zkir panics on wrapping Uint arithmetic after ledger 7 upgrade

[adamreynolds-io]

Context & versions

Compact: 0.27.110 (ledger 7) — broken
Compact: 0.27.0 (ledger 6) — worked

Steps to reproduce

Compile the following contract:

export ledger result_a: Boolean;

export circuit test(): [] {
   const a = ((255 as Uint<8>) + (1 as Uint<8>)) as Uint<8>;
   assert(a <= 255, "Not");

   result_a = a <= 255;
}

Actual behavior

Compilation crashes with a Rust panic in midnight-circuits 6.0.0:

thread 'main' panicked at .../midnight-circuits-6.0.0/src/field/native/native_gadget.rs:174:13:
Trying to convert Fq(0x0000000000000000000000000000000000000000000000000000000000000100) to an AssignedBounded less than 2^8!
Exception: zkir returned a non-zero exit status 101

The value 256 (result of 255 + 1 wrapping in Uint<8>) is 0x100, which exceeds the 8-bit bound expected by the circuit backend.

Expected behavior

Wrapping arithmetic on Uint<N> types that was valid under ledger 6 should continue to work under ledger 7, or the compiler should emit a clear error explaining the semantic change. The zkir binary should not panic.

---

Migrated from PM-21087

[adamreynolds-io]

Originally reported in Jira

[pataei]

Using compact 0.30.106 I got:

Compiling 1 circuits:
  circuit "test" |                                                                                                                                                       
thread 'main' (12863376) panicked at /private/tmp/nix-build-zkir-2.1.0.drv-0/cargo-vendor-dir/midnight-circuits-6.0.0/src/field/native/native_gadget.rs:174:13:
Trying to convert Fq(0x0000000000000000000000000000000000000000000000000000000000000100) to an AssignedBounded less than 2^8!
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Exception: zkir returned a non-zero exit status 101