Method take_safely

src/expr/src/relation.rs:1520–1543

Take ownership of `self`, leaving an empty `MirRelationExpr::Constant` with the optionally given scalar types. The given scalar types should be `base_eq` with the types that `typ()` would find. Keys and nullability are ignored in the given `SqlRelationType`, and instead we set the best possible key and nullability, since we are making an empty collection. If `typ` is not given, then this calls `.

(&mut self, typ: Option<ReprRelationType>)

    /// If `typ` is not given, then this calls `.typ()` (which is possibly expensive) to determine
    /// the correct type.
    pub fn take_safely(&mut self, typ: Option<ReprRelationType>) -> MirRelationExpr {
        if let Some(typ) = &typ {
            let self_typ = self.typ();
            soft_assert_no_log!(
                self_typ
                    .column_types
                    .iter()
                    .zip_eq(typ.column_types.iter())
                    .all(|(t1, t2)| t1.scalar_type == t2.scalar_type)
            );
        }
        let mut typ = typ.unwrap_or_else(|| self.typ());
        typ.keys = vec![vec![]];
        for ct in typ.column_types.iter_mut() {
            ct.nullable = false;
        }
        std::mem::replace(
            self,
            MirRelationExpr::Constant {
                rows: Ok(vec![]),
                typ,
            },
        )
    }

    /// Take ownership of `self`, leaving an empty `MirRelationExpr::Constant` with the given scalar
    /// types. Nullability is ignored in the given `SqlColumnType`s, and instead we set the best