Fix table.init text format parsing (#7654)

`table.init` text format has two forms:

- `table.init tableidx elemidx`
- `table.init elemidx` (`tableidx` is implicitly 0)

Because the optional `tableidx` is not the last (second) argument, we
have to parse one or two immediate arguments, and depending on how many
we parsed decide which one is `elemidx`.

Currently the code assumes first immediate argument is always a table,
which does not handle the second form.

Before this PR, `wasm-shell` fails to parse this functions in spec
tests:
https://github.com/WebAssembly/testsuite/blob/e05365077e13a1d86ffe77acfb1a835b7aa78422/bulk.wast#L207-L211

After the PR we're able to parse it, but we still can't parse the whole
file, because of other issues.

Error when parsing `bulk.wast`, before this PR:

```
209:6: error: expected elem index or identifier
```

After this PR:

```
250:33: error: unrecognized instruction
```

(The new error is about `elem.drop`, which we don't support yet, see
#7209)

Author: osa1

scripts/test/shared.py

@@ -416,12 +416,12 @@ def get_tests(test_dir, extensions=[], recursive=False):
     'address.wast',  # 64-bit offset allowed by memory64
     'align.wast',    # Alignment bit 6 used by multi-memory
     'binary.wast',   # memory.grow reserved byte a LEB in multi-memory
-    'bulk.wast',     # Requires table.init abbreviation with implicit table
+    'bulk.wast',     # Requires support for elem.drop
     'comments.wast',  # Issue with carriage returns being treated as newlines
     'const.wast',    # Hex float constant not recognized as out of range
     'conversions.wast',  # Promoted NaN should be canonical
     'data.wast',    # Constant global references allowed by GC
-    'elem.wast',    # Requires table.init abbreviation with implicit table
+    'elem.wast',    # Requires modeling empty declarative segments
     'f32.wast',     # Adding -0 and -nan should give a canonical NaN
     'f64.wast',     # Adding -0 and -nan should give a canonical NaN
     'float_exprs.wast',  # Adding 0 and NaN should give canonical NaN

src/parser/parsers.h

@@ -2130,10 +2130,33 @@ makeTableCopy(Ctx& ctx, Index pos, const std::vector<Annotation>& annotations) {
 template<typename Ctx>
 Result<>
 makeTableInit(Ctx& ctx, Index pos, const std::vector<Annotation>& annotations) {
+  // Note: binary and text formats for `table.init` are different. In both
+  // formats the table index is optional (with 0 as the default). When both the
+  // table and elem index are specified, the elem index comes first in the
+  // binary format, but second in the text format.
+
+  auto reset = ctx.in.getPos();
+
+  auto retry = [&]() -> Result<> {
+    // We're unable to parse the two argument format. Try one argument format
+    // with just elem index.
+    WithPosition with(ctx, reset);
+    auto elem = elemidx(ctx);
+    CHECK_ERR(elem);
+    MaybeResult<typename Ctx::TableIdxT> table = ctx.getTableFromIdx(0);
+    return ctx.makeTableInit(pos, annotations, table.getPtr(), *elem);
+  };
+
   auto table = maybeTableidx(ctx);
-  CHECK_ERR(table);
-  auto elem = elemidx(ctx);
-  CHECK_ERR(elem);
+  if (table.getErr()) {
+    return retry();
+  }
+
+  auto elem = maybeElemidx(ctx);
+  if (elem.getErr() || !elem) {
+    return retry();
+  }
+
   return ctx.makeTableInit(pos, annotations, table.getPtr(), *elem);
 }
 
@@ -2850,13 +2873,22 @@ template<typename Ctx> Result<typename Ctx::GlobalIdxT> globalidx(Ctx& ctx) {
 
 // elemidx ::= x:u32 => x
 //           | v:id => x (if elems[x] = v)
-template<typename Ctx> Result<typename Ctx::ElemIdxT> elemidx(Ctx& ctx) {
+template<typename Ctx>
+MaybeResult<typename Ctx::ElemIdxT> maybeElemidx(Ctx& ctx) {
   if (auto x = ctx.in.takeU32()) {
     return ctx.getElemFromIdx(*x);
   }
   if (auto id = ctx.in.takeID()) {
     return ctx.getElemFromName(*id);
   }
+  return {};
+}
+
+template<typename Ctx> Result<typename Ctx::ElemIdxT> elemidx(Ctx& ctx) {
+  if (auto idx = maybeElemidx(ctx)) {
+    CHECK_ERR(idx);
+    return *idx;
+  }
   return ctx.in.err("expected elem index or identifier");
 }