refactor(tvix/store/directorysvc): move from Vec<u8> to B3Digest

This introduces a new struct, B3Digest, which internally holds a
Vec<u8>, but only allows construction with 32 bytes.

It also implements display, which will print the base64 representation.
This should reduce some boilerplate when parsing Vec<u8>.

Change-Id: Ia91aa40cb691916773abc8f93e6ed79a5fd34863
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8592
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/store/src/digests.rs

@@ -0,0 +1,49 @@
+use data_encoding::BASE64;
+use thiserror::Error;
+
+// FUTUREWORK: make generic
+
+#[derive(PartialEq, Eq, Hash, Debug)]
+pub struct B3Digest(Vec<u8>);
+
+// TODO: allow converting these errors to crate::Error
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("invalid digest length: {0}")]
+    InvalidDigestLen(usize),
+}
+
+impl B3Digest {
+    // constructs a [B3Digest] from a [Vec<u8>].
+    // Returns an error if the digest has the wrong length.
+    pub fn from_vec(value: Vec<u8>) -> Result<Self, Error> {
+        if value.len() != 32 {
+            Err(Error::InvalidDigestLen(value.len()))
+        } else {
+            Ok(Self(Vec::from(value)))
+        }
+    }
+
+    // returns a copy of the inner [Vec<u8>].
+    pub fn to_vec(&self) -> Vec<u8> {
+        self.0.to_vec()
+    }
+}
+
+impl From<&[u8; 32]> for B3Digest {
+    fn from(value: &[u8; 32]) -> Self {
+        Self(value.to_vec())
+    }
+}
+
+impl Clone for B3Digest {
+    fn clone(&self) -> Self {
+        Self(self.0.to_owned())
+    }
+}
+
+impl std::fmt::Display for B3Digest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "b3:{}", BASE64.encode(self.0.as_slice()))
+    }
+}