Cache derive proc macro expansion with incremental query
This is a revival of https://github.com/rust-lang/rust/pull/129102, originally implemented by @futile. Since it looks like they are not active currently, I'd like to push this work forward.
The first commit is squashed and rebased work from the original PR, with author attribution to futile. The rest of the commits are some additional comments that I created mostly for myself to understand what happens here. I also did some cleanups based on Vadim's review comments on the original PR, plus I refactored the TLS access a bit using `scoped_tls`.
The biggest issue, as usually, are tests... I tried using `#[rustc_clean(..., loaded_from_disk = "derive_macro_expansion")]`, but the problem is that since this query cannot recover the original key from its hash, and thus its fingerprintstyle is `FingerprintStyle::Opaque`, [this](2fef0a30ae/compiler/rustc_incremental/src/persist/dirty_clean.rs (L388)) crashes when I try to use `loaded_from_disk`. Any suggestions from someone who actually understands the query system would be welcome 😅
TODO: document the new unstable flag
On a no-op change re-check of `octocrab 0.49` (which has a ton of `serde` derive proc macro invocations), this saves ~0.6s out of ~6s (so a ~10% win) on my PC.
r? @petrochenkov
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