In --debug mode, print partial wasm data that was read (#5356)

If wasm-opt or wasm-dis are given an invalid binary, after the error
message we can also print out the wasm we did manage to read. That
includes global stuff like imports and also all the functions up until
there. This can help debugging in some situations.

Only do this when --debug is passed as it can be very verbose and
in general users might not want it.

This is technically easy to do, it turns out, since we already use a
thrown exception on an error in parsing, and we fill up the wasm as
we go, so it just contains what we've read so far, and we can just
print it.

Fixes #5344

Also switch an existing test's comments to ;; from # which was
noticed here.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/tools/wasm-opt.cpp b/src/tools/wasm-opt.cpp
index effd085bc..8f958f164 100644
--- a/src/tools/wasm-opt.cpp
+++ b/src/tools/wasm-opt.cpp
@@ -279,7 +279,12 @@ int main(int argc, const char* argv[]) {
     } catch (ParseException& p) {
       p.dump(std::cerr);
       std::cerr << '\n';
-      Fatal() << "error parsing wasm";
+      if (options.debug) {
+        Fatal() << "error parsing wasm. here is what we read up to the error:\n"
+                << wasm;
+      } else {
+        Fatal() << "error parsing wasm (try --debug for more info)";
+      }
     } catch (MapParseException& p) {
       p.dump(std::cerr);
       std::cerr << '\n';