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 😅