uv-resolver: add "include" rules to `ResolverEnvironment`

When we generate conflict markers for each resolution after the
resolver runs, it turns out that generating them just from exclusion
rules is not sufficient.

For example, if `foo` and `bar` are declared as conflicting extras, then
we end up with the following forks:

    A: extra != 'foo'
    B: extra != 'bar'
    C: extra != 'foo' and extra != 'bar'

Now let's take an example where these forks don't share the same version
for all packages. Consider a case where `idna==3.9` is in forks A and C,
but where `idna==3.10` is in fork B. If we combine the markers in forks
A and C through disjunction, we get the following:

     idna==3.9: extra != 'foo' or (extra != 'foo' and extra != 'bar')
    idna==3.10: extra != 'bar'

Which simplifies to:

     idna==3.9: extra != 'foo'
    idna==3.10: extra != 'bar'

But these are clearly not disjoint. Both dependencies could be selected,
for example, when neither `foo` nor `bar` are active. We can remedy this
by keeping around the inclusion rules for each fork:

    A: extra != 'foo' and extra == 'bar'
    B: extra != 'bar' and extra == 'foo'
    C: extra != 'foo' and extra != 'bar'

And so for `idna`, we have:

     idna==3.9: (extra != 'foo' and extra == 'bar') or (extra != 'foo' and extra != 'bar')
    idna==3.10: extra != 'bar' and extra == 'foo'

Which simplifies to:

     idna==3.9: extra != 'foo'
    idna==3.10: extra != 'bar' and extra == 'foo'

And these *are* properly disjoint. There is no way for them both to be
active. This also correctly accounts for fork C where neither `foo` nor
`bar` are active, and yet, `idna==3.9` is still enabled but `idna==3.10`
is not. (In the [motivating example], this comes from `baz` being enabled.)
That is, this captures the idea that for `idna==3.10` to be installed,
there must actually be a specific extra that is enabled. That's what
makes it disjoint from `idna==3.9`.

We aren't quite done yet, because this does add *too many* conflict
markers to dependency edges that don't need it. In the next commit,
we'll add in our world knowledge to simplify these conflict markers.

[motivating example]: https://github.com/astral-sh/uv/issues/9289

crates/uv-resolver/src/resolver/environment.rs

@@ -95,6 +95,20 @@ enum Kind {
         initial_forks: Arc<[MarkerTree]>,
         /// The markers associated with this resolver fork.
         markers: MarkerTree,
+        /// Conflicting group inclusions.
+        ///
+        /// Note that inclusions don't play a role in predicates
+        /// like `ResolverEnvironment::included_by_group`. Instead,
+        /// only exclusions are considered.
+        ///
+        /// We record inclusions for two reasons. First is that if
+        /// we somehow wind up with an inclusion and exclusion rule
+        /// for the same conflict item, then we treat the resulting
+        /// fork as impossible. (You cannot require that an extra is
+        /// both included and excluded. Such a rule can never be
+        /// satisfied.) Second is that we use the inclusion rules to
+        /// write conflict markers after resolution is finished.
+        include: Arc<crate::FxHashbrownSet<ConflictItem>>,
         /// Conflicting group exclusions.
         exclude: Arc<crate::FxHashbrownSet<ConflictItem>>,
     },
@@ -134,6 +148,7 @@ impl ResolverEnvironment {
         let kind = Kind::Universal {
             initial_forks: initial_forks.into(),
             markers: MarkerTree::TRUE,
+            include: Arc::new(crate::FxHashbrownSet::default()),
             exclude: Arc::new(crate::FxHashbrownSet::default()),
         };
         ResolverEnvironment { kind }
@@ -201,6 +216,7 @@ impl ResolverEnvironment {
             Kind::Universal {
                 ref initial_forks,
                 markers: ref lhs,
+                ref include,
                 ref exclude,
             } => {
                 let mut markers = lhs.clone();
@@ -208,6 +224,7 @@ impl ResolverEnvironment {
                 let kind = Kind::Universal {
                     initial_forks: Arc::clone(initial_forks),
                     markers,
+                    include: Arc::clone(include),
                     exclude: Arc::clone(exclude),
                 };
                 ResolverEnvironment { kind }
@@ -215,22 +232,29 @@ impl ResolverEnvironment {
         }
     }
 
-    /// Returns a new resolver environment with the given groups excluded from
+    /// Returns a new resolver environment with the given groups included or
-    /// it.
+    /// excluded from it. An `Ok` variant indicates an include rule while an
+    /// `Err` variant indicates en exclude rule.
     ///
     /// When a group is excluded from a resolver environment,
     /// `ResolverEnvironment::included_by_group` will return false. The idea
     /// is that a dependency with a corresponding group should be excluded by
-    /// forks in the resolver with this environment.
+    /// forks in the resolver with this environment. (Include rules have no
+    /// effect in `included_by_group` since, for the purposes of conflicts
+    /// during resolution, we only care about what *isn't* allowed.)
+    ///
+    /// If calling this routine results in the same conflict item being both
+    /// included and excluded, then this returns `None` (since it would
+    /// otherwise result in a fork that can never be satisfied).
     ///
     /// # Panics
     ///
     /// This panics if the resolver environment corresponds to one and only one
     /// specific marker environment. i.e., "pip"-style resolution.
-    pub(crate) fn exclude_by_group(
+    pub(crate) fn filter_by_group(
         &self,
-        items: impl IntoIterator<Item = ConflictItem>,
+        rules: impl IntoIterator<Item = Result<ConflictItem, ConflictItem>>,
-    ) -> ResolverEnvironment {
+    ) -> Option<ResolverEnvironment> {
         match self.kind {
             Kind::Specific { .. } => {
                 unreachable!("environment narrowing only happens in universal resolution")
@@ -238,18 +262,34 @@ impl ResolverEnvironment {
             Kind::Universal {
                 ref initial_forks,
                 ref markers,
+                ref include,
                 ref exclude,
             } => {
+                let mut include: crate::FxHashbrownSet<_> = (**include).clone();
                 let mut exclude: crate::FxHashbrownSet<_> = (**exclude).clone();
-                for item in items {
+                for rule in rules {
-                    exclude.insert(item);
+                    match rule {
+                        Ok(item) => {
+                            if exclude.contains(&item) {
+                                return None;
+                            }
+                            include.insert(item);
+                        }
+                        Err(item) => {
+                            if include.contains(&item) {
+                                return None;
+                            }
+                            exclude.insert(item);
+                        }
+                    }
                 }
                 let kind = Kind::Universal {
                     initial_forks: Arc::clone(initial_forks),
                     markers: markers.clone(),
+                    include: Arc::new(include),
                     exclude: Arc::new(exclude),
                 };
-                ResolverEnvironment { kind }
+                Some(ResolverEnvironment { kind })
             }
         }
     }
@@ -266,6 +306,7 @@ impl ResolverEnvironment {
         let Kind::Universal {
             ref initial_forks,
             markers: ref _markers,
+            include: ref _include,
             exclude: ref _exclude,
         } = self.kind
         else {
@@ -332,9 +373,32 @@ impl ResolverEnvironment {
     pub(crate) fn try_universal_markers(&self) -> Option<UniversalMarker> {
         match self.kind {
             Kind::Specific { .. } => None,
-            Kind::Universal { ref markers, .. } => {
+            Kind::Universal {
-                // FIXME: Support conflicts.
+                ref markers,
-                Some(UniversalMarker::new(markers.clone(), MarkerTree::TRUE))
+                ref include,
+                ref exclude,
+                ..
+            } => {
+                let mut conflict_marker = MarkerTree::TRUE;
+                for item in exclude.iter() {
+                    if let Some(extra) = item.extra() {
+                        let operator = uv_pep508::ExtraOperator::NotEqual;
+                        let name = uv_pep508::MarkerValueExtra::Extra(extra.clone());
+                        let expr = uv_pep508::MarkerExpression::Extra { operator, name };
+                        let exclude_extra_marker = MarkerTree::expression(expr);
+                        conflict_marker.and(exclude_extra_marker);
+                    }
+                }
+                for item in include.iter() {
+                    if let Some(extra) = item.extra() {
+                        let operator = uv_pep508::ExtraOperator::Equal;
+                        let name = uv_pep508::MarkerValueExtra::Extra(extra.clone());
+                        let expr = uv_pep508::MarkerExpression::Extra { operator, name };
+                        let exclude_extra_marker = MarkerTree::expression(expr);
+                        conflict_marker.and(exclude_extra_marker);
+                    }
+                }
+                Some(UniversalMarker::new(markers.clone(), conflict_marker))
             }
         }
     }

crates/uv-resolver/src/resolver/mod.rs

@@ -2941,11 +2941,9 @@ impl Forks {
                 }
 
                 // Create a fork that excludes ALL extras.
-                let mut fork_none = fork.clone();
+                if let Some(fork_none) = fork.clone().filter(set.iter().cloned().map(Err)) {
-                for group in set.iter() {
+                    new.push(fork_none);
-                    fork_none = fork_none.exclude([group.clone()]);
                 }
-                new.push(fork_none);
 
                 // Now create a fork for each conflicting group, where
                 // that fork excludes every *other* conflicting group.
@@ -2955,13 +2953,18 @@ impl Forks {
                 // {foo, bar}, one that excludes {foo, baz} and one
                 // that excludes {bar, baz}.
                 for (i, _) in set.iter().enumerate() {
-                    let fork_allows_group = fork.clone().exclude(
+                    let fork_allows_group =
-                        set.iter()
+                        fork.clone()
-                            .enumerate()
+                            .filter(set.iter().cloned().enumerate().map(|(j, group)| {
-                            .filter(|&(j, _)| i != j)
+                                if i == j {
-                            .map(|(_, group)| group.clone()),
+                                    Ok(group)
-                    );
+                                } else {
-                    new.push(fork_allows_group);
+                                    Err(group)
+                                }
+                            }));
+                    if let Some(fork_allows_group) = fork_allows_group {
+                        new.push(fork_allows_group);
+                    }
                 }
             }
             forks = new;
@@ -3058,11 +3061,17 @@ impl Fork {
         self.conflicts.contains(&item)
     }
 
-    /// Exclude the given groups from this fork.
+    /// Include or Exclude the given groups from this fork.
     ///
     /// This removes all dependencies matching the given conflicting groups.
-    fn exclude(mut self, groups: impl IntoIterator<Item = ConflictItem>) -> Fork {
+    ///
-        self.env = self.env.exclude_by_group(groups);
+    /// If the exclusion rules would result in a fork with an unsatisfiable
+    /// resolver environment, then this returns `None`.
+    fn filter(
+        mut self,
+        rules: impl IntoIterator<Item = Result<ConflictItem, ConflictItem>>,
+    ) -> Option<Fork> {
+        self.env = self.env.filter_by_group(rules)?;
         self.dependencies.retain(|dep| {
             let Some(conflicting_item) = dep.package.conflicting_item() else {
                 return true;
@@ -3075,7 +3084,7 @@ impl Fork {
             }
             false
         });
-        self
+        Some(self)
     }
 }