Floating point operations from serde

[renatoalencar]

When running make debug I receive the following output:

cargo build --package tez_kernel --target wasm32-unknown-unknown --profile release --target-dir ./target/repl
    Finished release [optimized] target(s) in 0.32s
wasm-strip -o ./.bin/debug_kernel.wasm ./target/repl/wasm32-unknown-unknown/release/tez_kernel.wasm
docker run --rm -it --entrypoint=/bin/sh --name wasm-repl -v $PWD/.bin:/.bin tezos/tezos:master_f9675b19_20230303221946 -c '/usr/local/bin/octez-smart-rollup-wasm-debugger /.bin/debug_kernel.wasm --inputs /.bin/inputs.json'
Error
  Unknown exception: Error(_, "float instructions are forbidden")

make: *** [debug] Error 1

Running grep -nE 'f(32|64)\.' ./.bin/kernel.wat I can find a few occurrences of floating point instructions, and looking into the wat file I could find the following functions, with mangled names:

• $_ZN77_$LT$serde..__private..de..content..Content$u20$as$u20$core..clone..Clone$GT$5clone17h12cafb7e0f652464E: Which is the result of the Clone derive macro on https://github.com/serde-rs/serde/blob/master/serde/src/private/de.rs#L219
• $_ZN5serde9__private2de7content7Content10unexpected17h6a53c07a72eeeb43E: From https://github.com/serde-rs/serde/blob/master/serde/src/private/de.rs#L274
• $_ZN60_$LT$serde..de..Unexpected$u20$as$u20$core..fmt..Display$GT$3fmt17hcb6eab2fe1eecf7bE: From https://github.com/serde-rs/serde/blob/master/serde/src/de/mod.rs#L403
• $_ZN4core3fmt5float29float_to_decimal_common_exact17hd806da160a15dfd7E: From https://github.com/rust-lang/rust/blob/master/library/core/src/fmt/float.rs#L29, which probably comes from inlining some of those function calls inside it. I believe this one can be circumvented by having cfg(no_fp_fmt_parse) enabled. But probably could optimized out once float dependency is removed from Unexpected.
• $_ZN4core3fmt5float52_$LT$impl$u20$core..fmt..Display$u20$for$u20$f64$GT$3fmt17hbe7611732fc77d82E: From https://github.com/rust-lang/rust/blob/master/library/core/src/fmt/float.rs#L225

I'm not sure how to solve this, but I believe we could do something with conditional compilation directly on serde, since cfg(no_integer128) (example https://github.com/serde-rs/serde/blob/master/serde/src/de/mod.rs#L121) opens this possibility.