enable source installs for build dependencies

docs/reference/pip.rst

@@ -154,8 +154,9 @@ appropriately.
   installation of build dependencies from source has been disabled until a safe
   resolution of this issue is found.
 
-* ``pip<18.0`` does not support the use of environment markers and extras, only
-  version specifiers are respected.
+* ``pip<18.0``: only support installing build requirements from wheels, and
+  does not support the use of environment markers and extras (only version
+  specifiers are respected).
 
 
 Future Developments

news/5229.feature

@@ -0,0 +1 @@
+Add support for installing PEP 518 build dependencies from source.

src/pip/_internal/build_env.py

@@ -80,10 +80,13 @@ class BuildEnvironment(object):
         args = [
             sys.executable, '-m', 'pip', 'install', '--ignore-installed',
             '--no-user', '--prefix', self.path, '--no-warn-script-location',
-            '--only-binary', ':all:',
         ]
         if logger.getEffectiveLevel() <= logging.DEBUG:
             args.append('-v')
+        for format_control in ('no_binary', 'only_binary'):
+            formats = getattr(finder.format_control, format_control)
+            args.extend(('--' + format_control.replace('_', '-'),
+                         ','.join(sorted(formats or {':none:'}))))
         if finder.index_urls:
             args.extend(['-i', finder.index_urls[0]])
             for extra_index in finder.index_urls[1:]: