<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/79849>79849</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            `-frewrite-includes` produces malformed output when header ends with a backslash
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          AntonShalgachev
      </td>
    </tr>
</table>

<pre>
    ## Steps to reproduce
1. Create `header.h` (with the new line at the end):
```cpp
#define MY_MACRO_FUNC(X) (X) \

```
2. Create `main.cpp`:
```cpp
#include "header.h"
```
3. Compile:
```sh
clang++.exe -E -frewrite-includes main.cpp | clang++.exe -c -x c++ -
```

Clang 17.0.6 would output this error:
```
In file included from main.cpp:1:
./header.h:2:3: error: '#' is not followed by a macro parameter
    2 | # 2 "main.cpp" 2
      |   ^
1 error generated.
```

Latest clang (from git) would output this error:
```
In file included from main.cpp:1:
.\header.h:2:2: error: '#' is not followed by a macro parameter
    2 | #endif /* header.h expanded by -frewrite-includes */
      |  ^
main.cpp:1:2: error: unterminated conditional directive
    1 | #if defined(__CLANG_REWRITTEN_INCLUDES) /* header.h expanded by -frewrite-includes */
      |  ^
2 errors generated.
```

## Environment:
```
PS > clang++.exe --version
clang version 17.0.6
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Program Files\LLVM\bin
```
and
```
PS > T:\llvm-project\build\Debug\bin\clang++.exe --version
clang version 19.0.0git (https://github.com/llvm/llvm-project e3a38a75ddc6ff00301ec19a0e2488d00f2cc297)
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: T:\llvm-project\build\Debug\bin
```

## Additional information
The error seems to be caused by the missing empty line after the macro definition in the `-frewrite-includes` output:
```cpp
# 1 "<built-in>"
# 1 "main.cpp"
#if defined(__CLANG_REWRITTEN_INCLUDES) /* header.h expanded by -frewrite-includes */
#include "header.h"
#else /* header.h expanded by -frewrite-includes */
# 1 "main.cpp"
# 1 ".\\header.h" 1
#define MY_MACRO_FUNC(X) (X) \
#endif /* header.h expanded by -frewrite-includes */
# 2 "main.cpp" 2

```

I'm not that well familiar with the formal C++ standard, but from what I understand the code in `header.h` is valid (correct me if I'm wrong). Also the issue disappears if I remove the `-frewrite-includes` flag

## Fix
To get myself familiarzed with the clang codebase I've also prepared a fix that prevents an empty line from being swallowed by `OutputContentUpTo`:
[clang-rewrite-includes-fix.patch](https://github.com/llvm/llvm-project/files/14086035/clang-rewrite-includes-fix.patch)

If the described behavior is indeed unexpected, I can open a review with the fix (and additional unit tests of course)
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJy8V1tv2zoS_jX0y8ACTVmy_eAHR7EXAdK0aNK9PAW0OLK4S5ECSdnO_voDUrKdJmnantMewLAgXuby6ZtvSO6c3GnEJcmuSHY94p2vjV2utDf6vuZqx8sa96OtEU9LwlLCUrj32DrwBiy21oiuREKvCV1NEigsco9AclojF2iTmuQUCJsfpK_B1wgaD6CkRuA-vqMWhC1IuuptkJz2v7JthxGWCqzCjg__efywKj5_fNx8uSsIm_-bsAWcn1kxrP_aTv_KnofWcKmTYD-n7_uVulSdQCCMndNh7E0HaQKFaVqp8LVJV_cDpeJ6R9gVYVcJHhHGaxhXFg9WehwPvhycogMyK-DVlhLGRyj7ERi_GUr_X4SdMJklNMnhYDolwHS-7QLq0gFaa-zrUPvXGw2VVAhDTAIqa5pzYCRdTc47E8I2Z2zSFSPpKiXp6mwfCJtF2sxAOtDGQ2WUMgcUsH0CDg0vrYGWW96gR9sbBQBgMf9ANxbwPztnDNhlFcRVACRbDxzsPcMONVruUSTvYHTLPTrfgxyIFNPcSR_49Dshy4oXkLFfCxlqISsgbEPYCk6uAI8t16K38gbvCFuFHS-hPSP7IpevQ-60R9tIHRCH0mghvTSaKxDSYunlHi-GJ6cwZQV9ZQvC5o-Pxe3q7h-Pn9f_-nzz8LC-e7y5K26_XK_v-yr_pbmwPnL3YzQZZG-t99Ia3aD232LBp3sg6fp11Y73aJ00-pkOwDA0lGg_88DtDoN1OM7zx3w6bsvxQWphDm7cuH05rKotcgGNEajC2tY4eTzR0HmuFIprGb9LESLNik_W7CxvYCMVOpIVt7f__ECyYiv1m2lwLd5L76E3qtS-GbfW_BdLH4x1UgmSFde47XaD8az4OSgWCU3oTvpQjLX3rQuewqff7KSvu21SmoawTfA8PE4BAKY8nfNZJkSZVxWlKZ1gOVlwimw6nwtKK1aWbDELzeb3YP3jsHyXaitxLiCpK2Mb7s-QPYSmGTXOITaxD28RSt65vhxCU22kc1LvAJvWPw3ttvJo-8moH7HyohOQOo6TnL4updC-exF8v1WGqmaMpDFfP5aapOtLrzzNP1PxS5P9W1Tge92cpagc_kUP306ynwnK_1z8GYPJnzzk_AKRf6-3vkPRG8JmTWxLvuYeDqgUVLyRSnIL52NeJK2CYjipOM-14FYQVsC28313PIT9N9BpgTYuiDtLI0IbfXmGlA72XEkRsCiNDV0FGgRZQR_QwZogNIsEVsqZaEk61yEI6XjbIrcuLgaLjdnj-4yvFN-9UZabU-k_GNihh-bJoarO2f8fxQWAXtlCMlvuMAa5R-AhttZiyy0K4FDJYw9ja3GP2jvg-nnVRpy2GGrZHfjlFEBy-jFWZWG0R-2_tA_mq-NsdhUDGL_MblzJY9JyX9Yku_5plSVsU8UWwjaTKZ3nNM0I23zX00lyBwJVESCBrrRyG_LBmu-lseEbSy0QBXQajy2WHiNjbqDkGkyLGjhY3Es8PGOaPAZOBPrwi2x2WnoIpzsHpoLSdNYhYYuRWKZikS74CJeTGc2zaUrz6ahezvmUi3wrsKLTapHPtrRk05TSxTwXM5yIkVwyyqZ0whaTbDplacJyijnOKy4Ym1c0J1OKDZcqCYAlxu5GkX_L2WI-XYwU36Jy8ZLFWLgDxckgEdn1yC4jyNtu58iUKum8u1jx0itcfpOqww0s3BtUKDo8H1oPNepBGMIty_WQcdjy8n9OcVePOquWP02AGHhgQEzsjwAAAP__sP5KjA">