Add a test showing how cfg_eval can be used to gate serde_with attrib…

…utes

So far the story for cfg-gating serde_with attributes has not been
great, as shown in #331 or #355.
This adds a test showing how cfg_eval can be used to gate serde_with.

Part of #607

serde_with_test/Cargo.toml

@@ -8,6 +8,7 @@ rust-version = "1.60"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+cfg_eval = "0.1.2"
 s = {package = "serde", version = "1.0.75", features = ["derive"]}
 s_json = {package = "serde_json", version = "1.0.59"}
 s_with = {package = "serde_with", path = "../serde_with"}

serde_with_test/tests/serde_as_cfg.rs

@@ -0,0 +1,24 @@
+//! Test that cfg_eval helps in cfg-gating serde_with attributes
+
+// Ensure no prelude is available
+#![no_implicit_prelude]
+#![allow(dead_code, unused_imports)]
+
+#[cfg_attr(test, ::cfg_eval::cfg_eval, ::s_with::serde_as(crate = "::s_with"))]
+#[cfg_attr(test, derive(::s::Serialize, ::s::Deserialize))]
+#[derive(Debug)]
+#[serde(crate = "::s")]
+struct S {
+    #[cfg_attr(test, serde_as(as = "::s_with::DisplayFromStr"))]
+    int: i32,
+}
+
+#[test]
+fn serde_as_cfg_gated() {
+    let s = S { int: 42 };
+    // Check serialization
+    let s = ::s_json::to_string(&s).unwrap();
+    ::std::assert_eq!(s, r#"{"int":"42"}"#);
+    let s: S = ::s_json::from_str(&s).unwrap();
+    ::std::assert_eq!(s.int, 42);
+}