zcash_client_backend: Switch from async-trait to RPIT

RPIT (return-position `impl Trait`) was stabilized in Rust 1.75, and the
`dynosaur` crate providing dynamic dispatch support was published two
weeks ago. Now that our MSRV is 1.77, we can use both of these.

Cargo.toml

@@ -146,9 +146,11 @@ incrementalmerkletree-testing = "0.2"
 # - `arti-client` depends on `rusqlite`, and a version mismatch there causes a compilation
 #   failure due to incompatible `libsqlite3-sys` versions.
 arti-client = { version = "0.23", default-features = false, features = ["compression", "rustls", "tokio"] }
+dynosaur = "0.1.1"
 tokio = "1"
 tor-rtcompat = "0.23"
 tower = "0.4"
+trait-variant = "0.1"
 
 # ZIP 32
 aes = "0.8"

supply-chain/config.toml

@@ -414,6 +414,14 @@ criteria = "safe-to-deploy"
 version = "1.0.17"
 criteria = "safe-to-deploy"
 
+[[exemptions.dynosaur]]
+version = "0.1.1"
+criteria = "safe-to-deploy"
+
+[[exemptions.dynosaur_derive]]
+version = "0.1.1"
+criteria = "safe-to-deploy"
+
 [[exemptions.ecdsa]]
 version = "0.16.9"
 criteria = "safe-to-deploy"
@@ -1406,6 +1414,10 @@ criteria = "safe-to-deploy"
 version = "0.2.5"
 criteria = "safe-to-deploy"
 
+[[exemptions.trait-variant]]
+version = "0.1.2"
+criteria = "safe-to-deploy"
+
 [[exemptions.typed-index-collections]]
 version = "3.1.0"
 criteria = "safe-to-deploy"

zcash_client_backend/CHANGELOG.md

@@ -23,6 +23,10 @@ and this library adheres to Rust's notion of
 - A new feature flag, `non-standard-fees`, has been added. This flag is now
   required in order to make use of any types or methods that enable non-standard
   fee calculation.
+- `zcash_client_backend::tor::http::cryptex`:
+  - `LocalExchange`, a variant of the `Exchange` trait without `Send` bounds.
+  - `DynExchange`
+  - `DynLocalExchange`
 
 ### Changed
 - MSRV is now 1.77.0.
@@ -76,6 +80,9 @@ and this library adheres to Rust's notion of
   `ProposalDecodingError::FeeRuleNotSupported` has been added to replace it.
 - `zcash_client_backend::data_api::fees::fixed` is now available only via the
   use of the `non-standard-fees` feature flag.
+- `zcash_client_backend::tor::http::cryptex`:
+  - The `Exchange` trait is no longer object-safe. Replace any existing uses of
+    `dyn Exchange` with `DynExchange`.
 
 ### Removed
 - `zcash_client_backend::data_api`:

zcash_client_backend/Cargo.toml

@@ -95,8 +95,10 @@ nom = "7"
 #    `hyper::Error`, `hyper::http::Error`, `serde_json::Error`. We could avoid this with
 #    changes to error handling.
 arti-client = { workspace = true, optional = true }
+dynosaur = { workspace = true, optional = true }
 hyper = { workspace = true, optional = true, features = ["client", "http1"] }
 serde_json = { workspace = true, optional = true }
+trait-variant = { workspace = true, optional = true }
 
 # - Currency conversion
 rust_decimal = { workspace = true, optional = true }
@@ -179,7 +181,7 @@ sync = [
 ## operations.
 tor = [
     "dep:arti-client",
-    "dep:async-trait",
+    "dep:dynosaur",
     "dep:futures-util",
     "dep:http-body-util",
     "dep:hyper",
@@ -191,6 +193,7 @@ tor = [
     "dep:tokio",
     "dep:tokio-rustls",
     "dep:tor-rtcompat",
+    "dep:trait-variant",
     "dep:tower",
     "dep:webpki-roots",
 ]

zcash_client_backend/src/tor/http/cryptex.rs

@@ -1,6 +1,5 @@
 //! Cryptocurrency exchange rate APIs.
 
-use async_trait::async_trait;
 use futures_util::{future::join_all, join};
 use rand::{seq::IteratorRandom, thread_rng};
 use rust_decimal::Decimal;
@@ -26,8 +25,10 @@ pub mod exchanges {
 }
 
 /// An exchange that can be queried for ZEC data.
-#[async_trait]
-pub trait Exchange: 'static {
+#[trait_variant::make(Exchange: Send)]
+#[dynosaur::dynosaur(DynExchange = dyn Exchange)]
+#[dynosaur::dynosaur(DynLocalExchange = dyn LocalExchange)]
+pub trait LocalExchange {
     /// Queries data about the USD/ZEC pair.
     ///
     /// The returned bid and ask data must be denominated in USD, i.e. the latest bid and
@@ -55,8 +56,8 @@ impl ExchangeData {
 
 /// A set of [`Exchange`]s that can be queried for ZEC data.
 pub struct Exchanges {
-    trusted: Box<dyn Exchange>,
-    others: Vec<Box<dyn Exchange>>,
+    trusted: Box<DynExchange<'static>>,
+    others: Vec<Box<DynExchange<'static>>>,
 }
 
 impl Exchanges {
@@ -78,7 +79,7 @@ impl Exchanges {
     ///
     /// The `trusted` exchange will always have its data used, _if_ data is successfully
     /// obtained via Tor (i.e. no transient failures).
-    pub fn builder(trusted: impl Exchange) -> ExchangesBuilder {
+    pub fn builder(trusted: impl Exchange + 'static) -> ExchangesBuilder {
         ExchangesBuilder::new(trusted)
     }
 }
@@ -105,16 +106,16 @@ impl ExchangesBuilder {
     ///
     /// The `trusted` exchange will always have its data used, _if_ data is successfully
     /// obtained via Tor (i.e. no transient failures).
-    pub fn new(trusted: impl Exchange) -> Self {
+    pub fn new(trusted: impl Exchange + 'static) -> Self {
         Self(Exchanges {
-            trusted: Box::new(trusted),
+            trusted: DynExchange::boxed(trusted),
             others: vec![],
         })
     }
 
     /// Adds another [`Exchange`] as a data source.
-    pub fn with(mut self, other: impl Exchange) -> Self {
-        self.0.others.push(Box::new(other));
+    pub fn with(mut self, other: impl Exchange + 'static) -> Self {
+        self.0.others.push(DynExchange::boxed(other));
         self
     }
 

zcash_client_backend/src/tor/http/cryptex/binance.rs

@@ -1,4 +1,3 @@
-use async_trait::async_trait;
 use rust_decimal::Decimal;
 use serde::Deserialize;
 
@@ -44,7 +43,6 @@ struct BinanceData {
     count: u32,
 }
 
-#[async_trait]
 impl Exchange for Binance {
     async fn query_zec_to_usd(&self, client: &Client) -> Result<ExchangeData, Error> {
         // API documentation:

zcash_client_backend/src/tor/http/cryptex/coinbase.rs

@@ -1,4 +1,3 @@
-use async_trait::async_trait;
 use rust_decimal::Decimal;
 use serde::Deserialize;
 
@@ -31,7 +30,6 @@ struct CoinbaseData {
     conversions_volume: Option<Decimal>,
 }
 
-#[async_trait]
 impl Exchange for Coinbase {
     #[allow(dead_code)]
     async fn query_zec_to_usd(&self, client: &Client) -> Result<ExchangeData, Error> {

zcash_client_backend/src/tor/http/cryptex/gate_io.rs

@@ -1,4 +1,3 @@
-use async_trait::async_trait;
 use hyper::StatusCode;
 use rust_decimal::Decimal;
 use serde::Deserialize;
@@ -32,7 +31,6 @@ struct GateIoData {
     low_24h: Decimal,
 }
 
-#[async_trait]
 impl Exchange for GateIo {
     async fn query_zec_to_usd(&self, client: &Client) -> Result<ExchangeData, Error> {
         // API documentation:

zcash_client_backend/src/tor/http/cryptex/gemini.rs

@@ -1,4 +1,3 @@
-use async_trait::async_trait;
 use rust_decimal::Decimal;
 use serde::Deserialize;
 
@@ -30,7 +29,6 @@ struct GeminiData {
     ask: Decimal,
 }
 
-#[async_trait]
 impl Exchange for Gemini {
     async fn query_zec_to_usd(&self, client: &Client) -> Result<ExchangeData, Error> {
         // API documentation:

zcash_client_backend/src/tor/http/cryptex/ku_coin.rs

@@ -1,4 +1,3 @@
-use async_trait::async_trait;
 use rust_decimal::Decimal;
 use serde::Deserialize;
 
@@ -46,7 +45,6 @@ struct KuCoinResponse {
     data: KuCoinData,
 }
 
-#[async_trait]
 impl Exchange for KuCoin {
     async fn query_zec_to_usd(&self, client: &Client) -> Result<ExchangeData, Error> {
         // API documentation:

zcash_client_backend/src/tor/http/cryptex/mexc.rs

@@ -1,4 +1,3 @@
-use async_trait::async_trait;
 use rust_decimal::Decimal;
 use serde::Deserialize;
 
@@ -39,7 +38,6 @@ struct MexcData {
     closeTime: u64,
 }
 
-#[async_trait]
 impl Exchange for Mexc {
     async fn query_zec_to_usd(&self, client: &Client) -> Result<ExchangeData, Error> {
         // API documentation: