config: move message about leading spaces in config to config.py

When the config parser raises a ParseError, it uses the line that
failed to parse as the error message. It doesn't currently tell the
user anything about why it failed to parse. b13b99d39a46 (config:
highlight parse error caused by leading spaces (issue3214),
2014-03-16) added a checked based on the error *message* having
leading spaces. That has worked fine because only the config parser
uses the line itself as error message (I think the revset and fileset
parsers use more user-friendly proper messages). It still feels like a
hack. Let's make the config parser give a useful message about leading
whitespace instead. We should ideally follow up with more useful
messages for other parse errors in config files.

Differential Revision: https://phab.mercurial-scm.org/D9241

mercurial/config.py

@@ -200,7 +200,10 b' class config(object):'
                 self._unset.append((section, name))
                 continue
 
-            raise error.ParseError(l.rstrip(), (b"%s:%d" % (src, line)))
+            message = l.rstrip()
+            if l.startswith(b' '):
+                message = b"unexpected leading whitespace: %s" % message
+            raise error.ParseError(message, (b"%s:%d" % (src, line)))
 
     def read(self, path, fp=None, sections=None, remap=None):
         if not fp:

mercurial/dispatch.py

@@ -248,8 +248,6 b' def _formatparse(write, inst):'
             _(b"hg: parse error at %s: %s\n")
             % (pycompat.bytestr(inst.location), inst.message)
         )
-        if inst.message.startswith(b' '):
-            write(_(b"unexpected leading whitespace\n"))
     else:
         write(_(b"hg: parse error: %s\n") % inst.message)
         _reportsimilar(write, similar)

tests/test-config.t

@@ -25,8 +25,7 b' Test hint about invalid syntax from lead'
   >  key=value
   > EOF
   $ hg showconfig
-  hg: parse error at $TESTTMP/.hg/hgrc:1:  key=value
-  unexpected leading whitespace
+  hg: parse error at $TESTTMP/.hg/hgrc:1: unexpected leading whitespace:  key=value
   [255]
 
   $ cat > .hg/hgrc << EOF
@@ -34,8 +33,7 b' Test hint about invalid syntax from lead'
   > key=value
   > EOF
   $ hg showconfig
-  hg: parse error at $TESTTMP/.hg/hgrc:1:  [section]
-  unexpected leading whitespace
+  hg: parse error at $TESTTMP/.hg/hgrc:1: unexpected leading whitespace:  [section]
   [255]
 
 Reset hgrc

tests/test-hgrc.t

@@ -68,8 +68,7 b' issue1829: wrong indentation'
   $ echo '[foo]' > $HGRC
   $ echo '  x = y' >> $HGRC
   $ hg version
-  hg: parse error at $TESTTMP/hgrc:2:   x = y
-  unexpected leading whitespace
+  hg: parse error at $TESTTMP/hgrc:2: unexpected leading whitespace:   x = y
   [255]
 
   $ "$PYTHON" -c "from __future__ import print_function; print('[foo]\nbar = a\n b\n c \n  de\n fg \nbaz = bif cb \n')" \