Closure compiler warning about useless `null;`

[oboukli]

Closure compiler is warning that the null; line at the begging of JavaScript files is useless. Code won't build with strict compiler flags.

Works fine with emsdk 3.1.22, probably because it uses an older version of the Closure compiler, but not with 3.1.23.

Version of emscripten/emsdk:

docker run emscripten/emsdk emcc -v

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.23 (5ae63ce7dd955df449cb419bfe5afc51d1bd57f2)
clang version 16.0.0 (https://github.com/llvm/llvm-project 8b587113b746f31b63fd6473083df78cef30a72e)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /emsdk/upstream/bin

Failing command line in full:

Sample code:main.c

#include <stdio.h>

int main() {
  printf("hello, world!\n");
  return 0;
}

Command

docker run \
-v $(pwd):/src \
-v $(pwd)/emscripten_cache:/emsdk/upstream/emscripten/cache \
-v $(pwd)/emscripten_tmp:/tmp \
--workdir=/src emscripten/emsdk \
emcc -v -Wall -Werror -O3 \
--closure 1 \
-s ASSERTIONS=0 \
-s EXPORTED_FUNCTIONS='["_main"]' \
-s STRICT=1 \
-o main.js main.c

Output

 "/emsdk/upstream/bin/clang" --version
 "/emsdk/upstream/bin/clang" -target wasm32-unknown-emscripten -fignore-exceptions -fvisibility=default -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -Werror=implicit-function-declaration -I/emsdk/upstream/emscripten/cache/sysroot/include/SDL --sysroot=/emsdk/upstream/emscripten/cache/sysroot -Xclang -iwithsysroot/include/compat -v -Wall -Werror -O3 main.c -c -o /tmp/emscripten_temp_bscia4h5/main_0.o
clang version 16.0.0 (https://github.com/llvm/llvm-project 8b587113b746f31b63fd6473083df78cef30a72e)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /emsdk/upstream/bin
 (in-process)
 "/emsdk/upstream/bin/clang-16" -cc1 -triple wasm32-unknown-emscripten -emit-obj --mrelax-relocations -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name main.c -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-cpu generic -mllvm -treat-scalable-fixed-error-as-warning -debugger-tuning=gdb -v -fcoverage-compilation-dir=/src -resource-dir /emsdk/upstream/lib/clang/16.0.0 -I /emsdk/upstream/emscripten/cache/sysroot/include/SDL -isysroot /emsdk/upstream/emscripten/cache/sysroot -internal-isystem /emsdk/upstream/lib/clang/16.0.0/include -internal-isystem /emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /emsdk/upstream/emscripten/cache/sysroot/include -O3 -Werror=implicit-function-declaration -Wall -Werror -fdebug-compilation-dir=/src -ferror-limit 19 -fvisibility=default -fgnuc-version=4.2.1 -fignore-exceptions -vectorize-loops -vectorize-slp -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /tmp/emscripten_temp_bscia4h5/main_0.o -x c main.c
clang -cc1 version 16.0.0 based upon LLVM 16.0.0git default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
 /emsdk/upstream/emscripten/cache/sysroot/include/SDL
 /emsdk/upstream/emscripten/cache/sysroot/include/compat
 /emsdk/upstream/lib/clang/16.0.0/include
 /emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
 "/emsdk/upstream/bin/wasm-ld" -o main.wasm /tmp/emscripten_temp_bscia4h5/main_0.o -L/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten -lnoexit -lc -ldlmalloc -lcompiler_rt -lsockets --fatal-warnings -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --import-undefined --strip-debug --export-if-defined=main --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=__main_argc_argv --export=stackSave --export=stackRestore --export=stackAlloc --export=__wasm_call_ctors --export=__errno_location --export=__get_temp_ret --export=__set_temp_ret --export-table -z stack-size=5242880 --initial-memory=16777216 --no-entry --max-memory=16777216 --global-base=1024
 "/emsdk/upstream/bin/wasm-emscripten-finalize" --dyncalls-i64 --pass-arg=legalize-js-interface-exported-helpers main.wasm -o main.wasm --detect-features
 "/emsdk/node/14.18.2_64bit/bin/node" /emsdk/upstream/emscripten/src/compiler.js /tmp/tmpxi0cn2o6.json
 "/emsdk/upstream/bin/wasm-opt" --strip-dwarf --post-emscripten -O3 --low-memory-unused --zero-filled-memory --pass-arg=directize-initial-contents-immutable --strip-debug --strip-producers main.wasm -o main.wasm --mvp-features
 "/emsdk/node/14.18.2_64bit/bin/node" /emsdk/upstream/emscripten/tools/acorn-optimizer.js /tmp/emscripten_temp_bscia4h5/main.js AJSDCE --closureFriendly
 "/emsdk/node/14.18.2_64bit/bin/node" /emsdk/upstream/emscripten/tools/acorn-optimizer.js /tmp/emcc_acorn_info_9ysik92g.js emitDCEGraph noPrint --closureFriendly
 "/emsdk/upstream/bin/wasm-metadce" --graph-file=/tmp/emcc_dce_graph_6p5aj3bz.json main.wasm -o main.wasm --mvp-features
 "/emsdk/node/14.18.2_64bit/bin/node" /emsdk/upstream/emscripten/tools/acorn-optimizer.js /tmp/emcc_acorn_info_q3mi6lll.js applyDCEGraphRemovals --closureFriendly
 "/emsdk/node/14.18.2_64bit/bin/node" /emsdk/upstream/emscripten/tools/acorn-optimizer.js /tmp/emscripten_temp_bscia4h5/main.js.jso.js.jso.js AJSDCE --closureFriendly
 "/emsdk/upstream/bin/wasm-opt" --strip-dwarf --minify-imports-and-exports-and-modules main.wasm -o main.wasm --mvp-features
 "/emsdk/node/14.18.2_64bit/bin/node" /emsdk/upstream/emscripten/tools/acorn-optimizer.js /tmp/emcc_acorn_info_bm4m2tez.js applyImportAndExportNameChanges --closureFriendly
 "/emsdk/node/14.18.2_64bit/bin/node" --max_old_space_size=8192 /emsdk/upstream/emscripten/node_modules/.bin/google-closure-compiler --compilation_level ADVANCED_OPTIMIZATIONS --language_in ECMASCRIPT_2020 --language_out NO_TRANSPILE --emit_use_strict=false --externs /emsdk/upstream/emscripten/src/closure-externs/closure-externs.js --externs /emsdk/upstream/emscripten/src/closure-externs/node-externs.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/buffer.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/assert.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/string_decoder.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/os.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/tty.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/domain.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/process.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/readline.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/path.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/vm.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/dgram.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/events.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/util.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/dns.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/zlib.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/punycode.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/url.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/cluster.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/stream.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/repl.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/child_process.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/https.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/querystring.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/net.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/tls.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/fs.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/http.js --externs /emsdk/upstream/emscripten/third_party/closure-compiler/node-externs/core.js --js /tmp/emscripten_temp_bscia4h5/main.js.jso.js.jso.js.jso.js.jso.js --js_output_file tmp12ftzcj7.cc.js
building:ERROR: Closure compiler completed with warnings and -Werror=closure enabled, aborting!

building:ERROR: /tmp/emscripten_temp_bscia4h5/main.js.jso.js.jso.js.jso.js.jso.js:1:0: WARNING - [JSC_USELESS_CODE] Suspicious code. This code lacks side-effects. Is there a bug?
  1| null;
     ^^^^

0 error(s), 1 warning(s), 80.0% typed

building:WARNING: (rerun with -g1 linker flag for an unminified output)
emcc: error: closure compiler produced warnings and -W=error=closure enabled

[sbc100]

The change here is that we started including closure warnings in -Werror with #17878. What this means is likely that this warning was always present but it was previously being ignored. You can get the previous behaviour by disabling closure warnings with -Wno-closure.

I will also take a look at why this warning didn't show up in any of our tests (which generally run with -Werror).

[oboukli]

@sbc100, thank you for your reply.

Changing any of the compiler flags/configuration noted in the bug report above seems to eliminate the warning.

For example, setting -s ASSERTIONS=1. Another example is to remove -O3 or change it to -O2

[sbc100]

Fixed in #17964)