Rust "cannot find macro `X` in this scope" in CI

The compiler found no macro by that name in scope. The macro’s crate isn’t imported, a feature that exports it isn’t enabled, or the macro needs an explicit use on the 2018+ edition.

What this error means

cargo build fails with error: cannot find macro X in this scope, often with a help: suggesting an import. Frequently hits json!, format_args!-style helpers, or derive-adjacent macros after a refactor or upgrade.

cargo output

error: cannot find macro `json` in this scope
 --> src/main.rs:6:16
  |
6 |     let body = json!({ "ok": true });
  |                ^^^^
  = help: consider importing this macro:
          serde_json::json

Common causes

Macro not imported

On the 2018+ edition, macros are imported like items. json! needs use serde_json::json; (or a path call) rather than the old #[macro_use] extern crate.

Feature or crate providing the macro is absent

The macro lives behind a feature (e.g. a macros feature) or in a crate not declared in Cargo.toml, so it’s never brought into scope.

How to fix it

Import the macro

Bring the macro into scope with a use, as the help line suggests.

Rust

use serde_json::json;   // or call it by path: serde_json::json!(...)

Enable the feature that exports it

When a feature gates the macro, enable it on the dependency.

Cargo.toml

[dependencies]
tokio = { version = "1", features = ["macros", "rt-multi-thread"] }

How to prevent it

Import macros with use on modern editions instead of #[macro_use].
Enable the features that export the macros you call.
Run cargo check locally to catch macro-resolution errors early.