[jcranmer]

Contextual keywords are a growing thing in modern languages. Modern compilers don't tend to have strictly separated lexers and parsers, but instead use a combined lexer/parser model that feeds information back and forth between them. If your language is designed such that you aren't often in multiple potential parse states, then it's easy to feed into the lexer "get me the next token, and by the way, expect function attributes to be keywords right now."

Note that the requirement to not be in multiple potential parse states also tends to boil down to "build a language that's usually LL(1) or LALR(1)."