Forbid backslashes on compiler command lines. It is unfortunately the only reliable way.

build.py

@@ -46,6 +46,27 @@ known_shlib_kwargs = known_basic_kwargs.copy()
 known_shlib_kwargs.update({'version' : True,
                            'soversion' : True})
 
+backslash_explanation = \
+'''Compiler arguments have a backslash "\\" character. This is unfortunately not
+permitted. The reason for this is that backslash is a shell quoting character
+that behaves differently across different systems. Because of this is it not
+possible to make it work reliably across all the platforms Meson needs to
+support.
+
+There are several different ways of working around this issue. Most of the time
+you are using this to provide a -D define to your compiler. Try instead to
+create a config.h file and put all of your definitions in it using
+configure_file().
+
+Another approach is to move the backslashes into the source and have the other
+bits in the def. So you would have an arg -DPLAIN_TEXT="foo" and then in your
+C sources something like this:
+
+const char *fulltext = "\\\\" PLAIN_TEXT;
+
+We are fully aware that these are not really usable or pleasant ways to do
+this but it's the best we can do given the way shell quoting works.
+'''
 
 class InvalidArguments(coredata.MesonException):
     pass
@@ -513,6 +534,8 @@ class BuildTarget():
         for a in args:
             if not isinstance(a, (str, File)):
                 raise InvalidArguments('A non-string passed to compiler args.')
+            if isinstance(a, str) and '\\' in a:
+                raise InvalidArguments(backslash_explanation)
         if language in self.extra_args:
             self.extra_args[language] += args
         else:

test cases/common/104 stringdef/meson.build

@@ -0,0 +1,3 @@
+project('stringdef', 'c')
+
+test('stringdef', executable('stringdef', 'stringdef.c', c_args : '-DFOO="bar"'))

test cases/common/104 stringdef/stringdef.c

@@ -0,0 +1,10 @@
+#include<stdio.h>
+#include<string.h>
+
+int main(int argc, char **argv) {
+    if(strcmp(FOO, "bar")) {
+        printf("FOO is misquoted: %s\n", FOO);
+        return 1;
+    }
+    return 0;
+}