New database design

[nikomatsakis]

Prototype new database design. See the final commit for a write-up. This is ready to land but I was planning to do a bit more work, in particular removing the use of thread-local state.

Update: I've removed the thread-local state now.

KEY CHANGES

• No more Handle -- just clone the database; when you try to make mutation, it will cancel other clones
• No more thread-local state
• You can use salsa::DatabaseImpl as a default database if you don't need customizations
• The use of salsa::db is unchanged, you can still define your own database traits and types

[netlify[bot]]

✅ Deploy Preview for salsa-rs canceled.

Name	Link
🔨 Latest commit	1bce41f5d68903ca03c7e1222977c5c8f7df24d1
🔍 Latest deploy log	https://app.netlify.com/sites/salsa-rs/deploys/66af3a088db6e50008713708

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #538 will not alter performance

_{Comparing nikomatsakis:spindle2 (1bce41f) with master (7bdf51c)}

Summary

✅ 1 untouched benchmarks

[nikomatsakis]

@MichaReiser I've been AFK for a bit...hopefully will be getting more reliable internet access soon. I've been thinking it over and I think I can produce a variation of this branch where we have the old setup (i.e., you define a struct that contains the Storage), which should address the various concerns you've raised.

[MichaReiser]

No worries. I hope you're having a great time. I don't have a strong preference over either approach. I think both is fine. The only real blocking item right now is that I don't know how to implement Upcast.

[nikomatsakis]

@MichaReiser I retooled it now, take a look. I think you convinced me that the old approach was better. We now have:

• For simple uses, the built-in Database trait and DatabaseImpl struct.
• For more complex uses, you can define your own database trait and structs using #[salsa::db]; the struct must have storage: salsa::Storage<Self> as one of its fields.

[nikomatsakis]

I went ahead and pushed some other commits I had that make things miri-safe again (at least I think they do, let's see what CI has to say about it).