Rustc fails to resolve trait bounds

[weiznich]

Calling a trait function directly or with UFCS should be interchangeable if no additional type parameters need to be specified.
This code shows a example where this is not the case. As soon as line 76 is comment in the code will fail to compile with this pretty error message.

(The example code needs diesel master as dependency)

[sgrif]

This seems very likely to be related to #34260 to me, but it's extremely surprising that it doesn't overflow when using UFCS.

[Spoonbender]

Triage: I tried updating the code to modern-day diesel (1.4.8), and it seems to me the issue won't reproduce anymore using rustc 1.59.0 (9d1b2106e 2022-02-23)

@weiznich can you verify?

#[macro_use]
extern crate diesel;


use diesel::*;
use diesel::pg::{PgConnection, Pg};
use diesel::associations::HasTable;
use diesel::query_dsl::LoadQuery;
use diesel::insertable::Insertable;
use diesel::query_builder::InsertStatement;
use diesel::query_builder::UndecoratedInsertRecord;
use diesel::query_source::Table;
use diesel::result::QueryResult;



pub trait GstInsertable<'insert, T, Res, Ret = Res, E = Self> {
    fn insert(&'insert self, conn: &PgConnection) -> QueryResult<Ret>;
}

table!{
    users{
        id -> Integer,
        name-> Text,
    }
}

#[derive(Queryable)]
struct User {
    id: i32,
    name: String,
}

#[derive(Insertable)]
#[table_name = "users"]
struct NewUser<'a> {
    name: &'a str,
}

impl<'a, Res> GstInsertable<'a, users::table, Res> for NewUser<'a>
    where Res: Queryable<(diesel::types::Integer, diesel::types::Text), Pg>
{
    fn insert(&'a self, conn: &PgConnection) -> QueryResult<Res> {
        ::diesel::insert_into(users::table)
            .values(self)
            .get_result(conn)
    }
}

impl<'a, C, E, T, Res> GstInsertable<'a, T, Res, Vec<Res>, E> for C
where
    C: AsRef<[E]>,
    E: GstInsertable<'a, T, Res> + 'a + UndecoratedInsertRecord<T>,
    T: Table + HasTable<Table = T>,
    for<'b> InsertStatement<T, <&'b [E] as Insertable<T>>::Values>: LoadQuery<PgConnection, Res>,
    for<'b> &'b [E]: Insertable<T> + UndecoratedInsertRecord<T>,
{
    fn insert(&'a self, conn: &PgConnection) -> QueryResult<Vec<Res>> {
        ::diesel::insert_into(T::table())
            .values(self.as_ref())
            .get_results( conn)
    }
}


fn main() {
    let conn: PgConnection = unimplemented!();
    let new_user = NewUser { name: "jon" };

    let _: User = new_user.insert(&conn).unwrap();
    let users = &vec![new_user];

    // previously wasn't working, but now seems OK
    let _: Vec<User> = users.insert(&conn).unwrap();

    // but this is
    let _: Vec<User> = GstInsertable::insert(users, &conn).unwrap();
}

[weiznich]

I can verify that the bug is fixed with a recent rustc version.