Two parts:

1. Add a code: Option<Cow<'static, str>> field to garde::Error

2. Add a way to set the code for a specific rule via the proc macro.

The unresolved question here is syntax:

Option A:

#[garde(length(min = 1, code = "custom_error_code"))]

Option B:

#[garde(code("custom_error_code", length(min = 1)))]

(A) would require some hacks to "intercept" the code value during parsing. Both would work by intercepting the error returned by a rule and set its code field in the proc macro output. (B) seems easier to implement, but those coming from validator likely expect (A).