Fix E0798: Derive of #[serde]-like attribute should produce an item before calling set_proc_macro_attr

Wed, 03 Jun 2026 10:20:04 +0800

Fix E0798: Derive of `#[serde]`-like attribute should produce an item before calling `set_proc_macro_attr`

1. Symptoms

When compiling a Rust project that uses derive macros (commonly with #[serde(Serialize, Deserialize)] or custom derive macros), you encounter the following error:

error[E0798]: derive of `#[serde]`-like attribute should produce an item before calling `set_proc_macro_attr`
  --> src/lib.rs:12:1
   |
12 | #[derive(MyCustomDerive)]
   |  ^^^^^^^^^^^^^^^^^^^^^^
---

The error appears at the `#[derive(...)]` attribute site on a struct, enum, or other item. The compiler message is unambiguous: the derive macro attempted to call `set_proc_macro_attr` (a proc-macro2 function) before producing any output item.

Additional symptoms that may accompany this error:

```text
error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0798`.

The derive macro may fail to expand, leaving the original type untransformed.
If the macro is used on multiple items, every invocation triggers the same error.
The error is a hard compile-time failure — no binary or library is produced.

Example code that triggers E0798:

Derive-Macro on ErrorVault — Developer Error Code Dictionary

Fix E0798: Derive of #[serde]-like attribute should produce an item before calling set_proc_macro_attr

Fix E0798: Derive of #[serde]-like attribute should produce an item before calling set_proc_macro_attr

1. Symptoms

Fix E0798: Derive of `#[serde]`-like attribute should produce an item before calling `set_proc_macro_attr`