feat(es/plugin): Add experimental metadata field (swc-project#5254)

crates/swc_plugin/src/lib.rs

@@ -9,6 +9,10 @@ pub use swc_common::{
     },
 };
 
+pub mod collections {
+    pub use swc_common::collections::AHashMap;
+}
+
 pub mod comments {
     pub use swc_common::comments::{Comment, CommentKind, Comments};
     pub use swc_plugin_proxy::PluginCommentsProxy;
@@ -84,7 +88,22 @@ pub struct TransformPluginProgramMetadata {
     pub unresolved_mark: crate::syntax_pos::Mark,
     /// Stringified JSON value for relative context while running transform,
     /// like filenames.
-    /// /// This is readonly. Changing value in plugin doesn't affect host's
+    /// This is readonly. Changing value in plugin doesn't affect host's
     /// behavior.
     pub transform_context: String,
+    /// Non typed, extensible properties without breaking plugin compability
+    /// between host.
+    ///
+    /// Adding a new property to this metadata will be a breaking changes we
+    /// can't do freely.
+    /// Instead, we use this as a placeholder `@swc/core` may try new metadata.
+    /// Once it's proven to be stable with enough usecases, it'll be
+    /// promoted to actual property to TransformPluginProgramMetadata with
+    /// proper type support.
+    ///
+    /// There is no typed deserialization support for this unfortunately. Plugin
+    /// need to deserialize stringified values manually. In most cases this'll
+    /// be JSON type, but depends on the nature of the metadata it may
+    /// require different way to deserialize.
+    pub experimental: collections::AHashMap<String, String>,
 }

crates/swc_plugin_macro/src/lib.rs

@@ -115,7 +115,8 @@ fn handle_func(func: ItemFn) -> TokenStream {
                 source_map: swc_plugin::source_map::PluginSourceMapProxy,
                 plugin_config: config,
                 unresolved_mark: swc_plugin::syntax_pos::Mark::from_u32(unresolved_mark),
-                transform_context: context
+                transform_context: context,
+                experimental: swc_plugin::collections::AHashMap::default(),
             };
 
             // Take original plugin fn ident, then call it with interop'ed args