der: add class bits consts for Application and Private tag support (#1721)

Author: dishmaker

der/src/tag/class.rs

@@ -2,19 +2,30 @@
 
 use core::fmt;
 
+/// `UNIVERSAL`: built-in types whose meaning is the same in all
+/// applications.
+pub const CLASS_UNIVERSAL: u8 = 0b00000000;
+/// `APPLICATION`: types whose meaning is specific to an application.
+pub const CLASS_APPLICATION: u8 = 0b01000000;
+/// `CONTEXT-SPECIFIC`: types whose meaning is specific to a given
+/// structured type.
+pub const CLASS_CONTEXT_SPECIFIC: u8 = 0b10000000;
+/// `PRIVATE`: types whose meaning is specific to a given enterprise.
+pub const CLASS_PRIVATE: u8 = 0b11000000;
+
 /// Class of an ASN.1 tag.
 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
 #[repr(u8)]
 pub enum Class {
     /// `UNIVERSAL`: built-in types whose meaning is the same in all
     /// applications.
-    Universal = 0b00000000,
+    Universal = CLASS_UNIVERSAL,
 
     /// `APPLICATION`: types whose meaning is specific to an application,
     ///
     /// Types in two different applications may have the same
     /// application-specific tag and different meanings.
-    Application = 0b01000000,
+    Application = CLASS_APPLICATION,
 
     /// `CONTEXT-SPECIFIC`: types whose meaning is specific to a given
     /// structured type.
@@ -23,10 +34,10 @@ pub enum Class {
     /// with the same underlying tag within the context of a given structured
     /// type, and component types in two different structured types may have
     /// the same tag and different meanings.
-    ContextSpecific = 0b10000000,
+    ContextSpecific = CLASS_CONTEXT_SPECIFIC,
 
     /// `PRIVATE`: types whose meaning is specific to a given enterprise.
-    Private = 0b11000000,
+    Private = CLASS_PRIVATE,
 }
 
 impl fmt::Display for Class {
@@ -39,3 +50,26 @@ impl fmt::Display for Class {
         })
     }
 }
+
+impl Class {
+    /// Returns class as 2 most-significant bits (mask 0b11000000)
+    pub const fn bits(&self) -> u8 {
+        *self as u8
+    }
+
+    /// Returns class extracted from 2 most-significant bits (mask 0b11000000)
+    pub const fn from_bits(bits: u8) -> Self {
+        match (bits >> 6) & 0b11 {
+            0b00 => Class::Universal,
+            0b01 => Class::Application,
+            0b10 => Class::ContextSpecific,
+            0b11 => Class::Private,
+            _ => unreachable!(),
+        }
+    }
+}
+impl From<u8> for Class {
+    fn from(value: u8) -> Self {
+        Class::from_bits(value)
+    }
+}