YJIT: Generate specialized code for Symbol for objtostring (#12247)

* YJIT: Generate specialized code for Symbol for objtostring

Co-authored-by: John Hawthorn <john@hawthorn.email>

* Update yjit/src/codegen.rs

---------

Co-authored-by: John Hawthorn <john@hawthorn.email>
Co-authored-by: Maxime Chevalier-Boisvert <maximechevalierb@gmail.com>

yjit/src/codegen.rs

@@ -9922,6 +9922,35 @@ fn gen_objtostring(
         );
 
         // No work needed. The string value is already on the top of the stack.
+        Some(KeepCompiling)
+    } else if unsafe { RB_TYPE_P(comptime_recv, RUBY_T_SYMBOL) } && assume_method_basic_definition(jit, asm, comptime_recv.class_of(), ID!(to_s)) {
+        jit_guard_known_klass(
+            jit,
+            asm,
+            comptime_recv.class_of(),
+            recv,
+            recv.into(),
+            comptime_recv,
+            SEND_MAX_DEPTH,
+            Counter::objtostring_not_string,
+        );
+
+        extern "C" {
+            fn rb_sym2str(sym: VALUE) -> VALUE;
+        }
+
+        // Same optimization done in the interpreter: rb_sym_to_s() allocates a mutable string, but since we are only
+        // going to use this string for interpolation, it's fine to use the
+        // frozen string.
+        // rb_sym2str does not allocate.
+        let sym = recv;
+        let str = asm.ccall(rb_sym2str as *const u8, vec![sym]);
+        asm.stack_pop(1);
+
+        // Push the return value
+        let stack_ret = asm.stack_push(Type::TString);
+        asm.mov(stack_ret, str);
+
         Some(KeepCompiling)
     } else {
         let cd = jit.get_arg(0).as_ptr();

yjit/src/cruby.rs

@@ -823,6 +823,7 @@ pub(crate) mod ids {
         name: NULL               content: b""
         name: respond_to_missing content: b"respond_to_missing?"
         name: to_ary             content: b"to_ary"
+        name: to_s               content: b"to_s"
         name: eq                 content: b"=="
         name: include_p          content: b"include?"
     }