Migrate to thiserror (#7177)

* Migrate to thiserror

* Discourage the use of other modules' Result alias

`io::Result` set a bad precedent. Don't import other `Result`
aliases.

programs/vote/Cargo.toml

@@ -18,6 +18,7 @@ serde_derive = "1.0.103"
 solana-logger = { path = "../../logger", version = "0.22.0" }
 solana-metrics = { path = "../../metrics", version = "0.22.0" }
 solana-sdk = { path = "../../sdk", version = "0.22.0" }
+thiserror = "1.0"
 
 [lib]
 crate-type = ["lib", "cdylib"]

programs/vote/src/vote_instruction.rs

@@ -17,13 +17,21 @@ use solana_sdk::{
     system_instruction,
     sysvar::{self, clock::Clock, slot_hashes::SlotHashes, Sysvar},
 };
+use thiserror::Error;
 
 /// Reasons the stake might have had an error
-#[derive(Debug, Clone, PartialEq, FromPrimitive, ToPrimitive)]
+#[derive(Error, Debug, Clone, PartialEq, FromPrimitive, ToPrimitive)]
 pub enum VoteError {
+    #[error("vote already recorded or not in slot hashes history")]
     VoteTooOld,
+
+    #[error("vote slots do not match bank history")]
     SlotsMismatch,
+
+    #[error("vote hash does not match bank hash")]
     SlotHashMismatch,
+
+    #[error("vote has no slots, invalid")]
     EmptySlots,
 }
 impl<E> DecodeError<E> for VoteError {
@@ -32,22 +40,6 @@ impl<E> DecodeError<E> for VoteError {
     }
 }
 
-impl std::fmt::Display for VoteError {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        write!(
-            f,
-            "{}",
-            match self {
-                VoteError::VoteTooOld => "vote already recorded or not in slot hashes history",
-                VoteError::SlotsMismatch => "vote slots do not match bank history",
-                VoteError::SlotHashMismatch => "vote hash does not match bank hash",
-                VoteError::EmptySlots => "vote has no slots, invalid",
-            }
-        )
-    }
-}
-impl std::error::Error for VoteError {}
-
 #[derive(Serialize, Deserialize, Debug, PartialEq, Eq, Clone)]
 pub enum VoteInstruction {
     /// Initialize the VoteState for this `vote account`