Bug in pruning [sendrawtransaction RPC error -26 (Trailing bytes after final byte of witness)] #391

We've run into an issue pruning our contracts after upgrading simplicityhl to version 0.8.0.
Given that simplicity-lang was updated previously and this issue existed, we can provide contract examples related to this comment: BlockstreamResearch/SimplicityHL#391 (comment).

Errors

Simplex returns this kind of error after sending a formed transaction to a node:

    Error: Broadcast failed with HTTP 400 for http://127.0.0.1:52982/tx: sendrawtransaction RPC error -26: non-mandatory-script-verify-flag (Trailing bytes after final byte of witness)

SimplicityHL tests are failing with this error regarding the precision of the first encoded bytes.

bitstream had trailing bytes 0x04...

What happened?

We encountered this error in the simplicityhl-std repo across all our testing contracts. On the previous version of simplicity-lang(0.8.0), everything was fine.
Now I'm trying to migrate them to the latest version, but the migration is failing.

Why am I reporting this issue here?
After locally reverting the changes from this PR #379, everything works.
I'm sure that something went wrong in pruning after this commit: 4c7be1b.

Pruning doesn't prune redundant branches, and later decoding fails.
You can see this in our contracts because we have lots of redundant branches (we use them, but in certain places).
If we use values more carefully, we can have no inactive branches. All will be fine, as in this test single_none_without_inactive_branch_roundtrips.

Steps to Reproduce

I've created a minimal reproducible repository with replicated examples using both simplex and raw simplicityhl here: https://github.com/ikripaka/example.

To run tests, you can use the following commands.

• Ordinary cargo tests: cargo nextest run assert_original_none_256_encoding_roundtrip assert_none_256_witness_encoding_roundtrip single_none_without_inactive_branch_roundtrips minimal_pruning_encoding_roundtrip assert_none_256_manual_fail --no-fail-fast

• Simplex tests (To install simplex, use this guide: https://github.com/blockstreamResearch/smplx#installation.): simplex test minimal_pruning_none_happy_path__smplx_test assert_original_none_256_happy_path__smplx_test assert_none_256_happy_path__smplx_test --no-fail-fast

There you can find different contract configurations:

• asssert_original.simf, assert_original_lib.simf, assert_original_dispatch.simf - represent simplified version of real testing case;
• minimal_pruning.simf - represents minimal pruning failing encode roundtrip;
• asserts_bin.simf, asserts.simf - represent a simplified real testing case.

To build it on your own, run commands:

simplex clean
simplex build

# run tests
cargo nextest ....
simplex test ...

If I'm wrong, please tell me what pruning configuration I need to use. Then I'll correct it in simplex.