Query::map is broken because lazy norm on HRTBs equals confusion

[mehcode]

async fn test(conn: PgPool) {
    use crate::row::Row;
    let value = crate::query::query::<Postgres>("select 1 + 1")
        .map(|row: PgRow| row.get::<i32, _>(0))
        .fetch_all(&conn)
        .await
        .unwrap();
}

error[E0599]: no method named `fetch_all` found for struct `query::Map<'_, Postgres, impl std::marker::Send+FnMut<(<Postgres as Database>::Row,)>, PgArguments>` in the current scope
  --> sqlx-core/src/postgres/mod.rs:48:10
   |
48 |         .fetch_all(&conn)
   |          ^^^^^^^^^ method not found in `query::Map<'_, Postgres, impl std::marker::Send+FnMut<(<Postgres as Database>::Row,)>, PgArguments>`
   |
  ::: sqlx-core/src/query.rs:34:1
   |
34 | pub struct Map<'q, DB: Database, F, A> {
   | -------------------------------------- method `fetch_all` not found for this
   |
   = note: the method `fetch_all` exists but the following trait bounds were not satisfied:
           `<impl std::marker::Send+FnMut<(<Postgres as Database>::Row,)> as FnOnce<(PgRow,)>>::Output = std::result::Result<_, error::Error>`
           `impl std::marker::Send+FnMut<(<Postgres as Database>::Row,)>: FnMut<(PgRow,)>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0599`.
error: could not compile `sqlx-core`

From https://discord.com/channels/665528275556106240/665528275556106243/801701725001875458

[jplatte]

Looking into it. Have identified one test where .map is clearly better than .try_map, so there's going to be a test included with the fix too without me having to write a new one 🙂

I'm confident that I will be able to make map chaining work (i.e. I won't have to reinstantiate the previous hack).

[jplatte]

I dropped the exact test function from the issue description into sqlx-core/postgres/mod.rs and observed this:

    Checking sqlx-core v0.4.2 (/home/jplatte/Projekte/_work/sqlx/sqlx-core)
warning: unused variable: `value`
  --> sqlx-core/src/postgres/mod.rs:55:9
   |
55 |     let value = crate::query::query::<Postgres>("select 1 + 1")
   |         ^^^^^ help: if this is intentional, prefix it with an underscore: `_value`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: function is never used: `test`
  --> sqlx-core/src/postgres/mod.rs:53:10
   |
53 | async fn test(conn: PgPool) {
   |          ^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: 2 warnings emitted

    Finished dev [unoptimized + debuginfo] target(s) in 3.71s

on Rust 1.49.0. On a recent nightly, it was the same except for one additional unrelated warning.

This definitely needs more info.

[jplatte]

Apparently this is only happens with Rust <1.49.0. Since that version is still pretty new, it might make sense to document that the next release requires it.

[jplatte]

Copied from #1003, in case somebody wants to fix this for older versions of Rust:

Since map currently is implemented via try_map but try_map on its own works, a possible fix for older Rust versions should be renaming the Map type to TryMap and duplicating it to create a new Map type for the infallible map operation. This would mean a bunch of duplication, but shouldn't be too hard.