emit build-database: shell-escaped quotes inside the arguments array break consumers #655

Summary

mcpp emit build-database --format json puts shell-escaped quotes inside the arguments
array. A JSON arguments array is passed to exec without a shell, so its elements must be
unescaped; the backslashes reach the compiler as literal characters.

Consumers that honour arguments (clangd's module scanner, any tool driving the database) then fail
to parse every translation unit that uses a quoted macro define.

Reproduce

In a project whose build defines a macro to a quoted string — openxlings/xlings does, through its
vendored libarchive:

$ mcpp emit build-database --format json > db.json
$ python3 -c "
import json
d = json.load(open('db.json'))
def walk(o):
    if isinstance(o, dict):
        for v in o.values(): yield from walk(v)
    elif isinstance(o, list):
        for v in o: yield from walk(v)
    elif isinstance(o, str): yield o
for s in walk(d):
    if 'PLATFORM_CONFIG_H' in s: print(repr(s)); break
"
'-DPLATFORM_CONFIG_H=\\"mcpp_libarchive_config.h\\"'

The field is arguments, and its type is a list:

字段名: arguments | 类型: list
  元素: '-DPLATFORM_CONFIG_H=\\"mcpp_libarchive_config.h\\"'

So the actual element is:

-DPLATFORM_CONFIG_H=\"mcpp_libarchive_config.h\"

Expected

-DPLATFORM_CONFIG_H="mcpp_libarchive_config.h"

The quotes belong to the macro's value (the source does #include PLATFORM_CONFIG_H, which needs
"..." or <...>). Escaping is a shell concern, and arguments does not go through a shell.

What it breaks

With the backslashes literal, the macro expands to \"mcpp_libarchive_config.h\" and every TU
including it fails:

archive_platform.h:44:10: error: expected "FILENAME" or <FILENAME>
<built-in>:418:27: note: expanded from macro 'PLATFORM_CONFIG_H'
archive_platform.h:164:2: error: No suitable 32-bit unsigned integer type found for this platform
archive_platform.h:173:2: error: No suitable 32-bit signed integer type found for this platform

In openxlings/xlings this is every C file of libarchive, bzip2, lz4, xz, zlib and zstd. For a
language server the consequences are that those files never get an index (no navigation into them),
and the scanner retries and logs the failure repeatedly.

Environment

• mcpp 2026.9.16.1
• Project: openxlings/xlings (linux x86_64, gcc 16.1.0 profile)
• Consumer: mcpp-language-server, which reads arguments as exec-ready per the S1 build database
  contract

Note

If arguments is intended to carry shell-quoted text — i.e. consumers are expected to shell-split
and unescape it — then this is a documentation issue instead, and I would like to know, because the
distinction decides whether consumers should unescape defensively. command (a string) and
arguments (a list) usually differ exactly here, which is why we read it as exec-ready.