postprocess: Add TrimTransform and enable postprocessing for lua

[thedataking]

No description provided.

[thedataking]

Interesting failure I observed:

INFO:root:Comments transferred to Rust fn reverse:                
/// Reverse the stack segment from 'from' to 'to'
/// (auxiliary to 'lua_rotate')
/// Note that we move(copy) only the value inside the stack.
/// (We do not move additional fields that may exist.)
unsafe extern "C" fn reverse(mut L: *mut lua_State, mut from: StkId, mut to: StkId) {
    while from < to {
        let mut temp: TValue = TValue { value_:   Value { gc:   ::core::ptr::null_mut::<GCObject>() }, tt_:   0 };
        let mut io1: *mut TValue = &raw mut temp;
        let mut io2: *const TValue = &raw mut (*from).val;
        (*io1).value_ = (*io2).value_;
        (*io1).tt_ = (*io2).tt_;
        let mut io1_0: *mut TValue = &raw mut (*from).val;
        let mut io2_0: *const TValue = &raw mut (*to).val;
        (*io1_0).value_ = (*io2_0).value_;
        (*io1_0).tt_ = (*io2_0).tt_;
        let mut io1_1: *mut TValue = &raw mut (*to).val;
        let mut io2_1: *const TValue = &raw mut temp;
        (*io1_1).value_ = (*io2_1).value_;
        (*io1_1).tt_ = (*io2_1).tt_;
        from = from.offset(1);
        to = to.offset(-1);
    }
}

DEBUG:root:Running merge_rust with args: [PosixPath('/home/perl/Work/c2rust/tools/merge_rust/target/release/merge_rust'), '--update-only', PosixPath('repo/src/lapi.rs'), '/tmp/tmprchjvmuc']


thread 'main' (3666313) panicked at src/main.rs:38:44:
called `Result::unwrap()` on an `Err` value: At("parsing \"repo/src/lapi.rs\"", Syn(Error("cannot parse string into token stream")))
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::result::unwrap_failed
   3: merge_rust::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Using gemini-3.5-flash by the way.

Root cause: The broken block is in lua_gc. The match closes at tests/integration/tests/lua/repo/src/lapi.rs:2097, but the function itself does not close before the next #[no_mangle] at tests/integration/tests/lua/repo/src/lapi.rs:2099. A closing } is missing after return res;.

Potential solution:

• check syntax using cargo check or rustc before attempting merge_rust and retry transform or
• retry transform if merge_rust fails

[Crocodoctopus]

Looks generally good, but I want to mention two concerns, mostly related to prompting:

I tested TrimTransform on json-c and it was very optimistic with the trim, with many false positives (probably 80% of comments). Double check if this is or isn't the case for lua.

Also on json-c, large functions can cause TrimTransform to fail in very unpredictable ways. I've caught TrimTransform refactoring entire functions. Even with prompt changes, this issue can persists.

json-c is outside the scope of this PR, so I wont block this on that front.

[thedataking]

I tested TrimTransform on json-c and it was very optimistic with the trim, with many false positives (probably 80% of comments). Double check if this is or isn't the case for lua.

This is a problem, this PR shouldn't regress json-c. Did you see the false positives with gemini-3.1-flash-lite or gemini-3.5-flash or something else?

[Crocodoctopus]

gemini-3.1-flash-lite, gpt-5.5, and gpt-5.1 had this behavior. I haven't tried 3.5-flash.

Edit: Upon testing with 3.5-flash it looks correct.