fix: deprecate async-std-rt feature and document reqwless upgrade blocker

Deprecate the async-std-rt feature with a compile_error! directive
since async-std has been discontinued (RUSTSEC-2025-0052). Users
should migrate to the smol-rt feature, which is the recommended
successor.

The reqwless upgrade from 0.13.0 to 0.14.0 is blocked by an
embedded-tls/der incompatibility upstream. This keeps atomic-polyfill
(RUSTSEC-2023-0089) as a transitive dependency until the upstream
fix lands.

Closes #126
Relates-to #111

Author: e-desouza

Cargo.toml

@@ -78,6 +78,9 @@ embassy-time = { version = "0.3.2", optional = true }
 embedded-websocket-embedded-io = { version = "0.1.0", optional = true, default-features = false, features = [
     "embedded-io-async",
 ] }
+# Note: reqwless 0.14.0 introduces an embedded-tls/der incompatibility.
+# Upgrade blocked until embedded-tls resolves the der 0.8.0 heapless feature gate.
+# This keeps atomic-polyfill (RUSTSEC-2023-0089) as a transitive dependency.
 reqwless = { version = "0.13.0", optional = true }
 reqwest = { version = "0.12.7", optional = true, features = ["json"] }
 tokio-tungstenite = { version = "0.24.0", optional = true, features = [
@@ -86,6 +89,7 @@ tokio-tungstenite = { version = "0.24.0", optional = true, features = [
 embassy-futures = { version = "0.1.1" }
 embedded-nal-async = { version = "0.8.0", optional = true }
 actix-rt = { version = "2.10.0", optional = true }
+# Deprecated: async-std is discontinued (RUSTSEC-2025-0052). Use smol-rt instead.
 async-std = { version = "1.13.0", optional = true }
 futures-executor = { version = "0.3.30", optional = true }
 futures-timer = { version = "3.0.3", optional = true }

src/asynch/mod.rs

@@ -33,10 +33,11 @@ async fn wait_seconds(seconds: u64) {
         return;
     }
     #[cfg(feature = "async-std-rt")]
-    {
-        async_std::task::sleep(Duration::from_secs(seconds)).await;
-        return;
-    }
+    compile_error!(
+        "The async-std-rt feature is deprecated. async-std has been discontinued \
+         (RUSTSEC-2025-0052). Use the smol-rt feature instead, which is the recommended \
+         successor runtime."
+    );
     #[cfg(feature = "futures-rt")]
     {
         futures_timer::Delay::new(Duration::from_secs(seconds)).await;