Static rustc_ap_rustc_lint_defs::builtin::LEGACY_DERIVE_HELPERS

pub static LEGACY_DERIVE_HELPERS: &Lint

The legacy_derive_helpers lint detects derive helper attributes that are used before they are introduced.

Example

#[serde(rename_all = "camelCase")]
#[derive(Deserialize)]
struct S { /* fields */ }

produces:

warning: derive helper attribute is used before it is introduced
  --> $DIR/legacy-derive-helpers.rs:1:3
   |
 1 | #[serde(rename_all = "camelCase")]
   |   ^^^^^
...
 2 | #[derive(Deserialize)]
   |          ----------- the attribute is introduced here

Explanation

Attributes like this work for historical reasons, but attribute expansion works in left-to-right order in general, so, to resolve #[serde], compiler has to try to “look into the future” at not yet expanded part of the item , but such attempts are not always reliable.

To fix the warning place the helper attribute after its corresponding derive.

#[derive(Deserialize)]
#[serde(rename_all = "camelCase")]
struct S { /* fields */ }