Revert "do not expose that RawId is stored in a u32"

This reverts commit 9b106e9279.

src/raw_id.rs

@@ -2,8 +2,8 @@ use std::fmt;
 use std::num::NonZeroU32;
 
 /// The "raw-id" is used for interned keys in salsa -- it is basically
-/// a newtype'd integer. Typically, it is wrapped in a type of your
+/// a newtype'd u32. Typically, it is wrapped in a type of your own
-/// own devising. For more information about interned keys, see [the
+/// devising. For more information about interned keys, see [the
 /// interned key RFC][rfc].
 ///
 /// # Creating a `RawId`
@@ -18,23 +18,16 @@ use std::num::NonZeroU32;
 /// assert_eq!(raw_id1, raw_id2);
 /// ```
 ///
-/// # Converting to a usize
+/// # Converting to a u32 or usize
 ///
 /// Normally, there should be no need to access the underlying integer
 /// in a `RawId`. But if you do need to do so, you can convert to a
-/// `usize` using the `as_usize` method or the `From` impl.
+/// `usize` using the `as_u32` or `as_usize` methods or the `From` impls.
 ///
 /// ```
 /// # use salsa::RawId;
 /// let raw_id = RawId::from(22_u32);
-/// let value = raw_id.as_usize();
+/// let value = u32::from(raw_id);
-/// assert_eq!(value, 22);
-/// ```
-///
-/// ```
-/// # use salsa::RawId;
-/// let raw_id = RawId::from(22_u32);
-/// let value = usize::from(raw_id);
 /// assert_eq!(value, 22);
 /// ```
 ///
@@ -72,9 +65,34 @@ impl RawId {
         }
     }
 
+    /// Convert this raw-id into a u32 value.
+    ///
+    /// ```
+    /// # use salsa::RawId;
+    /// let raw_id = RawId::from(22_u32);
+    /// let value = raw_id.as_usize();
+    /// assert_eq!(value, 22);
+    /// ```
+    pub fn as_u32(self) -> u32 {
+        self.value.get() - 1
+    }
+
     /// Convert this raw-id into a usize value.
+    ///
+    /// ```
+    /// # use salsa::RawId;
+    /// let raw_id = RawId::from(22_u32);
+    /// let value = raw_id.as_usize();
+    /// assert_eq!(value, 22);
+    /// ```
     pub fn as_usize(self) -> usize {
-        (self.value.get() - 1) as usize
+        self.as_u32() as usize
+    }
+}
+
+impl From<RawId> for u32 {
+    fn from(raw: RawId) -> u32 {
+        raw.as_u32()
     }
 }