[clang-tools-extra] [clangd] Inject preceding includes for unity build targets (PR #180402)

[via cfe-commits]

likewhatevs wrote:

> Is what the kernel does in any way different from what build systems like cmake do for unity builds? 

I don't think so but let me get an example to make sure I'm not missing anything subtle.

This is what `compile_commands.json` looks like for one of the unity build files:
```
  {
    "command": "gcc-14 -Wp,-MMD,kernel/sched/.build_policy.o.d -nostdinc -I./arch/x86/include -I./arch/x86/include/generated -I./include -I./include -I./arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/compiler-version.h -include ./include/linux/kconfig.h -include ./include/linux/compiler_types.h -D__KERNEL__ -Werror -std=gnu11 -fshort-wchar -funsigned-char -fno-common -fno-PIE -fno-strict-aliasing -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -mno-sse4a -fcf-protection=branch -fno-jump-tables -m64 -falign-jumps=1 -falign-loops=1 -mno-80387 -mno-fp-ret-in-387 -mpreferred-stack-boundary=3 -mskip-rax-setup -march=x86-64 -mtune=generic -mno-red-zone -mcmodel=kernel -mstack-protector-guard-reg=gs -mstack-protector-guard-symbol=__ref_stack_chk_guard -Wno-sign-compare -fno-asynchronous-unwind-tables -mindirect-branch=thunk-extern -mindirect-branch-register -mindirect-branch-cs-prefix -mfunction-return=thunk-extern -fno-jump-tables -fpatchable-function-entry=16,16 -fno-delete-null-pointer-checks -O2 -fno-allow-store-data-races -fstack-protector-strong -ftrivial-auto-var-init=zero -fno-stack-clash-protection -pg -mrecord-mcount -mfentry -DCC_USING_FENTRY -fmin-function-alignment=16 -fstrict-flex-arrays=3 -fms-extensions -fno-strict-overflow -fno-stack-check -fconserve-stack -fno-builtin-wcslen -Wall -Wextra -Wundef -Werror=implicit-function-declaration -Werror=implicit-int -Werror=return-type -Werror=strict-prototypes -Wno-format-security -Wno-trigraphs -Wno-frame-address -Wno-address-of-packed-member -Wmissing-declarations -Wmissing-prototypes -Wframe-larger-than=2048 -Wno-main -Wno-dangling-pointer -Wvla-larger-than=1 -Wno-pointer-sign -Wcast-function-type -Wno-array-bounds -Wno-stringop-overflow -Wno-alloc-size-larger-than -Wimplicit-fallthrough=5 -Werror=date-time -Werror=incompatible-pointer-types -Werror=designated-init -Wenum-conversion -Wunused -Wno-unused-but-set-variable -Wno-unused-const-variable -Wno-packed-not-aligned -Wno-format-overflow -Wno-format-truncation -Wno-stringop-truncation -Wno-override-init -Wno-missing-field-initializers -Wno-type-limits -Wno-shift-negative-value -Wno-maybe-uninitialized -Wno-sign-compare -Wno-unused-parameter -g -Wno-unused-but-set-variable    -DKBUILD_MODFILE='\"kernel/sched/build_policy\"' -DKBUILD_BASENAME='\"build_policy\"' -DKBUILD_MODNAME='\"build_policy\"' -D__KBUILD_MODNAME=build_policy -c -o kernel/sched/build_policy.o kernel/sched/build_policy.c",
    "directory": "~/linux",
    "file": "~/linux/kernel/sched/build_policy.c"
  },
```

This is that file: https://elixir.bootlin.com/linux/v6.18/source/kernel/sched/build_policy.c#L50-L66 , the c files it includes (like `ext.c`) don't have their own entries in `compile_commands.json`.

> If so, then I believe the scope this affects is way larger as unity builds are available in multiple build systems. iiuc what this does is propagate the includes and defines from source1 etc. into the following files?

Yes

https://github.com/llvm/llvm-project/pull/180402