Trigger a warning if serde is disabled

CHANGELOG.md

@@ -8,6 +8,8 @@ All notable changes to MiniJinja are documented here.
 - Fixed a bug that caused cycle detection to trigger incorrectly when an included
   template extended from another template.  #538
 - Bumped the minimum version of `self_cell` to 1.0.4.  #540
+- MiniJinja will now warn if the `serde` feature is disabled.  This is in
+  anticipation of removing the serde dependency in the future.  #541
 
 ## 2.0.3
 

examples/build-script/Cargo.toml

@@ -5,4 +5,4 @@ edition = "2021"
 publish = false
 
 [build-dependencies]
-minijinja = { path = "../../minijinja", default-features = false, features = ["builtins"] }
+minijinja = { path = "../../minijinja", default-features = false, features = ["serde", "builtins"] }

examples/minimal/Cargo.toml

@@ -7,4 +7,4 @@ publish = false
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-minijinja = { path = "../../minijinja", default-features = false }
+minijinja = { path = "../../minijinja", features = ["serde"], default-features = false }

minijinja/Cargo.toml

@@ -25,6 +25,7 @@ default = [
     "multi_template",
     "adjacent_loop_items",
     "std_collections",
+    "serde",
 ]
 
 # API features
@@ -35,6 +36,7 @@ loader = ["self_cell", "memo-map"]
 unicode = ["unicode-ident", "unicase"]
 custom_syntax = ["dep:aho-corasick"]
 std_collections = []
+serde = []
 
 # Speedups
 key_interning = []

minijinja/src/lib.rs

@@ -150,6 +150,9 @@
 //!     filter's case insensitive comparison changes to using unicode and not
 //!     ASCII rules.  Without this features only ASCII identifiers can be used
 //!     for variable names and attributes.
+//!   - `serde`: enables or disables serde support.  In current versions of MiniJinja
+//!     it's not possible to disable serde but it will become possible.  To prevent
+//!     breakage, MiniJinja warns if this feature is disabled.
 //!
 //! - **Rust Functionality:**
 //!
@@ -246,6 +249,23 @@ pub use self::value::Value;
 pub use self::macros::__context;
 pub use self::vm::State;
 
+// fowards compatibility
+#[cfg(not(feature = "serde"))]
+const _: () = {
+    #[deprecated(
+        since = "2.0.4",
+        note = "Future versions of MiniJinja will require enabling \
+        the 'serde' feature to use serde types.  To silence this warning \
+        add 'serde' to the least of features of minijinja."
+    )]
+    #[allow(unused)]
+    fn enable_implicit_serde_support() {}
+
+    fn trigger_warning() {
+        enable_implicit_serde_support();
+    }
+};
+
 /// This module gives access to the low level machinery.
 ///
 /// This module is only provided by the `unstable_machinery` feature and does not