highlight for keyword contextually

Author: rhysd

syntax/rust.vim

@@ -15,7 +15,12 @@ endif
 " Syntax definitions {{{1
 " Basic keywords {{{2
 syn keyword   rustConditional match if else
-syn keyword   rustRepeat for loop while
+syn keyword   rustRepeat loop while
+" `:syn match` must be used to prioritize highlighting `for` keyword.
+syn match     rustRepeat /\<for\>/
+" Highlight `for` keyword in `impl ... for ... {}` statement. This line must
+" be put after previous `syn match` line to overwrite it.
+syn match     rustKeyword /\%(\<impl\>.\+\)\@<=\<for\>/
 syn keyword   rustTypedef type nextgroup=rustIdentifier skipwhite skipempty
 syn keyword   rustStructure struct enum nextgroup=rustIdentifier skipwhite skipempty
 syn keyword   rustUnion union nextgroup=rustIdentifier skipwhite skipempty contained