refactor: change more masking strategy structs to no-variant enums

as per #1973

crates/cards/src/validate.rs

@@ -72,7 +72,7 @@ impl<'de> Deserialize<'de> for CardNumber {
     }
 }
 
-pub struct CardNumberStrategy;
+pub enum CardNumberStrategy {}
 
 impl<T> Strategy<T> for CardNumberStrategy
 where

crates/masking/src/secret.rs

@@ -12,8 +12,8 @@ use crate::{strategy::Strategy, PeekInterface};
 /// To get access to value use method `expose()` of trait [`crate::ExposeInterface`].
 ///
 /// ## Masking
-/// Use the [`crate::strategy::Strategy`] trait to implement a masking strategy on a unit struct
-/// and pass the unit struct as a second generic parameter to [`Secret`] while defining it.
+/// Use the [`crate::strategy::Strategy`] trait to implement a masking strategy on a zero-variant
+/// enum and pass this enum as a second generic parameter to [`Secret`] while defining it.
 /// [`Secret`] will take care of applying the masking strategy on the inner secret when being
 /// displayed.
 ///
@@ -24,7 +24,7 @@ use crate::{strategy::Strategy, PeekInterface};
 /// use masking::Secret;
 /// use std::fmt;
 ///
-/// struct MyStrategy;
+/// enum MyStrategy {}
 ///
 /// impl<T> Strategy<T> for MyStrategy
 /// where

crates/masking/src/strategy.rs

@@ -7,7 +7,7 @@ pub trait Strategy<T> {
 }
 
 /// Debug with type
-pub struct WithType;
+pub enum WithType {}
 
 impl<T> Strategy<T> for WithType {
     fn fmt(_: &T, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -18,7 +18,7 @@ impl<T> Strategy<T> for WithType {
 }
 
 /// Debug without type
-pub struct WithoutType;
+pub enum WithoutType {}
 
 impl<T> Strategy<T> for WithoutType {
     fn fmt(_: &T, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {