[core] [macros] Add optional support for compact_str

[mcronce]

This is one of several "small string optimization" crates out there; I use it in particular because it seems to seems to effectively optimize both performance and size, including Option. Being able to pull short strings from a database without an extra allocation for each is a nice little win.

I wasn't able to make a generic impl Encode work (e.g. impl<'q, DB> Encode<'q, DB> for CompactString where DB: Database) due to lifetime issues without allocating a heap String temporarily, so I have separate impls for each database. Decode was no problem, though, and that makes me think I was probably missing something with Encode...

[mcronce]

I also wasn't able to make query_as! work with CompactString struct fields without doing AS "...: CompactString" in the query - that seems like it's just a limitation of the macros, but I'd love to be told I'm wrong there as well :)

[mcronce]

Never mind that bit about query_as!, I noticed something in grep output that I must have overlooked a dozen times and got it working

[abonander]

Instead of adding support for arbitrary string-wrapper crates, as upgrading to new versions of compact_str would be a breaking change, I'd rather look into ways to make it easier to create this wrapper type in your own code using #[derive(sqlx::Type)].

#[derive(sqlx::Type)]
#[sqlx(transparent, display_fromstr)]
pub struct MyStr(CompactString);

And then with some improvements to query_as! planned in the near-future, you wouldn't need to use a type override in the query as it would just validate that MyStr is compatible with SQL type TEXT and accept the mapping as-is.

Alternatively this might also work as a control attribute on #[derive(sqlx::FromRow)] like was being worked on in #1081, which I regret leaving open for so long.

[mcronce]

@abonander that sounds substantially better than what I came up with ;)

[lcmgh]

#[derive(sqlx::Type)]
#[sqlx(transparent, display_fromstr)]
pub struct MyStr(CompactString);

Hi @abonander . I am wondering if the chances for ending up on the stack remain the same for MyStr(CompactString) as for CompactString?

[mcronce]

@abonander so, trying to use #[sqlx(try_from = "String")] in a new project and it appears it doesn't work with query_as!. Can we revisit this?

[CosmicHorrorDev]

@mcronce Snowed in so can't personally check, but did you try using the custom type override as detailed here?

https://docs.rs/sqlx/latest/sqlx/macro.query.html#force-a-differentcustom-type

[mcronce]

@CosmicHorrorDev that requires Decode and Encode impls