Preserve 'C:\' as an absolute path in prefix.

mesonbuild/coredata.py

@@ -60,11 +60,9 @@ class UserStringOption(UserOption):
 
     def validate(self, value):
         if not isinstance(value, str):
-            raise MesonException('Value "%s" for string option "%s" is not a string.' % (str(newvalue), self.name))
-        if self.name == 'prefix':
-            if not os.path.isabs(value):
-                if len(value) >= 2 and value[1] != ':':
-                    raise MesonException('Prefix option value \'{0}\' must be an absolute path.'.format(value))
+            raise MesonException('Value "%s" for string option "%s" is not a string.' % (str(value), self.name))
+        if self.name == 'prefix' and not os.path.isabs(value):
+            raise MesonException('Prefix option value \'{0}\' must be an absolute path.'.format(value))
         if self.name in ('libdir', 'bindir', 'includedir', 'datadir', 'mandir', 'localedir') \
             and os.path.isabs(value):
             raise MesonException('Option %s must not be an absolute path.' % self.name)

mesonbuild/mesonmain.py

@@ -84,7 +84,12 @@ class MesonApp():
         if not os.path.isabs(options.prefix):
             raise RuntimeError('--prefix value \'{0}\' must be an absolute path: '.format(options.prefix))
         if options.prefix.endswith('/') or options.prefix.endswith('\\'):
-            options.prefix = options.prefix[:-1]
+            # On Windows we need to preserve the trailing slash if the
+            # string is of type 'C:\' because 'C:' is not an absolute path.
+            if len(options.prefix) == 3 and options.prefix[1] == ':':
+                pass
+            else:
+                options.prefix = options.prefix[:-1]
         self.meson_script_file = script_file
         self.options = options