rust-lang / rust-lang/rust

`cfg_accessible` is not processed before derive macros

Open
#148,513 1 comment 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

A-cfg A-macros A-proc-macros C-bug F-cfg_accessible needs-triage T-compiler
Dominant language
Rust
Stars
119k
Forks
16.1k
PR merge metrics
PR metrics pending

Description

I tried this code:

#![feature(cfg_accessible)]

#[derive(Clone)]
#[cfg_accessible(std::nonexistent)]
struct Thing;

I expected the code to compile. Instead, I got the following error:

error[E0412]: cannot find type `Thing` in this scope
 --> src/lib.rs:3:10
  |
3 | #[derive(Clone)]
  |          ^^^^^ not found in this scope

error[E0425]: cannot find value `Thing` in this scope
 --> src/lib.rs:3:10
  |
3 | #[derive(Clone)]
  |          ^^^^^ not found in this scope

Some errors have detailed explanations: E0412, E0425.
For more information about an error, try `rustc --explain E0412`.

Note that similar code using cfg instead of cfg_accessible compiles fine:

#![feature(cfg_accessible)]

#[derive(Clone)]
#[cfg(false)]
struct Thing;

From my testing, it seems that #[cfg] is processed before derive macros see the input. However, it is not processed before attribute macros see the input. However, cfg_accessible is treated as an attribute macros, and is processed from top-to-bottom along with other normal derive macros and attribute macros.

Meta

Reproducible on the playground with version 1.93.0-nightly (2025-11-04 f15a7f38580ddbdc1a23)

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  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

No repository file or test is named. Reproduce the playground example with the reported nightly version, then trace how cfg_accessible is handled relative to derive and attribute macros. Done means the shown cfg_accessible example compiles as expected, matching the behavior observed with cfg.

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
Mostly clear
Newbie friendliness
35/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.