TikZ 3 space between options and named angle in a label no more allowed

Question

Release comparison

In version 2.10 of TikZ you could write:

\coordinate [label={[red] center:$P$}] (x) at (0,0);

but in the latest release (version 3.0.0) you'll get an error (unknown function 'center') unless you write it without any space between the [options] and the <angle>:

\coordinate [label={[red]center:$P$}] (x) at (0,0);

On the contrary, you can still write it as before if the <angle> is a number:

\coordinate [label={[red] 30:$P$}] (x) at (0,0);

Question

Is this a bug or a desired behaviour?

MWE

\documentclass[tikz]{standalone}

\begin{document}
\begin{tikzpicture}
%\coordinate [label={[red] center:$P$}] (a) at (0,0); % this doesn't work
\coordinate [label={[red]center:$P$}]  (b) at (1,0);
\coordinate [label={[red] 30:$P$}]     (c) at (2,0);
\end{tikzpicture}
\end{document}

Answer 1

Amazingly finding the problem was much easier than using the \ignorespaces, thanks to David Carlisle on chat, realizing my stupid mistake that \zap@space should be utilized. More fun is here if you haven't yet I'm a cargo cult programmer ...

Anyway, the reason for the error is that the code is, with all respect, looking like unfinished there. The space gobbling is not performed so when you send a whitespace in front of center it is compared with a macro defined as

\def\tikz@on@text{center}

So when you \ifx them they don't match and TikZ thinks you sent an angle or an anchor. You know the rest. As a working example I tried to gobble the initial spaces for testing and David Carlisle fixed my gobble mechanism. The macro opening is the changed part that was originally located on line 701 of tikz.code.tex file.

\documentclass[]{article}
\usepackage{tikz}
\makeatletter

\tikzset{tikz@label@post/.code={% The code has 2 args here WHY? Line 701
  \edef\tikz@label@angle{\expandafter\expandafter\expandafter%
                         \zap@space%
                         \expandafter\tikz@label@angle\space \@empty}%
  \csname tikz@label@angle@is@\tikz@label@angle\endcsname%
  \ifx\tikz@label@angle\tikz@on@text%
    \def\tikz@node@at{\pgfpointanchor{\tikzlastnode}{center}}%
    \def\tikz@anchor{center}%
  \else%
    \iftikz@absolute%
      \pgftransformreset%
      \pgf@process{%
        \pgfpointshapeborder{\tikzlastnode}%
        {\pgfpointadd{\pgfpointanchor{\tikzlastnode}{center}}{\pgfpointpolar{\tikz@label@angle}{1pt}}}}%
      \edef\tikz@node@at{\noexpand\pgfqpoint{\the\pgf@x}{\the\pgf@y}}%
      \tikz@compute@direction{\tikz@label@angle}%
      \tikz@addtransform{\pgftransformshift{\pgfpointpolar{\tikz@label@angle}{#1}}}%  
    \else%
      \pgf@process{\pgfpointanchor{\tikzlastnode}{\tikz@label@angle}}%
      \edef\tikz@node@at{\noexpand\pgfqpoint{\the\pgf@x}{\the\pgf@y}}%
      \pgf@xb=\pgf@x%
      \pgf@yb=\pgf@y%
      \pgf@process{\pgfpointanchor{\tikzlastnode}{center}}%
      \pgf@xc=\pgf@x%
      \pgf@yc=\pgf@y%
      \tikz@label@simplefalse%
      \iftikz@fullytransformed%
        \tikz@label@simpletrue%
      \else
        \ifdim\pgf@xc=\pgf@xb\relax%
          \ifdim\pgf@yc=\pgf@yb\relax%
            \tikz@label@simpletrue%
          \fi%
        \fi%
      \fi%
      \iftikz@label@simple%  
        \tikz@compute@direction{\tikz@label@angle}%
        \tikz@addtransform{\pgftransformshift{\pgfpointpolar{\tikz@label@angle}{#1}}}%  
      \else%
        \pgf@process{\pgfpointnormalised{%
            \pgfpointdiff{\pgfpointtransformed{\pgfqpoint{\pgf@xc}{\pgf@yc}}}{\pgfpointtransformed{\pgfqpoint{\pgf@xb}{\pgf@yb}}}}}%
        \edef\pgf@marshal{%
          \noexpand\tikz@addtransform{\noexpand\pgftransformshift{\noexpand\pgfpointscale{#1}{
                \noexpand\pgfqpoint{\the\pgf@x}{\the\pgf@y}}}}}%
        \pgf@marshal%
        \pgf@xc=\pgf@x%
        \pgf@yc=\pgf@y%
        \pgf@x=\pgf@yc%
        \pgf@y=-\pgf@xc%
        \tikz@auto@anchor%
      \fi%
    \fi%
  \fi}
}
\makeatother

\begin{document}
\begin{tikzpicture}
\coordinate [label={[red]     center:$P$}] (a) at (0,0); % this works
\coordinate [label={[red]center:$P$}]  (b) at (1,0);
\coordinate [label={[red] 30:$P$}]     (c) at (2,0);
\end{tikzpicture}
\end{document}