[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the same time, the stringref proposal has been superseded by the imported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.

1 files changed, 2 insertions, 24 deletions

diff --git a/src/wasm-binary.h b/src/wasm-binary.h
index 9610aed9f..449c03699 100644
--- a/src/wasm-binary.h
+++ b/src/wasm-binary.h
@@ -369,10 +369,7 @@ enum EncodedType {
   exnref = -0x17,    // 0x69
   nullexnref = -0xc, // 0x74
   // string reference types
-  stringref = -0x19,        // 0x67
-  stringview_wtf8 = -0x1a,  // 0x66
-  stringview_wtf16 = -0x1e, // 0x62
-  stringview_iter = -0x1f,  // 0x61
+  stringref = -0x19, // 0x67
   // type forms
   Func = -0x20,     // 0x60
   Cont = -0x23,     // 0x5d
@@ -402,12 +399,6 @@ enum EncodedHeapType {
   struct_ = -0x15, // 0x6b
   array = -0x16,   // 0x6a
   string = -0x19,  // 0x67
-  // stringview/iter constants are identical to type, and cannot be duplicated
-  // here as that would be a compiler error, so add _heap suffixes. See
-  // https://github.com/WebAssembly/stringref/issues/12
-  stringview_wtf8_heap = -0x1a,  // 0x66
-  stringview_wtf16_heap = -0x1e, // 0x62
-  stringview_iter_heap = -0x1f,  // 0x61
 };
 
 namespace CustomSections {
@@ -1146,18 +1137,9 @@ enum ASTNodes {
   StringEncodeLossyUTF8 = 0x8d,
   StringEncodeWTF8 = 0x8e,
   StringNewUTF8Try = 0x8f,
-  StringAsWTF8 = 0x90,
-  StringViewWTF8Advance = 0x91,
-  StringViewWTF8Slice = 0x93,
   StringAsWTF16 = 0x98,
-  StringViewWTF16Length = 0x99,
   StringViewWTF16GetCodePoint = 0x9a,
   StringViewWTF16Slice = 0x9c,
-  StringAsIter = 0xa0,
-  StringViewIterNext = 0xa1,
-  StringViewIterAdvance = 0xa2,
-  StringViewIterRewind = 0xa3,
-  StringViewIterSlice = 0xa4,
   StringCompare = 0xa8,
   StringFromCodePoint = 0xa9,
   StringHash = 0xaa,
@@ -1777,18 +1759,14 @@ public:
   bool maybeVisitArrayFill(Expression*& out, uint32_t code);
   bool maybeVisitArrayInit(Expression*& out, uint32_t code);
   bool maybeVisitStringNew(Expression*& out, uint32_t code);
+  bool maybeVisitStringAsWTF16(Expression*& out, uint32_t code);
   bool maybeVisitStringConst(Expression*& out, uint32_t code);
   bool maybeVisitStringMeasure(Expression*& out, uint32_t code);
   bool maybeVisitStringEncode(Expression*& out, uint32_t code);
   bool maybeVisitStringConcat(Expression*& out, uint32_t code);
   bool maybeVisitStringEq(Expression*& out, uint32_t code);
-  bool maybeVisitStringAs(Expression*& out, uint32_t code);
-  bool maybeVisitStringWTF8Advance(Expression*& out, uint32_t code);
   bool maybeVisitStringWTF16Get(Expression*& out, uint32_t code);
-  bool maybeVisitStringIterNext(Expression*& out, uint32_t code);
-  bool maybeVisitStringIterMove(Expression*& out, uint32_t code);
   bool maybeVisitStringSliceWTF(Expression*& out, uint32_t code);
-  bool maybeVisitStringSliceIter(Expression*& out, uint32_t code);
   void visitSelect(Select* curr, uint8_t code);
   void visitReturn(Return* curr);
   void visitMemorySize(MemorySize* curr);