CompilerTool: support -###

[woodruffw]

Both GCC and Clang support -### as an option that enables an "explainer" mode: it prints out the steps that the compiler would have run, without actually running them.

For example:

clang -### -x c -

produces (on stderr):

Apple clang version 14.0.3 (clang-1403.0.22.14.1)
Target: arm64-apple-darwin22.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
 "/Library/Developer/CommandLineTools/usr/bin/clang" "-cc1" "-triple" "arm64-apple-macosx13.0.0" "-Wundef-prefix=TARGET_OS_" "-Wdeprecated-objc-isa-usage" "-Werror=deprecated-objc-isa-usage" "-Werror=implicit-function-declaration" "-emit-obj" "-mrelax-all" "--mrelax-relocations" "-disable-free" "-clear-ast-before-backend" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "-" "-mrelocation-model" "pic" "-pic-level" "2" "-mframe-pointer=non-leaf" "-fno-strict-return" "-ffp-contract=on" "-fno-rounding-math" "-funwind-tables=1" "-fobjc-msgsend-selector-stubs" "-target-sdk-version=13.3" "-fvisibility-inlines-hidden-static-local-var" "-target-cpu" "apple-m1" "-target-feature" "+v8.5a" "-target-feature" "+crc" "-target-feature" "+lse" "-target-feature" "+rdm" "-target-feature" "+crypto" "-target-feature" "+dotprod" "-target-feature" "+fp-armv8" "-target-feature" "+neon" "-target-feature" "+fp16fml" "-target-feature" "+ras" "-target-feature" "+rcpc" "-target-feature" "+zcm" "-target-feature" "+zcz" "-target-feature" "+fullfp16" "-target-feature" "+sm4" "-target-feature" "+sha3" "-target-feature" "+sha2" "-target-feature" "+aes" "-target-abi" "darwinpcs" "-fallow-half-arguments-and-returns" "-mllvm" "-treat-scalable-fixed-error-as-warning" "-debugger-tuning=lldb" "-target-linker-version" "857.1" "-fcoverage-compilation-dir=/Users/william" "-resource-dir" "/Library/Developer/CommandLineTools/usr/lib/clang/14.0.3" "-isysroot" "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk" "-I/usr/local/include" "-internal-isystem" "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/local/include" "-internal-isystem" "/Library/Developer/CommandLineTools/usr/lib/clang/14.0.3/include" "-internal-externc-isystem" "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include" "-internal-externc-isystem" "/Library/Developer/CommandLineTools/usr/include" "-Wno-reorder-init-list" "-Wno-implicit-int-float-conversion" "-Wno-c99-designator" "-Wno-final-dtor-non-final-class" "-Wno-extra-semi-stmt" "-Wno-misleading-indentation" "-Wno-quoted-include-in-framework-header" "-Wno-implicit-fallthrough" "-Wno-enum-enum-conversion" "-Wno-enum-float-conversion" "-Wno-elaborated-enum-base" "-Wno-reserved-identifier" "-Wno-gnu-folding-constant" "-fdebug-compilation-dir=/Users/william" "-ferror-limit" "19" "-stack-protector" "1" "-fstack-check" "-mdarwin-stkchk-strong-link" "-fblocks" "-fencode-extended-block-signature" "-fregister-global-dtors-with-atexit" "-fgnuc-version=4.2.1" "-no-opaque-pointers" "-fmax-type-align=16" "-fcommon" "-clang-vendor-feature=+disableNonDependentMemberExprInCurrentInstantiation" "-fno-odr-hash-protocols" "-clang-vendor-feature=+enableAggressiveVLAFolding" "-clang-vendor-feature=+revert09abecef7bbf" "-clang-vendor-feature=+thisNoAlignAttr" "-clang-vendor-feature=+thisNoNullAttr" "-mllvm" "-disable-aligned-alloc-awareness=1" "-D__GCC_HAVE_DWARF2_CFI_ASM=1" "-o" "/var/folders/k6/zk7vnkms1m73w0vrxjm7b2s40000gn/T/--d0fd1e.o" "-x" "c" "-"
 "/Library/Developer/CommandLineTools/usr/bin/ld" "-demangle" "-lto_library" "/Library/Developer/CommandLineTools/usr/lib/libLTO.dylib" "-no_deduplicate" "-dynamic" "-arch" "arm64" "-platform_version" "macos" "13.0.0" "13.3" "-syslibroot" "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk" "-o" "a.out" "-L/usr/local/lib" "/var/folders/k6/zk7vnkms1m73w0vrxjm7b2s40000gn/T/--d0fd1e.o" "-lSystem" "/Library/Developer/CommandLineTools/usr/lib/clang/14.0.3/lib/darwin/libclang_rt.osx.a"

and for GCC:

/opt/homebrew/opt/gcc/bin/gcc-13 -### -x c

Using built-in specs.
COLLECT_GCC=/opt/homebrew/opt/gcc/bin/gcc-13
COLLECT_LTO_WRAPPER=/opt/homebrew/Cellar/gcc/13.1.0/bin/../libexec/gcc/aarch64-apple-darwin22/13/lto-wrapper
Target: aarch64-apple-darwin22
Configured with: ../configure --prefix=/opt/homebrew/opt/gcc --libdir=/opt/homebrew/opt/gcc/lib/gcc/current --disable-nls --enable-checking=release --with-gcc-major-version-only --enable-languages=c,c++,objc,obj-c++,fortran --program-suffix=-13 --with-gmp=/opt/homebrew/opt/gmp --with-mpfr=/opt/homebrew/opt/mpfr --with-mpc=/opt/homebrew/opt/libmpc --with-isl=/opt/homebrew/opt/isl --with-zstd=/opt/homebrew/opt/zstd --with-pkgversion='Homebrew GCC 13.1.0' --with-bugurl=https://github.com/Homebrew/homebrew-core/issues --with-system-zlib --build=aarch64-apple-darwin22 --with-sysroot=/Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 13.1.0 (Homebrew GCC 13.1.0) 
COLLECT_GCC_OPTIONS='-mmacosx-version-min=13.0.0' '-asm_macosx_version_min=13.0' '-nodefaultexport' '-mlittle-endian' '-mabi=lp64' '-dumpdir' 'a-'
 /opt/homebrew/Cellar/gcc/13.1.0/bin/../libexec/gcc/aarch64-apple-darwin22/13/cc1 -quiet -iprefix /opt/homebrew/Cellar/gcc/13.1.0/bin/../lib/gcc/current/gcc/aarch64-apple-darwin22/13/ -D__DYNAMIC__ - -fPIC -quiet -dumpdir a- -dumpbase - "-mmacosx-version-min=13.0.0" -mlittle-endian "-mabi=lp64" -o /var/folders/k6/zk7vnkms1m73w0vrxjm7b2s40000gn/T//ccf1nCE4.s
COLLECT_GCC_OPTIONS='-mmacosx-version-min=13.0.0'  '-nodefaultexport' '-mlittle-endian' '-mabi=lp64' '-dumpdir' 'a-'
 as -arch arm64 "-mmacosx-version-min=13.0" -o /var/folders/k6/zk7vnkms1m73w0vrxjm7b2s40000gn/T//ccHzJxZ8.o /var/folders/k6/zk7vnkms1m73w0vrxjm7b2s40000gn/T//ccf1nCE4.s
COMPILER_PATH=/opt/homebrew/Cellar/gcc/13.1.0/bin/../libexec/gcc/aarch64-apple-darwin22/13/:/opt/homebrew/Cellar/gcc/13.1.0/bin/../libexec/gcc/
LIBRARY_PATH=/opt/homebrew/Cellar/gcc/13.1.0/bin/../lib/gcc/current/gcc/aarch64-apple-darwin22/13/:/opt/homebrew/Cellar/gcc/13.1.0/bin/../lib/gcc/current/gcc/:/opt/homebrew/Cellar/gcc/13.1.0/bin/../lib/gcc/current/gcc/aarch64-apple-darwin22/13/../../../
COLLECT_GCC_OPTIONS='-mmacosx-version-min=13.0.0'  '-nodefaultexport' '-mlittle-endian' '-mabi=lp64' '-dumpdir' 'a.'
 /opt/homebrew/Cellar/gcc/13.1.0/bin/../libexec/gcc/aarch64-apple-darwin22/13/collect2 -syslibroot /Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk/ -dynamic -arch arm64 -macosx_version_min 13.0.0 -o a.out -L/opt/homebrew/Cellar/gcc/13.1.0/bin/../lib/gcc/current/gcc/aarch64-apple-darwin22/13 -L/opt/homebrew/Cellar/gcc/13.1.0/bin/../lib/gcc/current/gcc -L/opt/homebrew/Cellar/gcc/13.1.0/bin/../lib/gcc/current/gcc/aarch64-apple-darwin22/13/../../.. /var/folders/k6/zk7vnkms1m73w0vrxjm7b2s40000gn/T//ccHzJxZ8.o -lemutls_w -lgcc -lSystem -lgcc -no_compact_unwind -rpath "@loader_path" -rpath /opt/homebrew/Cellar/gcc/13.1.0/lib/gcc/current/gcc/aarch64-apple-darwin22/13 -rpath /opt/homebrew/Cellar/gcc/13.1.0/lib/gcc/current/gcc -rpath /opt/homebrew/Cellar/gcc/13.1.0/lib/gcc/current

Notably, these have completely different formats.