Allow backtracking to before a specific package

src/internal/core.rs

@@ -82,7 +82,7 @@ impl<DP: DependencyProvider> State<DP> {
     ) {
         let dep_incompats =
             self.add_incompatibility_from_dependencies(package, version.clone(), dependencies);
-        self.partial_solution.add_package_version_incompatibilities(
+        self.partial_solution.add_package_version_dependencies(
             package,
             version.clone(),
             dep_incompats,
@@ -249,7 +249,8 @@ impl<DP: DependencyProvider> State<DP> {
         }
     }
 
-    /// Backtracking.
+    /// After a conflict occurred, backtrack the partial solution to a given decision level, and add
+    /// the incompatibility if it was new.
     fn backtrack(
         &mut self,
         incompat: IncompDpId<DP>,
@@ -265,6 +266,21 @@ impl<DP: DependencyProvider> State<DP> {
         }
     }
 
+    /// Manually backtrack before the given package was selected.
+    ///
+    /// This can be used to switch the order of packages if the previous prioritization was bad.
+    ///
+    /// Returns the number of the decisions that were backtracked, or `None` if the package was not
+    /// decided on yet.
+    pub fn backtrack_package(&mut self, package: Id<DP::P>) -> Option<u32> {
+        let base_decision_level = self.partial_solution.current_decision_level();
+        let new_decision_level = self.partial_solution.backtrack_package(package).ok()?;
+        // Remove contradicted incompatibilities that depend on decisions we just backtracked away.
+        self.contradicted_incompatibilities
+            .retain(|_, dl| *dl <= new_decision_level);
+        Some(base_decision_level.0 - new_decision_level.0)
+    }
+
     /// Add this incompatibility into the set of all incompatibilities.
     ///
     /// PubGrub collapses identical dependencies from adjacent package versions

src/internal/partial_solution.rs

@@ -6,6 +6,7 @@
 use std::fmt::{Debug, Display};
 use std::hash::BuildHasherDefault;
 
+use log::debug;
 use priority_queue::PriorityQueue;
 use rustc_hash::FxHasher;
 
@@ -377,12 +378,37 @@ impl<DP: DependencyProvider> PartialSolution<DP> {
         self.has_ever_backtracked = true;
     }
 
-    /// We can add the version to the partial solution as a decision
-    /// if it doesn't produce any conflict with the new incompatibilities.
-    /// In practice I think it can only produce a conflict if one of the dependencies
-    /// (which are used to make the new incompatibilities)
-    /// is already in the partial solution with an incompatible version.
-    pub(crate) fn add_package_version_incompatibilities(
+    /// Backtrack the partial solution before a particular package was selected.
+    ///
+    /// This can be used to switch the order of packages if the previous prioritization was bad.
+    ///
+    /// Returns the new decision level on success and an error if the package was not decided on
+    /// yet.
+    pub(crate) fn backtrack_package(&mut self, package: Id<DP::P>) -> Result<DecisionLevel, ()> {
+        let Some(decision_level) = self.package_assignments.get_index_of(&package) else {
+            return Err(());
+        };
+        let decision_level = DecisionLevel(decision_level as u32);
+        if decision_level > self.current_decision_level {
+            return Err(());
+        }
+        debug!(
+            "Package backtracking ot decision level {}",
+            decision_level.0
+        );
+        self.backtrack(decision_level);
+        Ok(decision_level)
+    }
+
+    /// Add a package version as decision if none of its dependencies conflicts with the partial
+    /// solution.
+    ///
+    /// If the resolution never backtracked before, a fast path adds the package version directly
+    /// without checking dependencies.
+    ///
+    /// Returns the incompatibility that caused the current version to be rejected, if a conflict
+    /// in the dependencies was found.
+    pub(crate) fn add_package_version_dependencies(
         &mut self,
         package: Id<DP::P>,
         version: DP::V,