fix: make serde a non-optional dependency of pallas-primitives (#779)

The crate gated `serde` behind the `json` feature, but used
`serde::{Serialize, Deserialize}` unconditionally across the era
models, so building with `default-features = false` failed to
compile. Since `pallas-codec` and `pallas-crypto` already depend on
serde unconditionally, gating it here gave no real benefit anyway.

The `json` feature now gates only `serde_json`. A CI job checks
pallas-primitives in isolation, since a workspace-wide
`--no-default-features` build unifies the feature back on.

Fixes #740

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: scarmuega

.github/workflows/ci.yml

@@ -92,6 +92,20 @@ jobs:
       - uses: Swatinem/rust-cache@v2
       - run: cargo check --workspace --all-targets ${{ matrix.flags }}
 
+  isolated-features:
+    name: Isolated no-default-features
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: true
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@v2
+      # A workspace-wide --no-default-features build unifies features across
+      # members, so it always pulls pallas-primitives in with `json` enabled.
+      # Check the crate in isolation to catch missing serde feature gating.
+      - run: cargo check -p pallas-primitives --no-default-features
+
   msrv:
     name: MSRV (1.88)
     runs-on: ubuntu-latest

pallas-primitives/Cargo.toml

@@ -15,14 +15,14 @@ authors.workspace = true
 hex.workspace = true
 pallas-crypto.workspace = true
 pallas-codec.workspace = true
-serde = { workspace = true, optional = true }
+serde = { workspace = true }
 serde_json = { workspace = true, optional = true }
 
 [dev-dependencies]
 proptest = { workspace = true, features = ["alloc"] }
 test-case = "3.3.1"
 
 [features]
-json = ["serde", "serde_json"]
+json = ["serde_json"]
 default = ["json"]
 relaxed = ["pallas-crypto/relaxed"]