Add a comment explaining the issues in ZIPs 205 and 208

And add the network to the difficulty filter error.

zebra-consensus/src/block/check.rs

@@ -70,12 +70,19 @@ pub fn difficulty_is_valid(
         ))?;
     }
 
-    // Difficulty filter
+    // The difficulty filter is also context-free.
+    //
+    // ZIP 205 and ZIP 208 incorrectly describe testnet minimum difficulty blocks
+    // as a change to the difficulty filter. But in `zcashd`, it is implemented
+    // as a change to the difficulty adjustment algorithm. So we don't need to
+    // do anything special for testnet here.
+    // For details, see https://github.com/zcash/zips/issues/416
     if hash > &difficulty_threshold {
         Err(BlockError::DifficultyFilter(
             *height,
             *hash,
             difficulty_threshold,
+            network,
         ))?;
     }
 

zebra-consensus/src/block/tests.rs

@@ -234,7 +234,8 @@ fn difficulty_validation_failure() -> Result<(), Report> {
     // Validate the block
     let result = check::difficulty_is_valid(&block.header, Network::Mainnet, &height, &bad_hash)
         .unwrap_err();
-    let expected = BlockError::DifficultyFilter(height, bad_hash, difficulty_threshold);
+    let expected =
+        BlockError::DifficultyFilter(height, bad_hash, difficulty_threshold, Network::Mainnet);
     assert_eq!(expected, result);
 
     Ok(())

zebra-consensus/src/error.rs

@@ -108,10 +108,13 @@ pub enum BlockError {
         zebra_chain::work::difficulty::ExpandedDifficulty,
     ),
 
-    #[error("block {0:?} has a hash {1:?} that is easier than the difficulty threshold {2:?}")]
+    #[error(
+        "block {0:?} on {3:?} has a hash {1:?} that is easier than its difficulty threshold {2:?}"
+    )]
     DifficultyFilter(
         zebra_chain::block::Height,
         zebra_chain::block::Hash,
         zebra_chain::work::difficulty::ExpandedDifficulty,
+        zebra_chain::parameters::Network,
     ),
 }