gc is needed too, now, for string optimization

README.md

@@ -201,7 +201,9 @@ There are a few differences between Binaryen IR and the WebAssembly language:
      stringref operations, before any default optimization passes. Those
      stringref operations can then be optimized (e.g., a concat of constants
      turns into a concatenated constant). When we are about to finish running
-     default optimizations, we lower stringref back into string builtins.
+     default optimizations, we lower stringref back into string builtins. (Note:
+     reference types and GC must also be enabled, as imported string operations
+     depend on GC arrays.)
 
 As a result, you might notice that round-trip conversions (wasm => Binaryen IR
 => wasm) change code a little in some corner cases.

test/lit/string-builtins.wast

@@ -1,11 +1,18 @@
 ;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.
 
 ;; Run normal -O2, which should lift, optimize, and lower strings, if the
-;; string builtins feature is enabled. We optimize in the last two here.
+;; string builtins feature is enabled (GC is required, as well, as the lowering
+;; emits functions using arrays). We optimize in the last two here.
 
-;; RUN: foreach %s %t wasm-opt -O2 --enable-reference-types -S -o - | filecheck %s --check-prefix=MVP
-;; RUN: foreach %s %t wasm-opt -O2 -all -S -o - | filecheck %s --check-prefix=ALL
-;; RUN: foreach %s %t wasm-opt -O2 --enable-reference-types --enable-string-builtins -S -o - | filecheck %s --check-prefix=ESB
+;; RUN: foreach %s %t wasm-opt -O2 -S -o - --enable-reference-types \
+;; RUN:    | filecheck %s --check-prefix=MVP
+
+;; RUN: foreach %s %t wasm-opt -O2 -S -o - -all \
+;; RUN:    | filecheck %s --check-prefix=ALL
+
+;; RUN: foreach %s %t wasm-opt -O2 -S -o - --enable-reference-types \
+;; RUN:    --enable-string-builtins --enable-gc \
+;; RUN:    | filecheck %s --check-prefix=ESB
 
 (module
   (type $array16 (array (mut i16)))
@@ -46,7 +53,7 @@
 
   ;; ESB:      (export "string.concat" (func $string.concat))
 
-  ;; ESB:      (func $string.concat (result (ref extern))
+  ;; ESB:      (func $string.concat (type $0) (result (ref extern))
   ;; ESB-NEXT:  (global.get $"string.const_\"foobar\"")
   ;; ESB-NEXT: )
   (func $string.concat (export "string.concat") (result (ref extern))