rust-lang / rust-lang/rust

proc_macro::quote does not work with interpolated constant inside repetition

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A-proc-macros C-bug F-proc_macro_quote T-libs
Dominant language
Rust
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Description

A constant N can be interpolated as $N inside quote, but not inside a repetition. I believe this should be supported.

#![feature(proc_macro_quote)]

use proc_macro::{quote, TokenStream};

const N: usize = 8;

#[proc_macro]
pub fn repro(_input: TokenStream) -> TokenStream {
    let ok = '?';
    let array = ['0', '1', '2'];
    let tokens = quote! {
        // Okay
        ... $ok $N ...

        // Okay
        $($ok $array)*

        // Fail
        $($N $array)*
    };
    eprintln!("{}", tokens);
    TokenStream::new()
}
error[E0530]: let bindings cannot shadow constants
  --> src/lib.rs:19:12
   |
 5 | const N: usize = 8;
   | ------------------- the constant `N` is defined here
...
19 |         $($N $array)*
   |            ^ cannot be named the same as a constant

Expected output:

... '?' 8usize ... '?' '0' '?' '1' '?' '2' 8usize '0' 8usize '1' 8usize '2'

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First steps

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  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the reproducer in src/lib.rs and run the proc_macro::quote example to confirm the failure and expected token output. Trace how quote handles interpolation inside repetitions, using the diagnostic and linked tracking issue #54722 as context. Done means constants interpolate correctly inside repetitions without the shadowing error.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Clearly specified
Newbie friendliness
48/100

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