[Parser] Parse if-else in the new wat parser and IRBuilder (#5963)

Parse both the straight-line and folded versions of if, including the
abbreviations that allow omitting the else clause. In the IRBuilder, generalize
the scope stack to be able to track scopes other than blocks and add methods for
visiting the beginnings of ifs and elses.

1 files changed, 17 insertions, 2 deletions

diff --git a/src/parser/contexts.h b/src/parser/contexts.h
index 210945e8d..dae938f7f 100644
--- a/src/parser/contexts.h
+++ b/src/parser/contexts.h
@@ -312,7 +312,12 @@ struct NullInstrParserCtx {
   InstrT makeBlock(Index, std::optional<Name>, BlockTypeT) {
     return Ok{};
   }
-  InstrT finishBlock(Index, InstrsT) { return Ok{}; }
+  template<typename BlockTypeT>
+  InstrT makeIf(Index, std::optional<Name>, BlockTypeT) {
+    return Ok{};
+  }
+  InstrT visitEnd(Index, InstrsT) { return Ok{}; }
+  InstrT visitElse(Index) { return Ok{}; }
 
   InstrT makeUnreachable(Index) { return Ok{}; }
   InstrT makeNop(Index) { return Ok{}; }
@@ -994,10 +999,20 @@ struct ParseDefsCtx : TypeParserCtx<ParseDefsCtx> {
                                        type.getSignature().results));
   }
 
-  Result<> finishBlock(Index pos, InstrsT) {
+  Result<> makeIf(Index pos, std::optional<Name> label, HeapType type) {
+    // TODO: validate labels?
+    // TODO: Move error on input types to here?
+    return withLoc(
+      pos,
+      irBuilder.makeIf(label ? *label : Name{}, type.getSignature().results));
+  }
+
+  Result<> visitEnd(Index pos, InstrsT) {
     return withLoc(pos, irBuilder.visitEnd());
   }
 
+  Result<> visitElse(Index pos) { return withLoc(pos, irBuilder.visitElse()); }
+
   Result<> makeUnreachable(Index pos) {
     return withLoc(pos, irBuilder.makeUnreachable());
   }