Document token spacing

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先从 lexer 章节和现有的 macro.proc.token.conversion.from-proc_macro 小节开始，然后检查 rustc_parse/src/parser/mod.rs 中 break_and_eat 附近的内容。将语法摘要中对连接 token 的处理与参考模式和借用运算符进行比较，并查看关联的 issue 和 pull request。当 Reference 明确定义 token 的间距、拆分或粘合方式，以及 lexer/parser 的边界时，即可视为完成。

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A-grammar A-lexer

I think the behavior around token splitting should be documented in some way, though I'm uncertain exactly how. The general idea is that the lexer chapter defines tokens, such as <-, which are then split in certain situations to smaller tokens. AFAIK, the reference doesn't really define how this works. I also don't know how much of this is part of the lexer versus the parser. I suspect it needs to be addressed in both depending on which approach we take.

A question I have is whether it should be documented as tokens being split, or tokens being glued, since I could imagine either approach is viable.

I think in rustc, currently the primary method that is used to handle this is break_and_eat. However, I may be misremembering if there are other places where this is handled.

IIUC, the places where this splitting happens is somewhat arbitrary, in terms of places where people thought it would be helpful. There's nothing that prevents the parser from avoiding those splitting methods.

There is a brief mention in macro.proc.token.conversion.from-proc_macro how proc-macro output is glued/split. The proc-macro Spacing type briefly touches on it as well.

Note that reference patterns and borrow operators specifically include the && token in their grammar. I don't remember the exact details about why we did it that way (that is, why did we do this just for && and no other token?). Depending on how we approach this, we may need to consider how that might need to change.

Note that I included some of the jointed tokens in the syntax summary. We may want to remove some. For example, I have >>= and >= linking to generics.

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