Avoid mutation of snapshot's config -- that's spooky action at a
distance. Instead, copy it over to a local variable.
This points out a minor architecture problem, which we won't fix right
away.
Various `ide`-level config structs, like `AssistConfig`, are geared
towards one-shot use when calling a specific methods. On the other
hand, the large `Config` struct in `rust-analyzer` is a long-term
config store.
The fact that `Config` stores `AssistConfig` is accidental -- a better
design would probably be to just store `ConfigData` inside `Config`
and create various `Config`s on the fly out of it.
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