use std::borrow::Cow; use std::fmt::Debug; use std::iter::once; pub trait SqlType: Sized { fn from_sql() -> Result<Self, ()>; fn to_sql(&self) -> Result<String, ()>; } pub trait Table { type Columns; } pub trait TopLevelTable: Table { const TABLE_NAME: &'static str; } pub trait ColumnType: Into<<Self::Table as Table>::Columns> + 'static + Copy + Debug { type Table: Table; type ColumnType; fn column_sql() -> Cow<'static, str>; } pub trait ProvidesColumns<C> { type ColumnIds; type Output; fn column_ids(&self, columns: &C) -> Self::ColumnIds; fn result_columns( &self, columns: &C, column_ids: &Self::ColumnIds, ) -> impl Iterator<Item = Cow<'static, str>>; fn output_from_row( &self, column: &C, column_ids: &Self::ColumnIds, ) -> Result<Self::Output, ()>; } impl<T, C> ProvidesColumns<C> for T where T: TopLevelTable, C: ColumnType<Table = T>, C::ColumnType: SqlType, { type ColumnIds = usize; type Output = C::ColumnType; fn column_ids(&self, _columns: &C) -> Self::ColumnIds { todo!() } fn result_columns( &self, columns: &C, column_ids: &Self::ColumnIds, ) -> impl Iterator<Item = Cow<'static, str>> { once(format!("t{}.{} AS c{column_ids}", "cool", columns.column_sql()).into()) } fn output_from_row( &self, _column: &C, column_ids: &Self::ColumnIds, ) -> Result<Self::Output, ()> { todo!() } } impl<C1, C2, T> ProvidesColumns<(C1, C2)> for T where T: ProvidesColumns<C1>, T: ProvidesColumns<C2>, { type ColumnIds = ( <T as ProvidesColumns<C1>>::ColumnIds, <T as ProvidesColumns<C2>>::ColumnIds, ); type Output = ( <T as ProvidesColumns<C1>>::Output, <T as ProvidesColumns<C2>>::Output, ); fn column_ids( &self, columns: &(C1, C2), ) -> Self::ColumnIds { ( <T as ProvidesColumns<C1>>::column_ids(self, &columns.0), <T as ProvidesColumns<C2>>::column_ids(self, &columns.1), ) } fn result_columns( &self, columns: &(C1, C2), column_ids: &Self::ColumnIds, ) -> impl Iterator<Item = Cow<'static, str>> { let c1_sql = <T as ProvidesColumns<C1>>::result_columns(self, &columns.0, &column_ids.0); let c2_sql = <T as ProvidesColumns<C2>>::result_columns(self, &columns.1, &column_ids.1); c1_sql.chain(c2_sql) } fn output_from_row( &self, column: &(C1, C2), column_ids: &Self::ColumnIds, ) -> Result<Self::Output, ()> { let c1_output = <T as ProvidesColumns<C1>>::output_from_row( self, &column.0, &column_ids.0, )?; let c2_output = <T as ProvidesColumns<C2>>::output_from_row( self, &column.1, &column_ids.1, )?; Ok((c1_output, c2_output)) } }

$ cargo +1.77 build Compiling bisect-thing v0.1.0 (/tmp/bisect-thing) error[E0599]: no method named `column_sql` found for reference `&C` in the current scope --> src/lib.rs:62:65 | 62 | once(format!("t{}.{} AS c{column_ids}", "cool", columns.column_sql()).into()) | --------^^^^^^^^^^-- | | | | | this is an associated function, not a method | help: use associated function syntax instead: `C::column_sql()` | = note: found the following associated functions; to be used as methods, functions must have a `self` parameter note: the candidate is defined in the trait `ColumnType` --> src/lib.rs:22:5 | 22 | fn column_sql() -> Cow<'static, str>; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ For more information about this error, try `rustc --explain E0599`. error: could not compile `bisect-thing` (lib) due to 1 previous error