Method analyse_optimistic

src/transform/src/analysis.rs:426–462

Analysis that starts optimistically but is only correct at a fixed point. Will fail out to `analyse_pessimistic` if the lattice is missing, or `self.fuel` is exhausted. When successful, the result indicates whether new information was produced for `exprs.last()`.

(
            &mut self,
            exprs: &[&MirRelationExpr],
            lower: usize,
            upper: usize,
            depends: &Derived,
            lattice: &dyn crate::analysis::Lattice<A

        /// Will fail out to `analyse_pessimistic` if the lattice is missing, or `self.fuel` is exhausted.
        /// When successful, the result indicates whether new information was produced for `exprs.last()`.
        fn analyse_optimistic(
            &mut self,
            exprs: &[&MirRelationExpr],
            lower: usize,
            upper: usize,
            depends: &Derived,
            lattice: &dyn crate::analysis::Lattice<A::Value>,
        ) -> Result<bool, ()> {
            // Repeatedly re-evaluate the whole tree bottom up, until no changes of fuel spent.
            let mut changed = true;
            while changed {
                changed = false;

                // Bail out if we have done too many `LetRec` passes in this analysis.
                self.fuel -= 1;
                if self.fuel == 0 {
                    return Err(());
                }

                // Track if repetitions may be required, to avoid iteration if they are not.
                let mut is_recursive = false;
                // Update each derived value and track if any have changed.
                for index in lower..upper {
                    let value = self.derive(exprs[index], index, depends);
                    changed = lattice.meet_assign(&mut self.results[index], value) || changed;
                    if let MirRelationExpr::LetRec { .. } = &exprs[index] {
                        is_recursive = true;
                    }
                }

                // Un-set the potential loop if there was no recursion.
                if !is_recursive {
                    changed = false;
                }
            }
            Ok(true)
        }

        /// Analysis that starts conservatively and can be stopped at any point.
        ///