Library: Add compatibility for legacy Tokio 0.2 (#40)

Adds support to the library for tokio 0.2 backward-compatibility. This should hopefully benefit, and prevent lavalink-rs from being blocked on this feature.

These can be reached using, e.g., `gateway-tokio-02`, `driver-tokio-02`, `serenity-rustls-tokio-02`, and `serenity-native-tokio-02` features.

Naturally, this requires some jiggering about with features and the underlying CI, which has been taken care of. Twilight can't be handled in this way, as their last tokio 0.2 version uses the deprecated Discord Gateway v6.

Cargo.toml

@@ -28,6 +28,13 @@ features = ["tokio-runtime"]
 optional = true
 version = "0.11"
 
+[dependencies.async-tungstenite-compat]
+package = "async-tungstenite"
+default-features = false
+features = ["tokio-runtime"]
+optional = true
+version = "0.9"
+
 [dependencies.audiopus]
 optional = true
 version = "0.2"
@@ -62,7 +69,7 @@ version = "0.8"
 
 [dependencies.serenity]
 optional = true
-version = "0.10"
+version = "^0.10.2"
 default-features = false
 features = ["voice", "gateway"]
 
@@ -83,6 +90,12 @@ optional = true
 version = "1.0"
 default-features = false
 
+[dependencies.tokio-compat]
+optional = true
+package = "tokio"
+version = "0.2"
+default-features = false
+
 [dependencies.twilight-gateway]
 optional = true
 version = "0.3"
@@ -115,20 +128,35 @@ criterion = "0.3"
 utils = { path = "utils" }
 
 [features]
+# Core features
 default = [
     "serenity-rustls",
     "driver",
     "gateway",
 ]
 gateway = [
+    "gateway-core",
+    "tokio/sync",
+]
+gateway-core = [
     "dashmap",
     "flume",
     "parking_lot",
-    "tokio/sync",
 ]
 driver = [
-    "async-trait",
     "async-tungstenite",
+    "driver-core",
+    "tokio/fs",
+    "tokio/io-util",
+    "tokio/macros",
+    "tokio/net",
+    "tokio/process",
+    "tokio/rt",
+    "tokio/sync",
+    "tokio/time",
+]
+driver-core = [
+    "async-trait",
     "audiopus",
     "byteorder",
     "discortp",
@@ -138,21 +166,13 @@ driver = [
     "serenity-voice-model",
     "spin_sleep",
     "streamcatcher",
-    "tokio/fs",
-    "tokio/io-util",
-    "tokio/macros",
-    "tokio/net",
-    "tokio/process",
-    "tokio/rt",
-    "tokio/sync",
-    "tokio/time",
     "typemap_rev",
     "url",
     "uuid",
     "xsalsa20poly1305",
 ]
-rustls = ["async-tungstenite/tokio-rustls"]
-native = ["async-tungstenite/tokio-native-tls"]
+rustls = ["async-tungstenite/tokio-rustls", "rustls-marker"]
+native = ["async-tungstenite/tokio-native-tls", "native-marker"]
 serenity-rustls = ["serenity/rustls_backend", "rustls", "gateway", "serenity-deps"]
 serenity-native = ["serenity/native_tls_backend", "native", "gateway", "serenity-deps"]
 twilight-rustls = ["twilight", "twilight-gateway/rustls", "rustls", "gateway"]
@@ -162,9 +182,41 @@ simd-zlib = ["twilight-gateway/simd-zlib"]
 stock-zlib = ["twilight-gateway/stock-zlib"]
 serenity-deps = ["async-trait"]
 
+rustls-marker = []
+native-marker = []
+
+# Tokio 0.2 Compatibility features
+# These should probably be dropped around the same time as serenity drop them.
+rustls-tokio-02 = ["async-tungstenite-compat/tokio-rustls", "rustls-marker", "tokio-02-marker"]
+native-tokio-02 = ["async-tungstenite-compat/tokio-native-tls", "native-marker", "tokio-02-marker"]
+serenity-rustls-tokio-02 = ["serenity/rustls_tokio_0_2_backend", "rustls-tokio-02", "gateway-tokio-02", "serenity-deps"]
+serenity-native-tokio-02 = ["serenity/native_tls_tokio_0_2_backend", "native-tokio-02", "gateway-tokio-02", "serenity-deps"]
+gateway-tokio-02 = [
+    "gateway-core",
+    "tokio-02-marker",
+    "tokio-compat/sync",
+]
+driver-tokio-02 = [
+    "async-tungstenite-compat",
+    "driver-core",
+    "tokio-02-marker",
+    "tokio-compat/fs",
+    "tokio-compat/io-util",
+    "tokio-compat/macros",
+    "tokio-compat/net",
+    "tokio-compat/process",
+    "tokio-compat/rt-core",
+    "tokio-compat/sync",
+    "tokio-compat/time",
+]
+tokio-02-marker = []
+
+# Behaviour altering features.
 youtube-dlc = []
 builtin-queue = []
 
+# Used for docgen/testing/benchmarking.
+full-doc = ["default", "twilight-rustls", "builtin-queue", "stock-zlib"]
 internals = []
 
 [[bench]]
@@ -179,4 +231,4 @@ required-features = ["internals"]
 harness = false
 
 [package.metadata.docs.rs]
-features = ["default", "twilight-rustls", "builtin-queue", "stock-zlib"]
+features = ["full-doc"]