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On non-Windows platforms, --sysroot can be used to make the compiler use
a single, hermetic directory for all header and library files.
This is useful, but difficult to do on Windows. After D95472 it's
possible to achieve this with two flags:
out/gn/bin/clang-cl win.c -fuse-ld=lld \
/vctoolsdir path/to/VC/Tools/MSVC/14.26.28801 \
/winsdkdir path/to/win_sdk
But that's still cumbersome: It requires two flags instead of one, and
it requires writing down the (changing) VC/Tools/MSVC version.
This adds a new `/winsysroot <dir>` flag that's effectively an alias to
these two flags. With this, building against a hermetic Windows
toolchain only needs:
out/gn/bin/clang-cl win.c -fuse-ld=lld /winsysroot path
`/winsysroot <dir>` is the same as adding
/vctoolsdir <dir>/VC/Tools/MSVC/<vctoolsver>
/winsdkdir <dir>/Windows Kits/<winsdkmajorversion>
`<vctoolsver>` is taken from `/vctoolsversion` if passed, or else it's
the name of the directory in `<dir>/VC/Tools/MSVC` that's the highest
numeric tuple.
`<winsdkmajorversion>` is the major version in /winsdkversion if passed,
else it's the name of the directory in `<dir>/Windows Kits` that's the
highest number.
So `/winsysroot <path>` requires this subfolder structure:
path/
VC/
Tools/
MSVC/
14.26.28801 (or another number)
include/
...
Windows Kits/
10/
Include/
10.0.19041.0/ (or another number)
um/
...
Lib/
10.0.19041.0/ (or another number)
um/
x64/
...
...
Differential Revision: https://reviews.llvm.org/D95534
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