tls_codec: feature for conditional deserialization derivation

.github/workflows/tls_codec.yml

@@ -71,3 +71,4 @@ jobs:
       - uses: RustCrypto/actions/cargo-hack-install@master
       - run: cargo hack test --feature-powerset
       - run: cargo hack test -p tls_codec_derive --feature-powerset --test encode\* --test decode\*
+      - run: cargo hack test -p tls_codec_derive --feature-powerset --doc

tls_codec/Cargo.toml

@@ -27,12 +27,16 @@ criterion = { version = "0.5", default-features = false }
 regex = "1.8"
 
 [features]
-default = [ "std" ]
-arbitrary = [ "std", "dep:arbitrary" ]
-derive = [ "tls_codec_derive" ]
-serde = [ "std", "dep:serde" ]
+default = ["std"]
+arbitrary = ["std", "dep:arbitrary"]
+derive = ["tls_codec_derive"]
+serde = ["std", "dep:serde"]
 mls = [] # In MLS variable length vectors are limited compared to QUIC.
-std = [ "tls_codec_derive?/std" ]
+std = ["tls_codec_derive?/std"]
+conditional_deserialization = [
+    "derive",
+    "tls_codec_derive/conditional_deserialization",
+]
 
 [[bench]]
 name = "tls_vec"

tls_codec/derive/Cargo.toml

@@ -23,5 +23,6 @@ tls_codec = { path = "../" }
 trybuild = "1"
 
 [features]
-default = [ "std" ]
+default = ["std"]
+conditional_deserialization = ["syn/full"]
 std = []