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Jonathan Brouwer
85cc4b247b
`rustc_queries` simplifications
Queries have two ways of specifying code snippets, in `desc` and `cache_on_disk_if` blocks. An example:
```rust
query check_liveness(key: LocalDefId) -> &'tcx rustc_index::bit_set::DenseBitSet<abi::FieldIdx> {
arena_cache
desc { |tcx| "checking liveness of variables in `{}`", tcx.def_path_str(key.to_def_id()) }
cache_on_disk_if(tcx) { tcx.is_typeck_child(key.to_def_id()) }
}
```
If you need to use `tcx` in the snippet, you can use an explicit binding. But there are multiple problems with this.
- The syntax used is different in the two snippets: `|tcx|` within the block vs. `(tcx)` outside the block. (!!)
- Bug 1: In `desc` snippets you can leave out the `|tcx|` and still use `tcx`. Several existing queries do this.
- Bug 2: In `desc` snippets you can always use `key` in the snippet to refer to the key, even if that's not the identifier used in the query head.
- Finally, you can bind `tcx` and not use it, without a warning. Several existing queries do this.
I think explicit `tcx` binding is silly. Many queries need `tcx` and this macro is already super-magical, so just making `tcx` implicitly available seems fine, rather than making the query writer jump through a little syntactic hoop. This makes both the query definitions and the proc macro simpler.
r? @petrochenkov
For more information about how rustc works, see the rustc dev guide.