diesel/query_source/
peano_numbers.rs

1//! A simple implementation of peano numbers.
2//!
3//! This is used to enforce that columns can only be selected from a given from
4//! clause if their table appears exactly one time.
5
6/// A table never appears in the from clause.
7#[allow(missing_debug_implementations, missing_copy_implementations)]
8pub struct Never;
9
10/// A table appears in the from clause exactly one time.
11#[allow(missing_debug_implementations, missing_copy_implementations)]
12pub struct Once;
13
14/// A table appears in the from clause two or more times.
15#[allow(missing_debug_implementations, missing_copy_implementations)]
16pub struct MoreThanOnce;
17
18/// Add two peano numbers together.
19///
20/// This is used to determine the number of times a table appears in a from
21/// clause when the from clause contains a join.
22pub trait Plus<T> {
23    /// The result of adding these numbers together
24    type Output;
25}
26
27impl<T> Plus<T> for Never {
28    type Output = T;
29}
30
31impl<T> Plus<T> for MoreThanOnce {
32    type Output = Self;
33}
34
35impl Plus<Never> for Once {
36    type Output = Self;
37}
38
39impl Plus<Once> for Once {
40    type Output = MoreThanOnce;
41}
42
43impl Plus<MoreThanOnce> for Once {
44    type Output = MoreThanOnce;
45}
diesel/query_source/peano_numbers.rs

diesel/query_source/
peano_numbers.rs
