babe: small compilation fix (#3764)

* babe: fix type on find_pre_digest call

* fork-tree: optimize prune

* babe: fix test compilation

substrate/core/consensus/babe/src/lib.rs

@@ -896,7 +896,7 @@ impl<B, E, Block, I, RA, PRA> BlockImport<Block> for BabeBlockImport<B, E, Block
 						.expect("best finalized hash was given by client; \
 								finalized headers must exist in db; qed");
 
-					find_pre_digest::<Block>(&finalized_header)
+					find_pre_digest::<Block::Header>(&finalized_header)
 						.expect("finalized header must be valid; \
 								valid blocks have a pre-digest; qed")
 						.slot_number()

substrate/core/consensus/babe/src/tests.rs

@@ -625,7 +625,7 @@ fn importing_epoch_change_block_prunes_tree() {
 		};
 
 		let mut proposer = environ.init(&parent_header).unwrap();
-		let parent_pre_digest = find_pre_digest::<Block>(&parent_header).unwrap();
+		let parent_pre_digest = find_pre_digest(&parent_header).unwrap();
 
 		let pre_digest = sr_primitives::generic::Digest {
 			logs: vec![

substrate/core/utils/fork-tree/src/lib.rs

@@ -115,10 +115,11 @@ impl<H, N, V> ForkTree<H, N, V> where
 
 			// we found the deepest ancestor of the finalized block, so we prune
 			// out any children that don't include the finalized block.
-			root.children.retain(|node| {
+			let children = std::mem::replace(&mut root.children, Vec::new());
+			root.children = children.into_iter().filter(|node| {
 				node.number == *number && node.hash == *hash ||
 					node.number < *number && is_descendent_of(&node.hash, hash).unwrap_or(false)
-			});
+			}).take(1).collect();
 
 			self.roots = vec![root];
 		}