Support custom derives on Request and Response

Replace the bespoke "derive_serde" with a more flexible
"derive = [<path>, <path>, ...]" form.

Deprecate the old "derive_serde" form, and emit deprecation warnings.

Add trybuild tests in a separate test crate to ensure the deprecation,
features, and error messages all behave as expected. This technique of a
separate trybuild test crate is borrowed from tokio/tests-build.
Update CI to run this new crate's tests under the serde matrix.

.github/workflows/main.yml

@@ -56,11 +56,12 @@ jobs:
           access_token: ${{ github.token }}
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
-      - run: >
-          cargo test --manifest-path tarpc/Cargo.toml
-          ${{ matrix.serde }} ${{ matrix.tokio }} ${{ matrix.serde-transport }}
-          ${{ matrix.serde-transport-json }} ${{ matrix.serde-transport-bincode }}
-          ${{ matrix.tcp }} ${{ matrix.unix }}
+      - run: |
+          cargo test --manifest-path tarpc/Cargo.toml \
+          ${{ matrix.serde }} ${{ matrix.tokio }} ${{ matrix.serde-transport }} \
+          ${{ matrix.serde-transport-json }} ${{ matrix.serde-transport-bincode }} \
+          ${{ matrix.tcp }} ${{ matrix.unix }} \
+          cargo test --manifest-path tests-build/Cargo.toml $ {{ matrix.serde }}
 
   list-examples:
     name: List Examples

Cargo.toml

@@ -5,6 +5,7 @@ members = [
     "example-service",
     "tarpc",
     "plugins",
+    "tests-build",
 ]
 
 [profile.dev]

plugins/Cargo.toml

@@ -23,6 +23,7 @@ travis-ci = { repository = "google/tarpc" }
 proc-macro2 = "1.0"
 quote = "1.0"
 syn = { version = "2.0", features = ["full"] }
+proc-macro-warning = "1"
 
 [lib]
 proc-macro = true