diesel 2.3.8

A safe, extensible ORM and Query Builder for PostgreSQL, SQLite, and MySQL
Documentation 
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use std::marker::PhantomData;

use crate::expression::array_comparison::InExpression;
use crate::expression::*;
use crate::query_builder::*;
use crate::result::QueryResult;

/// This struct tells our type system that the whatever we put in `values`
/// will be handled by SQL as an expression of type `ST`.
/// It also implements the usual `SelectableExpression` and `AppearsOnTable` traits
/// (which is useful when using this as an expression). To enforce correctness here, it checks
/// the dedicated [`ValidSubselect`]. This however does not check that the `SqlType` of
/// [`SelectQuery`], matches `ST`, so appropriate constraints should be checked in places that
/// construct Subselect. (It's not always equal, notably .single_value() makes `ST` nullable, and
/// `exists` checks bounds on `SubSelect<T, Bool>` although there is actually no such subquery in
/// the final SQL.)
#[derive(Debug, Copy, Clone, QueryId)]
pub struct Subselect<T, ST> {
    values: T,
    _sql_type: PhantomData<ST>,
}

impl<T, ST> Subselect<T, ST> {
    pub(crate) fn new(values: T) -> Self {
        Self {
            values,
            _sql_type: PhantomData,
        }
    }
}

impl<T: SelectQuery, ST> Expression for Subselect<T, ST>
where
    ST: SqlType + TypedExpressionType,
{
    // This is useful for `.single_value()`
    type SqlType = ST;
}

impl<T, ST: SqlType> InExpression for Subselect<T, ST> {
    type SqlType = ST;
    fn is_empty(&self) -> bool {
        false
    }
    fn is_array(&self) -> bool {
        false
    }
}

impl<T, ST, QS> SelectableExpression<QS> for Subselect<T, ST>
where
    Subselect<T, ST>: AppearsOnTable<QS>,
    T: ValidSubselect<QS>,
{
}

impl<T, ST, QS> AppearsOnTable<QS> for Subselect<T, ST>
where
    Subselect<T, ST>: Expression,
    T: ValidSubselect<QS>,
{
}

// FIXME: This probably isn't sound. The subselect can reference columns from
// the outer query, and is affected by the `GROUP BY` clause of the outer query
// identically to using it outside of a subselect
impl<T, ST, GB> ValidGrouping<GB> for Subselect<T, ST> {
    type IsAggregate = is_aggregate::Never;
}

impl<T, ST, DB> QueryFragment<DB> for Subselect<T, ST>
where
    DB: Backend,
    T: QueryFragment<DB>,
{
    fn walk_ast<'b>(&'b self, mut out: AstPass<'_, 'b, DB>) -> QueryResult<()> {
        self.values.walk_ast(out.reborrow())?;
        Ok(())
    }
}

pub trait ValidSubselect<QS> {}