cargo: Document that cargo subprojects is an experimental feature

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

@@ -316,6 +316,8 @@ foo-bar-1.0 = foo_bar_dep
 ```
 ### Cargo wraps
 
+*This is experimental and has no backwards or forwards compatibility guarantees.*
+
 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 +1,6 @@
+## 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 feature and future releases might do breaking
+changes.

mesonbuild/interpreter/interpreter.py

@@ -1043,6 +1043,8 @@ def _do_subproject_cargo(self, subp_name: str, subdir: str,
                              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)