Add basic custom derive lowering

crates/ra_hir_expand/src/proc_macro.rs

@@ -0,0 +1,32 @@
+//! Proc Macro Expander stub
+
+use crate::{db::AstDatabase, LazyMacroId, MacroCallKind, MacroCallLoc};
+use ra_db::CrateId;
+
+#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
+pub struct ProcMacroExpander {
+    krate: CrateId,
+}
+
+impl ProcMacroExpander {
+    pub fn new(krate: CrateId) -> ProcMacroExpander {
+        ProcMacroExpander { krate }
+    }
+
+    pub fn expand(
+        &self,
+        db: &dyn AstDatabase,
+        id: LazyMacroId,
+        _tt: &tt::Subtree,
+    ) -> Result<tt::Subtree, mbe::ExpandError> {
+        let loc: MacroCallLoc = db.lookup_intern_macro(id);
+        let name = match loc.kind {
+            MacroCallKind::FnLike(_) => return Err(mbe::ExpandError::ConversionError),
+            MacroCallKind::Attr(_, name) => name,
+        };
+
+        dbg!(name);
+
+        unimplemented!()
+    }
+}