Add a flag to opt in to the old WAT parser (#6536)

This flag is intended to help users gracefully migrate to the new wat parser. It
will be removed again not too long after the new wat parser is enabled by
default in wasm-opt.

src/tools/wasm-opt.cpp

@@ -220,6 +220,13 @@ int main(int argc, const char* argv[]) {
          [&outputSourceMapUrl](Options* o, const std::string& argument) {
            outputSourceMapUrl = argument;
          })
+    .add(
+      "--deprecated-wat-parser",
+      "",
+      "Use the old, deprecated WAT parser. This option will be removed soon!",
+      WasmOptOption,
+      Options::Arguments::Zero,
+      [](Options*, const std::string&) { useNewWATParser = false; })
     .add("--new-wat-parser",
          "",
          "Use the experimental new WAT parser",

test/lit/deprecated-wat.wast

@@ -0,0 +1,24 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.
+
+;; RUN: wasm-opt --deprecated-wat-parser -all %s -S -o - | filecheck %s
+
+(module
+ ;; CHECK:      (type $0 (func (param i32)))
+
+ ;; CHECK:      (type $1 (func (result i32)))
+
+ ;; CHECK:      (import "env" "bad" (func $wrong (type $0) (param i32)))
+
+ ;; CHECK:      (func $foo (type $1) (result i32)
+ ;; CHECK-NEXT:  (i32.const 0)
+ ;; CHECK-NEXT: )
+ (func $foo (result i32)
+  ;; Non-standard block result format
+  (block i32
+   (i32.const 0)
+  )
+ )
+
+ ;; Non-standard import after declared function
+ (import $wrong "env" "bad" (param i32))
+)

test/lit/help/wasm-opt.test

@@ -72,6 +72,10 @@
 ;; CHECK-NEXT:   --output-source-map-url,-osu                  Emit specified string as source
 ;; CHECK-NEXT:                                                 map URL
 ;; CHECK-NEXT:
+;; CHECK-NEXT:   --deprecated-wat-parser                       Use the old, deprecated WAT
+;; CHECK-NEXT:                                                 parser. This option will be
+;; CHECK-NEXT:                                                 removed soon!
+;; CHECK-NEXT:
 ;; CHECK-NEXT:   --new-wat-parser                              Use the experimental new WAT
 ;; CHECK-NEXT:                                                 parser
 ;; CHECK-NEXT: