cargo: Document that cargo subprojects is an experimental feature

docs/markdown/Wrap-dependency-system-manual.md

@@ -297,6 +297,9 @@ fallback to use the subproject, assuming it uses
 
 ### CMake wraps
 
+**Note**: This is experimental and has no backwards or forwards compatibility guarantees.
+See [Meson's rules on mixing build systems](Mixing-build-systems.md).
+
 Since the CMake module does not know the public name of the provided
 dependencies, a CMake `.wrap` file cannot use the `dependency_names = foo`
 syntax. Instead, the `dep_name = <target_name>_dep` syntax should be used, where
@@ -316,6 +319,9 @@ foo-bar-1.0 = foo_bar_dep
 ```
 ### Cargo wraps
 
+**Note**: This is experimental and has no backwards or forwards compatibility guarantees.
+See [Meson's rules on mixing build systems](Mixing-build-systems.md).
+
 Cargo subprojects automatically override the `<package_name>-<version>-rs` dependency
 name:
 - `package_name` is defined in `[package] name = ...` section of the `Cargo.toml`.

docs/markdown/snippets/cargo_exp.md

@@ -0,0 +1,8 @@
+## Cargo subprojects is experimental
+
+Cargo subprojects was intended to be experimental with no stability guarantees.
+That notice was unfortunately missing from documentation. Meson will now start
+warning about usage of experimental features and future releases might do breaking
+changes.
+
+This is aligned with our general policy regarding [mixing build systems](Mixing-build-systems.md).

mesonbuild/interpreter/interpreter.py

@@ -1043,6 +1043,8 @@ class Interpreter(InterpreterBase, HoldableObject):
                              kwargs: kwtypes.DoSubproject) -> SubprojectHolder:
         from .. import cargo
         FeatureNew.single_use('Cargo subproject', '1.3.0', self.subproject, location=self.current_node)
+        mlog.warning('Cargo subproject is an experimental feature and has no backwards compatibility guarantees.',
+                     once=True, location=self.current_node)
         with mlog.nested(subp_name):
             ast, options = cargo.interpret(subp_name, subdir, self.environment)
             self.coredata.update_project_options(options, subp_name)