_x]
    | ^^^^^^^^^^ not found in this scope

Manually replacing annotated function with pub fn args_test ( x : u32 ) { println ! ( "{}" , x ) } gives no errors, the code is fine.

The fun escalated when I removed the nightly proc-macro2 feature: real macro started working again, but the boiled-down print_x still failed with the same error.

It's really confusing, I see no logic behind these errors. I know, if it's not a Syn problem, but could anybody help me with it? Thank you!

[dtolnay]

This has to do with whether the x token is spanned to resolve at the macro call site or at the macro def site. Check out rust-lang/rust#45934 for some background, and see this example of a macro that uses def site and call site spans correctly.

[CodeSandwich]

You're right, that's exactly my problem, thank you very much!