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| pub trait DistinctDsl { | |
| type Output; | |
| fn distinct(self) -> Self::Output; | |
| } | |
| #[derive(Default)] | |
| pub struct SelectStatement<D = NoDistinctClause, FU = NoForUpdateClause> { | |
| distinct: D, | |
| for_update: FU, | |
| } | |
| impl<FU> SelectStatement<NoDistinctClause, FU> { | |
| pub fn for_update(self) -> SelectStatement<NoDistinctClause, ForUpdateClause> { | |
| SelectStatement { | |
| distinct: self.distinct, | |
| for_update: ForUpdateClause, | |
| } | |
| } | |
| } | |
| pub struct DistinctClause; | |
| #[derive(Default)] | |
| pub struct NoDistinctClause; | |
| pub struct ForUpdateClause; | |
| #[derive(Default)] | |
| pub struct NoForUpdateClause; | |
| impl<D> DistinctDsl for SelectStatement<D> { | |
| type Output = SelectStatement<DistinctClause>; | |
| fn distinct(self) -> Self::Output { | |
| SelectStatement { | |
| distinct: DistinctClause, | |
| for_update: self.for_update, | |
| } | |
| } | |
| } | |
| pub trait AsQuery { | |
| type Query; | |
| fn as_query(self) -> Self::Query; | |
| } | |
| impl<D, FU> AsQuery for SelectStatement<D, FU> { | |
| type Query = Self; | |
| fn as_query(self) -> Self::Query { | |
| self | |
| } | |
| } | |
| pub trait SelectStatementDefinitelyDoesntImplementMe: AsQuery {} | |
| // This is the thing that causes problems. Since `DistinctDsl` is implemented only when the `FU` | |
| // slot is a specific type, someone could theoretically write `impl<D> | |
| // SelectStatementDefinitelyDoesntImplementMe for SelectStatement<D, MyLocalType>`, which *would* | |
| // make this infinitely recursive. However, that case gets evaluated even when we are looking at | |
| // the concrete type `SelectStatement<NoDistinctClause, ForUpdateClause>, which we can definitively | |
| // say *does not* implement `SelectStatementDefinitelyDoesntImplementMe` | |
| impl<T> DistinctDsl for T | |
| where | |
| T: SelectStatementDefinitelyDoesntImplementMe, | |
| T::Query: DistinctDsl, | |
| { | |
| type Output = <T::Query as DistinctDsl>::Output; | |
| fn distinct(self) -> Self::Output { | |
| self.as_query().distinct() | |
| } | |
| } | |
| pub fn stuff() { | |
| // UFCS is important here. If we use the method syntax, it just resolves to the method not | |
| // existing (this discrepancy is incredibly annoying, the overflow should either not be masked | |
| // there, or we should consistently treat overflow as "this impl doesn't apply") | |
| DistinctDsl::distinct(SelectStatement::default().for_update()) | |
| } |
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