You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: pg_on_conflict_requires_valid_conflict_target.rs
    path of file: ./repos/diesel/diesel_compile_tests/tests/fail
    
    the code of the file until where you have to start completion: extern crate diesel;

use diesel::*;

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

table! {
    posts {
        id -> Integer,
        title -> VarChar,
    }
}

#[derive(Insertable)]
#[diesel(table_name = users)]
pub struct NewUser(#[diesel(column_name = name)] &'static str);

define_sql_function!(fn lower(x: diesel::sql_types::Text) -> diesel::sql_types::Text);

fn main() {
    use self::users::dsl::*;
    let mut connection = PgConnection::establish("postgres://localhost").unwrap();

    let valid_
    
fn main() {
    use self::users::dsl::*;
    let mut connection = PgConnection::establish("postgres://localhost").unwrap();

    let valid_insert = insert_into(users)
        .values(&NewUser("Sean"))
        .on_conflict(id)
        .do_nothing()
        .execute(&mut connection);
    // Sanity check, no error

    let column_from_other_table = insert_into(users)
        .values(&NewUser("Sean"))
        .on_conflict(posts::id);

    let expression_using_column_from_other_table = insert_into(users)
        .values(&NewUser("Sean"))
        .on_conflict(lower(posts::title));

    let random_non_expression = insert_into(users)
        .values(&NewUser("Sean"))
        .on_conflict("id");
}