[Strings] Remove stringview types and instructions (#6579)

The stringview types from the stringref proposal have three irregularities that
break common invariants and require pervasive special casing to handle properly:
they are supertypes of `none` but not subtypes of `any`, they cannot be the
targets of casts, and they cannot be used to construct nullable references. At
the same time, the stringref proposal has been superseded by the imported
strings proposal, which does not have these irregularities. The cost of
maintaing and improving our support for stringview types is no longer worth the
benefit of supporting them.

Simplify the code base by entirely removing the stringview types and related
instructions that do not have analogues in the imported strings proposal and do
not make sense in the absense of stringviews.

Three remaining instructions, `stringview_wtf16.get_codeunit`,
`stringview_wtf16.slice`, and `stringview_wtf16.length` take stringview operands
in the stringref proposal but cannot be removed because they lower to operations
from the imported strings proposal. These instructions are changed to take
stringref operands in Binaryen IR, and to allow a graceful upgrade path for
users of these instructions, the text and binary parsers still accept but ignore
`string.as_wtf16`, which is the instruction used to convert stringrefs to
stringviews. The binary writer emits code sequences that use scratch locals and `string.as_wtf16` to keep the output valid.

Future PRs will further align binaryen with the imported strings proposal
instead of the stringref proposal, for example by making `string` a subtype of
`extern` instead of a subtype of `any` and by removing additional instructions
that do not have analogues in the imported strings proposal.

2 files changed, 3 insertions, 44 deletions

diff --git a/src/tools/fuzzing/fuzzing.cpp b/src/tools/fuzzing/fuzzing.cpp
index 3378d0745..d6f8e0f12 100644
--- a/src/tools/fuzzing/fuzzing.cpp
+++ b/src/tools/fuzzing/fuzzing.cpp
@@ -29,13 +29,6 @@ namespace wasm {
 
 namespace {
 
-bool canBeNullable(HeapType type) {
-  // V8 does not accept nullable string views.
-  return type != HeapType::stringview_wtf8 &&
-         type != HeapType::stringview_wtf16 &&
-         type != HeapType::stringview_iter;
-}
-
 } // anonymous namespace
 
 TranslateToFuzzReader::TranslateToFuzzReader(Module& wasm,
@@ -2654,16 +2647,6 @@ Expression* TranslateToFuzzReader::makeBasicRef(Type type) {
       }
       WASM_UNREACHABLE("bad switch");
     }
-    case HeapType::stringview_wtf16:
-      // We fully support wtf16 strings.
-      return builder.makeStringAs(StringAsWTF16,
-                                  makeTrappingRefUse(HeapType::string));
-    case HeapType::stringview_wtf8:
-    case HeapType::stringview_iter:
-      // We do not have interpreter support for wtf8 and iter, so emit something
-      // that does not return, with a block that casts to the type the parent
-      // expects.
-      return builder.makeBlock({_makeunreachable()}, type);
     case HeapType::none:
     case HeapType::noext:
     case HeapType::nofunc:
@@ -2719,9 +2702,6 @@ Expression* TranslateToFuzzReader::makeCompoundRef(Type type) {
     if (funcContext && !funcContext->typeLocals[type].empty()) {
       return makeLocalGet(type);
     }
-    if (!canBeNullable(heapType)) {
-      return makeConst(type);
-    }
     return builder.makeRefAs(RefAsNonNull, builder.makeRefNull(heapType));
   }
 
@@ -2834,11 +2814,10 @@ Expression* TranslateToFuzzReader::makeStringConcat() {
 }
 
 Expression* TranslateToFuzzReader::makeStringSlice() {
-  // StringViews cannot be non-nullable.
-  auto* ref = make(Type(HeapType::stringview_wtf16, NonNullable));
+  auto* ref = makeTrappingRefUse(HeapType::string);
   auto* start = make(Type::i32);
   auto* end = make(Type::i32);
-  return builder.makeStringSliceWTF(StringSliceWTF16, ref, start, end);
+  return builder.makeStringSliceWTF(ref, start, end);
 }
 
 Expression* TranslateToFuzzReader::makeStringEq(Type type) {
@@ -2866,8 +2845,7 @@ Expression* TranslateToFuzzReader::makeStringMeasure(Type type) {
 Expression* TranslateToFuzzReader::makeStringGet(Type type) {
   assert(type == Type::i32);
 
-  // StringViews cannot be non-nullable.
-  auto* ref = make(Type(HeapType::stringview_wtf16, NonNullable));
+  auto* ref = makeTrappingRefUse(HeapType::string);
   auto* pos = make(Type::i32);
   return builder.makeStringWTF16Get(ref, pos);
 }
@@ -4277,12 +4255,6 @@ HeapType TranslateToFuzzReader::getSubType(HeapType type) {
         return HeapType::exn;
       case HeapType::string:
         return HeapType::string;
-      case HeapType::stringview_wtf8:
-        return HeapType::stringview_wtf8;
-      case HeapType::stringview_wtf16:
-        return HeapType::stringview_wtf16;
-      case HeapType::stringview_iter:
-        return HeapType::stringview_iter;
       case HeapType::none:
       case HeapType::noext:
       case HeapType::nofunc:
diff --git a/src/tools/fuzzing/heap-types.cpp b/src/tools/fuzzing/heap-types.cpp
index 3d0d29153..08b4c4453 100644
--- a/src/tools/fuzzing/heap-types.cpp
+++ b/src/tools/fuzzing/heap-types.cpp
@@ -395,9 +395,6 @@ struct HeapTypeGeneratorImpl {
         case HeapType::ext:
         case HeapType::exn:
         case HeapType::string:
-        case HeapType::stringview_wtf8:
-        case HeapType::stringview_wtf16:
-        case HeapType::stringview_iter:
         case HeapType::none:
         case HeapType::noext:
         case HeapType::nofunc:
@@ -460,17 +457,7 @@ struct HeapTypeGeneratorImpl {
       case HeapType::string:
         candidates.push_back(HeapType::any);
         break;
-      case HeapType::stringview_wtf8:
-      case HeapType::stringview_wtf16:
-      case HeapType::stringview_iter:
-        break;
       case HeapType::none:
-        if (features.hasStrings() && rand.oneIn(10)) {
-          candidates.push_back(HeapType::stringview_wtf8);
-          candidates.push_back(HeapType::stringview_wtf16);
-          candidates.push_back(HeapType::stringview_iter);
-          break;
-        }
         return pickSubAny();
       case HeapType::nofunc:
         return pickSubFunc();