gccrs: derive: Add dummy builtin transcriber for builtin derive macros.

Fixes #2257.

Derive builtins do not need a real transcriber, but still need one. It
will however never be called since builtin derive macros get expanded
differently, and benefit from knowing on what kind of items they are
applied (struct, enums, unions) rather than receiving a list of tokens
like regular builtin macros.

gcc/rust/ChangeLog:

	* expand/rust-macro-builtins.cc (MacroBuiltin::dummy): New function.
	* expand/rust-macro-builtins.h: Declare it.

gcc/testsuite/ChangeLog:

	* rust/compile/derive_macro7.rs: New test.

3 files changed, 35 insertions, 0 deletions

diff --git a/gcc/rust/expand/rust-macro-builtins.cc b/gcc/rust/expand/rust-macro-builtins.cc
index f457750..33b813c 100644
--- a/gcc/rust/expand/rust-macro-builtins.cc
+++ b/gcc/rust/expand/rust-macro-builtins.cc
@@ -105,6 +105,22 @@ std::unordered_map<
     {"test_case", MacroBuiltin::sorry},
     {"global_allocator", MacroBuiltin::sorry},
     {"cfg_accessible", MacroBuiltin::sorry},
+    /* Derive builtins do not need a real transcriber, but still need one. It
+       will however never be called since builtin derive macros get expanded
+       differently, and benefit from knowing on what kind of items they are
+       applied (struct, enums, unions) rather than receiving a list of tokens
+       like regular builtin macros */
+    {"RustcEncodable", MacroBuiltin::proc_macro_builtin},
+    {"RustcDecodable", MacroBuiltin::proc_macro_builtin},
+    {"Clone", MacroBuiltin::proc_macro_builtin},
+    {"Copy", MacroBuiltin::proc_macro_builtin},
+    {"Debug", MacroBuiltin::proc_macro_builtin},
+    {"Default", MacroBuiltin::proc_macro_builtin},
+    {"Eq", MacroBuiltin::proc_macro_builtin},
+    {"PartialEq", MacroBuiltin::proc_macro_builtin},
+    {"Ord", MacroBuiltin::proc_macro_builtin},
+    {"PartialOrd", MacroBuiltin::proc_macro_builtin},
+    {"Hash", MacroBuiltin::proc_macro_builtin},
 };
 
 // FIXME: This should return an Optional
@@ -932,4 +948,11 @@ MacroBuiltin::sorry (Location invoc_locus, AST::MacroInvocData &invoc)
   return AST::Fragment::create_error ();
 }
 
+AST::Fragment
+MacroBuiltin::proc_macro_builtin (Location, AST::MacroInvocData &)
+{
+  // nothing to do!
+  return AST::Fragment::create_error ();
+}
+
 } // namespace Rust
diff --git a/gcc/rust/expand/rust-macro-builtins.h b/gcc/rust/expand/rust-macro-builtins.h
index 0d8c4db..3ad10a3 100644
--- a/gcc/rust/expand/rust-macro-builtins.h
+++ b/gcc/rust/expand/rust-macro-builtins.h
@@ -159,6 +159,10 @@ public:
 				     AST::MacroInvocData &invoc);
 
   static AST::Fragment sorry (Location invoc_locus, AST::MacroInvocData &invoc);
+
+  /* Builtin procedural macros do not work directly on tokens, but still need an
+   * empty builtin transcriber to be considered proper builtin macros */
+  static AST::Fragment proc_macro_builtin (Location, AST::MacroInvocData &);
 };
 } // namespace Rust
 
diff --git a/gcc/testsuite/rust/compile/derive_macro7.rs b/gcc/testsuite/rust/compile/derive_macro7.rs
new file mode 100644
index 0000000..108e3cf
--- /dev/null
+++ b/gcc/testsuite/rust/compile/derive_macro7.rs
@@ -0,0 +1,8 @@
+#![feature(rustc_attrs)]
+
+#[rustc_builtin_macro]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[allow_internal_unstable(core_intrinsics, libstd_sys_internals)]
+pub macro RustcDecodable($item:item) {
+    /* compiler built-in */
+}
\ No newline at end of file