Method make_nonrecursive

src/expr/src/relation.rs:2079–2123

Replaces `LetRec` nodes with a stack of `Let` nodes. In each `Let` binding, uses of `Get` in `value` that are not at strictly greater identifiers are rewritten to be the constant collection. This makes the computation perform exactly "one" iteration. This was used only temporarily while developing `LetRec`.

(self: &mut MirRelationExpr)

    ///
    /// This was used only temporarily while developing `LetRec`.
    pub fn make_nonrecursive(self: &mut MirRelationExpr) {
        let mut deadlist = BTreeSet::new();
        let mut worklist = vec![self];
        while let Some(expr) = worklist.pop() {
            if let MirRelationExpr::LetRec {
                ids,
                values,
                limits: _,
                body,
            } = expr
            {
                let ids_values = values
                    .drain(..)
                    .zip_eq(ids)
                    .map(|(value, id)| (*id, value))
                    .collect::<Vec<_>>();
                *expr = body.take_dangerous();
                for (id, mut value) in ids_values.into_iter().rev() {
                    // Remove references to potentially recursive identifiers.
                    deadlist.insert(id);
                    value.visit_pre_mut(|e| {
                        if let MirRelationExpr::Get {
                            id: crate::Id::Local(id),
                            typ,
                            ..
                        } = e
                        {
                            let typ = typ.clone();
                            if deadlist.contains(id) {
                                e.take_safely(Some(typ));
                            }
                        }
                    });
                    *expr = MirRelationExpr::Let {
                        id,
                        value: Box::new(value),
                        body: Box::new(expr.take_dangerous()),
                    };
                }
                worklist.push(expr);
            } else {
                worklist.extend(expr.children_mut().rev());
            }
        }
    }

    /// For each Id `id'` referenced in `expr`, if it is larger or equal than `id`, then record in
    /// `expire_whens` that when `id'` is redefined, then we should expire the information that