Export SQL numeric operations from diesel::expression::ops::numeric , and auto-type them.

[dessalines]

Discussed in #4461

^{Originally posted by dessalines January 28, 2025}
I'd like to use a subtract operation be used as a select_expression.

This gets the number of unread comments in a post: post_aggregates::comments - post_actions::read_comments_amount

This unfortunately can't use auto_type, and I also can't strictly define the type, since the Sub numeric operation is in a private module : diesel::expression::ops::numeric::Sub

My output type should be something like:

diesel::expression::ops::numeric::Sub<
  post_aggregates::comments,
  post_actions::read_comments_amount
>

Is there any way to get the type from a numeric operation?

Thanks in advance.

[weiznich]

This requires us to add the following type definitions including documentation to diesel::expression::helper_types:

pub type Add<L, R> = <L as ::core::ops::Add<R>>::Output;
pub type Sub<L, R> = <L as ::core::ops::Sub<R>>::Output;
pub type Mul<L, R> = <L as ::core::ops::Mul<R>>::Output;
pub type Div<L, R> = <L as ::core::ops::Div<R>>::Output;