unescaped macros

You can decide to make - "active" only within math mode, and look ahead for a >.

\documentclass{article}
\mathchardef\minuschar=\mathcode`-\relax
\mathcode`-="8000\relax
\begingroup\makeatletter\lccode`~=`-\lowercase{\endgroup
  \def~{\@ifnextchar{>}{\rightarrow\@gobble}{\minuschar}}}
\begin{document}
We can try $-\int$, $-\sum$, $x->0$, $x- >0$ etc.
\end{document}

When TeX encounters a character with catcode 11 or 12 (letter or other) in math mode, it looks up the character's mathcode. That's a number between "0000 and "8000 (hexadecimal). The value "8000 is treated specially to mean "use the corresponding active character instead". I then defined - to look ahead for a >, and either put a \rightarrow and \@gobble the >, or put the minus character.

The easiest solution, as was observed in one of the comments is to directly type →. Both ConTeXt MkIV and LaTeX + unicode-math give the right output when using the correct unicode input.

If you are using LuaTeX, it is also possible to parse the input and convert -> to →. Here is an example in ConTeXt that uses the translate module

\usemodule[translate]

\translateinput[->][→]
\translateinput[=>][⇒]

\enableinputtranslation

\starttext
$x - y = 0 => x = y$
\stoptext    

Just for the record,

\usepackage{newunicodechar}
\newunicodechar{→}{\to}

allows for

$x→0$

with pdflatex (the file must be coded in UTF-8 and \usepackage[utf8]{inputenc} must be used, not utf8x). It also works with xelatex or lualatex (without unicode-math, of course, otherwise it would be nonsense).