Store offline data in one file per query / remove sqlx-data.json

[jplatte]

Query data is now stored in .sqlx/{query_hash}.json directly by the macro invocations, rather than first writing to target/sqlx/{input_span_hash}.json and then collecting those into sqlx-data.json separately.

• save offline data (if .sqlx exists)
• load offline data
• create .sqlx when running cargo sqlx prepare
• diff target/sqlx with .sqlx in cargo sqlx prepare --check
• update documentation

Closes #570.

[jplatte]

@mehcode @abonader it would be nice if one of you could do an initial review now, to find any issues with the general approach I took early. There are TODO comments for everything that's not done yet.

The only thing I'm not sure about is how support for workspaces with multiple crates using SQLx'es offline feature should work. I think it makes sense for there not to be one merged .sqlx directory as there was one merged sqlx-data.json before. We could probably detect whether there are multiple crates using sqlx with features = ["offline"] in the workspace and trigger the more complicated clean (maybe just of those crates) + check with RUSTFLAGS logic based on that. By the way, why does the cargo rustc code in there pass --emit dep-info,metadata but the cargo check code doesn't?

[mehcode]

@jplatte What's left here? Is it just the bikeshedding over where we output the query data files?

[jplatte]

No, I still need to get back to this and finish the implementation. I think all open questions were resolved. See the PR description for what's left to do.

[abonander]

I think you'd still need a separate program to go through and remove stale query data to keep the folder size from growing indefinitely. Perhaps we could do something with build scripts where we put all the boilerplate in a function that the user can call in a single line, e.g.

fn main() {
    sqlx::build_script().unwrap();
}

which would:

• emit cargo:rerun-if-changed:... for the files in .sqlx/ as well as all query_file!() paths (which would fix that invalidation problem) and migration files as well: Force recompile when migrations changed #681
• if SQLX_OFFLINE is not set but DATABASE_URL is, wipe .sqlx/ to remove stale data

[jplatte]

I know the branch name says rip-sqlx-prepare but I'm not actually planning to remove it anymore. I think the build script might be problematic if multiple compilations run at once, so I won't work on that in this PR. Also I think recompiling when a query_file! path changes shouldn't require a build script I think.

[jplatte]

Huh, so even if a branch rename happens through GitHubs web UI, corresponding PRs are just closed. That's stupid...

[jplatte]

I wanted to re-open after renaming the branch back, but apparently that's not possible either. New PR is up: #1183.