decl_macro. no field 0 on type. a field with a similar name exists: 0

[planetoryd]

I tried this code:

https://github.com/planetoryd/rust_bug_missing_field

#![feature(decl_macro)]

make_struct!(Uh);

macro make_struct ($id:ident) {
    pub struct $id(pub i32);
}

fn main() {
    let k = Uh(2);
    println!("{}", k.0);
}

The equivalent code without macro works.

hash@hash-pc /s/e/missing_field (master)> cargo b
   Compiling missing_field v0.1.0 (/space/experim/missing_field)
error[E0609]: no field `0` on type `Uh`
  --> src/main.rs:11:22
   |
11 |     println!("{}", k.0);
   |                      ^ help: a field with a similar name exists: `0`

For more information about this error, try `rustc --explain E0609`.
error: could not compile `missing_field` (bin "missing_field") due to previous error
warning: build failed, waiting for other jobs to finish...

Meta

rustc --version --verbose:

rustc 1.75.0-nightly (e0d7ed1f4 2023-10-01)
binary: rustc
commit-hash: e0d7ed1f453fb54578cc96dfea859b0e7be15016
commit-date: 2023-10-01
host: x86_64-unknown-linux-gnu
release: 1.75.0-nightly
LLVM version: **17.0.2**

[planetoryd]

Methods don't work either.

[fmease]

This works as intended, this is macro hygiene at play. The tuple field 0 declared inside the macro is invisible to the outside world.

Typically if you'd like to make such an identifier accessible, you'd turn it into a parameter like $field:ident. However, for tuple structs, I don't actually know if there's a way to do that :/

To elaborate, the (implicit) identifier 0 inside the macro is different from the identifier 0 found in main, they differ by syntactic context. For example, in this playground I seemingly define a struct with two (!) fields field which works because they live in a different context.

[fmease]

If you're fine with changing your tuple struct to a normal struct, then you can fix your code like this: Playground.

Alternatively, you can turn the macro 2.0 into a macro_rules! macro which follows different hygiene rules.

[fmease]

Let's leave this open since the diagnostic could be improved.