introduce `fallible_impl_from` lint with exeptions (#4609)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.cargo/config.toml

@@ -50,7 +50,10 @@ rustflags = [
     "-Aclippy::try_err",
 
     # Links in public docs can point to private items.
-    "-Arustdoc::private_intra_doc_links"
+    "-Arustdoc::private_intra_doc_links",
+
+    # Panics
+    "-Wclippy::fallible_impl_from",
 
     # TODOs:
     # `cargo fix` might help do these fixes,

zebra-chain/src/orchard/keys.rs

@@ -1,7 +1,7 @@
 //! Orchard key types.
 //!
 //! <https://zips.z.cash/protocol/nu5.pdf#orchardkeycomponents>
-
+#![allow(clippy::fallible_impl_from)]
 #![allow(dead_code)]
 
 #[cfg(test)]
@@ -308,7 +308,9 @@ impl From<SpendValidatingKey> for [u8; 32] {
 
 impl From<SpendAuthorizingKey> for SpendValidatingKey {
     fn from(ask: SpendAuthorizingKey) -> Self {
-        let sk = redpallas::SigningKey::<SpendAuth>::try_from(<[u8; 32]>::from(ask)).unwrap();
+        let sk = redpallas::SigningKey::<SpendAuth>::try_from(<[u8; 32]>::from(ask)).expect(
+            "a scalar converted to byte array and then converted back to a scalar should not fail",
+        );
 
         Self(redpallas::VerificationKey::from(&sk))
     }

zebra-chain/src/sapling/keys.rs

@@ -8,6 +8,7 @@
 //! [ps]: https://zips.z.cash/protocol/protocol.pdf#saplingkeycomponents
 //! [3.1]: https://zips.z.cash/protocol/protocol.pdf#addressesandkeys
 #![allow(clippy::unit_arg)]
+#![allow(clippy::fallible_impl_from)]
 #![allow(dead_code)]
 
 #[cfg(test)]
@@ -507,7 +508,9 @@ impl From<AuthorizingKey> for [u8; 32] {
 
 impl From<SpendAuthorizingKey> for AuthorizingKey {
     fn from(ask: SpendAuthorizingKey) -> Self {
-        let sk = redjubjub::SigningKey::<SpendAuth>::try_from(<[u8; 32]>::from(ask)).unwrap();
+        let sk = redjubjub::SigningKey::<SpendAuth>::try_from(<[u8; 32]>::from(ask)).expect(
+            "a scalar converted to byte array and then converted back to a scalar should not fail",
+        );
         Self(redjubjub::VerificationKey::from(&sk))
     }
 }

zebra-chain/src/work/u256.rs

@@ -3,6 +3,7 @@
 // it raises a lot of issues in the macro.
 #![allow(clippy::all)]
 #![allow(clippy::range_plus_one)]
+#![allow(clippy::fallible_impl_from)]
 
 use uint::construct_uint;
 

zebra-network/src/protocol/external/message.rs

@@ -352,7 +352,11 @@ where
             // use specific varieties of `RejectReason`.
             ccode: RejectReason::Other,
 
-            reason: e.source().unwrap().to_string(),
+            reason: if let Some(reason) = e.source() {
+                reason.to_string()
+            } else {
+                String::from("")
+            },
 
             // Allow this to be overridden but not populated by default, methinks.
             data: None,