feat(tvix/store): refactor digest conversion

This refactors how the original digest type (Vec<u8>) is converted
to [u8; PATH_HASH_SIZE].

Change-Id: I9441470a3a199620fcf328f2b7c890ca6ae93bde
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7710
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/Cargo.lock

@@ -2145,8 +2145,8 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "blake3",
- "data-encoding",
  "clap 4.0.32",
+ "data-encoding",
  "lazy_static",
  "prost",
  "prost-build",

tvix/store/src/nixpath.rs

@@ -49,13 +49,9 @@ impl NixPath {
 
         NixPath::validate_characters(&s[name_offset..])?;
 
-        // copy the digest:Vec<u8> to a [u8; PATH_HASH_SIZE]
-        let mut buffer: [u8; PATH_HASH_SIZE] = [0; PATH_HASH_SIZE];
-        buffer[..PATH_HASH_SIZE].copy_from_slice(&digest[..PATH_HASH_SIZE]);
-
         Ok(NixPath {
             name: s[name_offset..].to_string(),
-            digest: buffer,
+            digest: digest.try_into().expect("size is known"),
         })
     }