1238: Macro queries r=edwin0cheng a=matklad
In https://github.com/rust-analyzer/rust-analyzer/pull/1231, I've added aggressive clean up of `ast_id_to_node` query.
The result of this query is a `SyntaxTree`, and we don't want to retain syntax trees in memory unless absolutely necessary.
Moreover, `SyntaxTree` has identity equality semantics, meaning that we'll get a diffferent syntax tree for a file after every reparse. That means that `ast_id_to_node` query should not genereally be used in HIR, unless it is behind some kind of salsa firewall, like the `raw` module of name resoulution.
However, that PR resulted in the abysmal performance: turns out we were using ast_id_to_node quite heavily in hir when expanding macros!
So this PR installs the more incremental-friendly query structure:
* converting source to token tree is now a query; changing source without affecting token-trees will now preserve macro expansions
* expand macro (tt -> tt) is now a query as well, so we cache macro expansions *before* parsing them into item lists or expressions, which is nice: we can cache expansion without knowing the calling context!
r? @edwin0cheng
Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>
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