Decode with postgres: hr bound for fields

[lcnr]

fixes #3185

// sqlx = { git = "https://github.com/launchbadge/sqlx", features = ["postgres"] }
use sqlx::Type;
#[derive(Type)]
struct Foo {
    a: u32,
}

expands to the following impl for Decode

impl<'r> Decode<'r, Postgres> for Foo
where
    i32: Decode<'r, Postgres>,
    i32: Type<Postgres>,
{
    fn decode(
        value: Postgres::PgValueRef<'r>,
    ) -> Result<Self, Box<dyn Error + 'static + Send + Sync>> {
        let mut decoder = PgRecordDecoder::new(value)?;
        let a = decoder.try_decode::<i32>()?;
        Ok(Foo { a })
    }
}

decoder.try_decode has the following bound:

sqlx/sqlx-postgres/src/types/record.rs

Line 97 in 03926de

T: for<'a> Decode<'a, Postgres> + Type<Postgres>,

The where-bound only provides Decode for a concrete lifetime 'r however. This previously compiled because the compiler eagerly rejected the where-bound and used the more general impl. This behavior has changed in rust-lang/rust#119820.

---

It would have already been broken before rust-lang/rust#119820 if the type of a were generic, e.g.

struct Foo<T> {
    a: T,
}

However, this currently results in an unrelated error as the expansion does not add the generic arguments to Foo, resulting in

error[E0107]: missing generics for struct `Foo`
 --> src/main.rs:3:8
  |
3 | struct Foo<T> {
  |        ^^^ expected 1 generic argument
  |
note: struct defined here, with 1 generic parameter: `T`
 --> src/main.rs:3:8
  |
3 | struct Foo<T> {
  |        ^^^ -
help: add missing generic argument
  |
3 | struct Foo<T><T> {
  |           +++

---

I first tried to change try_decode to not require a higher-ranked bound, requiring T: Decode<'r, Postgres> instead. However, we need the higher ranked bound as T::decode is called with a local buffer:

sqlx/sqlx-postgres/src/types/record.rs

Lines 184 to 201 in 03926de

	let buf = if element.is_empty() && !quoted {
	// completely empty input means NULL
	None
	} else {
	Some(element.as_bytes())
	};
	// NOTE: we do not call [`accepts`] or give a chance to from a user as
	// TEXT sequences are not strongly typed
	T::decode(PgValueRef {
	// NOTE: We pass `0` as the type ID because we don't have a reasonable value
	// we could use.
	type_info: PgTypeInfo::with_oid(Oid(0)),
	format: self.fmt,
	value: buf,
	row: None,
	})

I therefore ended up changing the derive to require a higher ranked bound instead. I tested this locally but did not add a test for this as I didn't immediately understand your test suite 😅

[lcnr]

We're delaying this breaking change by one version, so it will take another 8 weeks until sqlx will break on stable: rust-lang/rust#125629

I would really sqlx to work around this breakage to minimize the impact on downstream crates. If there are concerns or issues with the approach in this PR or you'd like more clarity about the decision making behind the breakage, please reach out to me. I am available either in this issue directly, via zulip (https://rust-lang.zulipchat.com), or via email (rust@lcnr.de). We can also talk about this in sync if desired.

Explicitly pinging @abonander as you're the main maintainer of sqlx from what I can tell.

[abonander]

Superceded by #2940