can: Only allow valid Id s to be constructed

Use newtypes for the enum variants so they cannot be constructed
anymore. Implement the `From` trait for the variants to access the id
value as integer.
timokroeger/embedded-can#7

src/can.rs

@@ -1,34 +1,96 @@
 //! Controller Area Network
+/// Standard 11bit Identifier (0..=0x7FF)
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub struct StandardId(u16);
+
+impl StandardId {
+    /// Creates a new standard identifier.
+    pub fn new(id: u16) -> Result<StandardId, ()> {
+        if id <= 0x7FF {
+            Ok(StandardId(id))
+        } else {
+            Err(())
+        }
+    }
+}
+
+impl core::convert::From<StandardId> for u16 {
+    fn from(id: StandardId) -> u16 {
+        id.0
+    }
+}
+
+impl core::convert::From<StandardId> for u32 {
+    fn from(id: StandardId) -> u32 {
+        id.0 as u32
+    }
+}
+
+impl ExtendedId {
+    /// Creates a new extended identifier.
+    pub fn new(id: u32) -> Result<ExtendedId, ()> {
+        if id <= 0x1FFF_FFFF {
+            Ok(ExtendedId(id))
+        } else {
+            Err(())
+        }
+    }
+}
+
+impl core::convert::From<ExtendedId> for u32 {
+    fn from(id: ExtendedId) -> u32 {
+        id.0
+    }
+}
+
+/// Extended 29bit Identifier (0..=0x1FFF_FFFF)
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub struct ExtendedId(u32);
 
 /// CAN Identifier
+///
+/// The variants are wrapped in newtypes so they can only be costructed with valid values.
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub enum Id {
     /// Standard 11bit Identifier (0..=0x7FF)
-    Standard(u32),
+    Standard(StandardId),
 
     /// Extended 29bit Identifier (0..=0x1FFF_FFFF)
-    Extended(u32),
+    Extended(ExtendedId),
 }
 
 impl Id {
-    /// Returs true when the identifier is valid, false otherwise.
-    pub fn valid(self) -> bool {
-        match self {
-            Id::Standard(id) if id <= 0x7FF => true,
-            Id::Extended(id) if id <= 0x1FFF_FFFF => true,
-            _ => false,
-        }
+    /// Creates a new standard identifier.
+    pub fn new_standard(id: u16) -> Result<Id, ()> {
+        Ok(StandardId::new(id)?.into())
+    }
+
+    /// Creates a new extended identifier.
+    pub fn new_extended(id: u32) -> Result<Id, ()> {
+        Ok(ExtendedId::new(id)?.into())
+    }
+}
+
+impl core::convert::From<StandardId> for Id {
+    fn from(id: StandardId) -> Id {
+        Id::Standard(id)
+    }
+}
+
+impl core::convert::From<ExtendedId> for Id {
+    fn from(id: ExtendedId) -> Id {
+        Id::Extended(id)
     }
 }
 
 /// A CAN2.0 Frame
 pub trait Frame: Sized {
     /// Creates a new frame.
-    /// Returns an error when the the identifier is not valid or the data slice is too long.
+    /// Returns an error when the data slice is too long.
     fn new(id: Id, data: &[u8]) -> Result<Self, ()>;
 
     /// Creates a new remote frame (RTR bit set).
-    /// Returns an error when the the identifier is  or the data length code (DLC) not valid.
+    /// Returns an error when the data length code (DLC) is not valid.
     fn new_remote(id: Id, dlc: usize) -> Result<Self, ()>;
 
     /// Returns true if this frame is a extended frame.