Add basic bultin macro infrastructure

crates/ra_hir_expand/src/builtin_macro.rs

@@ -0,0 +1,26 @@
+//! Builtin macro
+use crate::{ast, name, AstId, BuiltinMacro, CrateId, MacroDefId};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum BuiltinExpander {
+    Line
+}
+
+impl BuiltinExpander {
+    pub fn expand(&self, _tt: &tt::Subtree) -> Result<tt::Subtree, mbe::ExpandError> {
+        Err(mbe::ExpandError::UnexpectedToken)
+    }
+}
+
+pub fn find_builtin_macro(
+    ident: &name::Name,
+    krate: CrateId,
+    ast_id: AstId<ast::MacroCall>,
+) -> Option<MacroDefId> {
+    // FIXME: Better registering method
+    if ident == &name::LINE {
+        Some(MacroDefId::BuiltinMacro(BuiltinMacro { expander: BuiltinExpander::Line, krate, ast_id }))
+    } else {
+        None
+    }
+}