diff --git a/src/expand/assert.cpp b/src/expand/assert.cpp
index 4fd67b43..de7dd472 100644
--- a/src/expand/assert.cpp
+++ b/src/expand/assert.cpp
@@ -83,7 +83,6 @@ class CExpander_assert:
         else if( tok == TOK_EOF )
         {
             ::std::stringstream ss;
-            ss << "assertion failed: ";
             n->print(ss);
 
             toks.push_back( Token(InterpolatedFragment(InterpolatedFragment::EXPR, n.release())) );
@@ -93,6 +92,8 @@ class CExpander_assert:
             toks.push_back( Token(TOK_IDENT, RcString::new_interned("panic")) );
             toks.push_back( Token(TOK_EXCLAM) );
             toks.push_back( Token(TOK_PAREN_OPEN) );
+            toks.push_back( Token(TOK_STRING, std::string("assertion failed: {}")) );
+            toks.push_back( Token(TOK_COMMA) );
             toks.push_back( Token(TOK_STRING, ss.str()) );
             toks.push_back( Token(TOK_PAREN_CLOSE) );
         }
diff --git a/src/expand/format_args.cpp b/src/expand/format_args.cpp
index 7f45f6f0..e9377d81 100644
--- a/src/expand/format_args.cpp
+++ b/src/expand/format_args.cpp
@@ -856,6 +856,19 @@ class CFormatArgsExpander:
         return expand_format_args(sp, crate, lex, /*add_newline=*/false);
     }
 };
+class CConstFormatArgsExpander:
+    public ExpandProcMacro
+{
+    ::std::unique_ptr<TokenStream> expand(const Span& sp, const ::AST::Crate& crate, const TokenTree& tt, AST::Module& mod) override
+    {
+        Token   tok;
+
+        auto lex = TTStream(sp, ParseState(), tt);
+        lex.parse_state().module = &mod;
+
+        return expand_format_args(sp, crate, lex, /*add_newline=*/false);
+    }
+};
 
 class CFormatArgsNlExpander:
     public ExpandProcMacro
@@ -872,5 +885,6 @@ class CFormatArgsNlExpander:
 };
 
 STATIC_MACRO("format_args", CFormatArgsExpander);
+STATIC_MACRO("const_format_args", CConstFormatArgsExpander);
 STATIC_MACRO("format_args_nl", CFormatArgsNlExpander);
 
diff --git a/src/expand/mod.cpp b/src/expand/mod.cpp
index c0212dae..726c2b30 100644
--- a/src/expand/mod.cpp
+++ b/src/expand/mod.cpp
@@ -281,6 +281,7 @@ ::std::unique_ptr<TokenStream> Expand_Macro_Inner(
         return ProcMacro_Invoke(mi_span, crate, mac_path, input_tt);
         }
     TU_ARMA(BuiltinProcMacro, proc_mac) {
+        ASSERT_BUG(mi_span, proc_mac, "null BuiltinProcMacro? " << path);
         auto e = input_ident == ""
             ? proc_mac->expand(mi_span, crate, input_tt, mod)
             : proc_mac->expand_ident(mi_span, crate, input_ident, input_tt, mod)