Docs/Message improvements (eg, "no method named `on_conflict` found for type OwnedBatchInsert")

[bnewbold]

I had a piece of code like this:

let new_abstracts: Vec<AbstractsRow> = ...
insert_into(abstracts::table)
                    .values(new_abstracts)
                    .on_conflict(abstracts::sha1)
                    .do_nothing()
                    .execute(conn)?;

which was giving me errrors like:

error[E0599]: no method named `on_conflict` found for type `diesel::query_builder::InsertStatement<database_schema::abstracts::table, diesel::insertable::OwnedBatchInsert<diesel::query_builder::ValuesClause<(diesel::insertable::ColumnInsertValue<database_schema::abstracts::columns::sha1, diesel::expression::bound::Bound<diesel::sql_types::Text, std::string::String>>, diesel::insertable::ColumnInsertValue<database_schema::abstracts::columns::content, diesel::expression::bound::Bound<diesel::sql_types::Text, std::string::String>>), database_schema::abstracts::table>>>` in the current scope
   --> src/api_server.rs:811:22
    |
811 |                     .on_conflict(abstracts::sha1)
    |                      ^^^^^^^^^^^
    |
    = note: the method `on_conflict` exists but the following trait bounds were not satisfied:
            `diesel::insertable::OwnedBatchInsert<diesel::query_builder::ValuesClause<(diesel::insertable::ColumnInsertValue<database_schema::abstracts::columns::sha1, diesel::expression::bound::Bound<diesel::sql_types::Text, std::string::String>>, diesel::insertable::ColumnInsertValue<database_schema::abstracts::columns::content, diesel::expression::bound::Bound<diesel::sql_types::Text, std::string::String>>), database_schema::abstracts::table>> : diesel::query_builder::UndecoratedInsertRecord<database_schema::abstracts::table>`

which I found really opaque. Eventually I found that passing the rows by reference (.values(&new_abstracts)) fixed the error.

I'd like to contribute an improved error message and/or note in the documentation for on_conflict(), as a first-time contributor, but I don't know enough about diesel internals to know if this makes sense. The error message looks like a rust type-system thing, and I don't know if there is a way to hook in to that.

I'd be happy to contribute more such error message improvements if it's possible. I have enjoyed using Diesel for this project, but have run in to a lot of very long compile error messages, and think it would be a huge win if they could be improved.

Versions

• Rust: 1.27.2
• Diesel: 1.3.2
• Database: Postgres 10
• Operating System: Linux (Debian stretch)
• Feature Flags: diesel = { version = "1.3.2", features = ["postgres", "uuid", "serde_json", "chrono", "r2d2"] }

[bnewbold]

Splunking around some more, I see previous discussion of some other compile-time messages, notably in #151 ("Examine our compiler errors, and see if we can improve them") and #573 ("See if we can catch common errors with a lint").

[sgrif]

Unfortunately, this is an error generated by Rust in general and not something we have any control over. Feel free to open an issue on the Rust repo if you think this is actionable for them.

[sgrif]

(The problem is likely that you forgot an & in the call to values which we do try and call out will generally lead to more opaque than average error messages)