Add Cast operator

diesel/src/expression/cast.rs

@@ -0,0 +1,152 @@
+//! SQL `CAST(expr AS sql_type)` expression support
+
+use crate::expression::{AppearsOnTable, Expression, SelectableExpression, ValidGrouping};
+use crate::query_source::aliasing::{AliasSource, FieldAliasMapper};
+use crate::result::QueryResult;
+use crate::{query_builder, query_source, sql_types};
+
+use std::marker::PhantomData;
+
+pub(crate) mod private {
+    use super::*;
+
+    #[derive(Debug, Clone, Copy, diesel::query_builder::QueryId, sql_types::DieselNumericOps)]
+    pub struct Cast<E, ST> {
+        pub(super) expr: E,
+        pub(super) sql_type: PhantomData<ST>,
+    }
+}
+pub(crate) use private::Cast;
+
+impl<E, ST> Cast<E, ST> {
+    pub(crate) fn new(expr: E) -> Self {
+        Self {
+            expr,
+            sql_type: PhantomData,
+        }
+    }
+}
+
+impl<E, ST, GroupByClause> ValidGrouping<GroupByClause> for Cast<E, ST>
+where
+    E: ValidGrouping<GroupByClause>,
+{
+    type IsAggregate = E::IsAggregate;
+}
+
+impl<E, ST, QS> SelectableExpression<QS> for Cast<E, ST>
+where
+    Cast<E, ST>: AppearsOnTable<QS>,
+    E: SelectableExpression<QS>,
+{
+}
+
+impl<E, ST, QS> AppearsOnTable<QS> for Cast<E, ST>
+where
+    Cast<E, ST>: Expression,
+    E: AppearsOnTable<QS>,
+{
+}
+
+impl<E, ST> Expression for Cast<E, ST>
+where
+    E: Expression,
+    ST: sql_types::SingleValue,
+{
+    type SqlType = ST;
+}
+impl<E, ST, DB> query_builder::QueryFragment<DB> for Cast<E, ST>
+where
+    E: query_builder::QueryFragment<DB>,
+    DB: diesel::backend::Backend,
+    ST: KnownCastSqlTypeName<DB>,
+{
+    fn walk_ast<'b>(&'b self, mut out: query_builder::AstPass<'_, 'b, DB>) -> QueryResult<()> {
+        out.push_sql("CAST(");
+        self.expr.walk_ast(out.reborrow())?;
+        out.push_sql(" AS ");
+        out.push_sql(ST::sql_type_name());
+        out.push_sql(")");
+        Ok(())
+    }
+}
+
+/// We know what to write as `sql_type` in the `CAST(expr AS sql_type)` SQL for
+/// `Self`
+///
+/// That is what is returned by `Self::sql_type_name()`
+#[diagnostic::on_unimplemented(
+    note = "In order to use `CAST`, it is necessary that Diesel knows how to express the name \
+		of this type in the given backend.",
+    note = "This can be PRed into Diesel if the type is a standard SQL type."
+)]
+pub trait KnownCastSqlTypeName<DB> {
+    /// What to write as `sql_type` in the `CAST(expr AS sql_type)` SQL for
+    /// `Self`
+    fn sql_type_name() -> &'static str;
+}
+impl<ST, DB> KnownCastSqlTypeName<DB> for sql_types::Nullable<ST>
+where
+    ST: KnownCastSqlTypeName<DB>,
+{
+    fn sql_type_name() -> &'static str {
+        <ST as KnownCastSqlTypeName<DB>>::sql_type_name()
+    }
+}
+
+macro_rules! type_name {
+    ($($backend: ty: $backend_feature: literal { $($type: ident => $val: literal,)+ })*) => {
+        $(
+            $(
+				#[cfg(feature = $backend_feature)]
+                impl KnownCastSqlTypeName<$backend> for sql_types::$type {
+                    fn sql_type_name() -> &'static str {
+                        $val
+                    }
+                }
+            )*
+        )*
+    };
+}
+type_name! {
+    diesel::pg::Pg: "postgres_backend" {
+        Int4 => "int4",
+        Int8 => "int8",
+        Text => "text",
+    }
+    diesel::mysql::Mysql: "mysql_backend" {
+        Int4 => "integer",
+        Int8 => "integer",
+        Text => "char",
+    }
+    diesel::sqlite::Sqlite: "sqlite" {
+        Int4 => "integer",
+        Int8 => "bigint",
+        Text => "text",
+    }
+}
+
+impl<S, E, ST> FieldAliasMapper<S> for Cast<E, ST>
+where
+    S: AliasSource,
+    E: FieldAliasMapper<S>,
+{
+    type Out = Cast<<E as FieldAliasMapper<S>>::Out, ST>;
+    fn map(self, alias: &query_source::Alias<S>) -> Self::Out {
+        Cast {
+            expr: self.expr.map(alias),
+            sql_type: self.sql_type,
+        }
+    }
+}
+
+/// Marker trait: this SQL type (`Self`) can be casted to the target SQL type
+/// (`ST`) using `CAST(expr AS target_sql_type)`
+pub trait CastsTo<ST> {}
+impl<ST1, ST2> CastsTo<sql_types::Nullable<ST2>> for sql_types::Nullable<ST1> where ST1: CastsTo<ST2>
+{}
+
+impl CastsTo<sql_types::Int8> for sql_types::Int4 {}
+impl CastsTo<sql_types::Int4> for sql_types::Int8 {}
+impl CastsTo<sql_types::Text> for sql_types::Int4 {}
+impl CastsTo<sql_types::Text> for sql_types::Int8 {}

diesel/src/expression/helper_types.rs

@@ -90,6 +90,10 @@ pub type NotBetween<Lhs, Lower, Upper> = Grouped<
 /// [`lhs.concat(rhs)`](crate::expression_methods::TextExpressionMethods::concat())
 pub type Concat<Lhs, Rhs> = Grouped<super::operators::Concat<Lhs, AsExpr<Rhs, Lhs>>>;
 
+/// The return type of
+/// [`expr.cast<ST>()`](crate::expression_methods::ExpressionMethods::cast())
+pub type Cast<Expr, ST> = super::cast::Cast<Expr, ST>;
+
 /// The return type of
 /// [`expr.desc()`](crate::expression_methods::ExpressionMethods::desc())
 pub type Desc<Expr> = super::operators::Desc<Expr>;

diesel/src/expression/mod.rs

@@ -37,6 +37,7 @@ pub(crate) mod nullable;
 #[macro_use]
 pub(crate) mod operators;
 mod case_when;
+pub mod cast;
 pub(crate) mod select_by;
 mod sql_literal;
 pub(crate) mod subselect;

diesel/src/expression_methods/global_expression_methods.rs

@@ -2,7 +2,7 @@ use crate::dsl;
 use crate::expression::array_comparison::{AsInExpression, In, NotIn};
 use crate::expression::grouped::Grouped;
 use crate::expression::operators::*;
-use crate::expression::{assume_not_null, nullable, AsExpression, Expression};
+use crate::expression::{assume_not_null, cast, nullable, AsExpression, Expression};
 use crate::sql_types::{SingleValue, SqlType};
 
 /// Methods present on all expressions, except tuples
@@ -437,6 +437,47 @@ pub trait ExpressionMethods: Expression + Sized {
         ))
     }
 
+    /// Generates a `CAST(expr AS sql_type)` expression
+    ///
+    /// It is necessary that the expression's SQL type can be casted to the
+    /// target SQL type (represented by the [`CastsTo`](cast::CastsTo) trait),
+    /// and that we know how the corresponding SQL type is named for the
+    /// specific backend (represented by the
+    /// [`KnownCastSqlTypeName`](cast::KnownCastSqlTypeName) trait).
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # include!("../doctest_setup.rs");
+    /// #
+    /// # fn main() {
+    /// #     run_test().unwrap();
+    /// # }
+    /// #
+    /// # fn run_test() -> QueryResult<()> {
+    /// #     use schema::animals::dsl::*;
+    /// #     let connection = &mut establish_connection();
+    /// #
+    /// use diesel::sql_types;
+    ///
+    /// let data = diesel::select(
+    ///     12_i32
+    ///         .into_sql::<sql_types::Int4>()
+    ///         .cast::<sql_types::Text>(),
+    /// )
+    /// .first::<String>(connection)?;
+    /// assert_eq!("12", data);
+    /// #     Ok(())
+    /// # }
+    /// ```
+    fn cast<ST>(self) -> dsl::Cast<Self, ST>
+    where
+        ST: SingleValue,
+        Self::SqlType: cast::CastsTo<ST>,
+    {
+        cast::Cast::new(self)
+    }
+
     /// Creates a SQL `DESC` expression, representing this expression in
     /// descending order.
     ///