Remove extra forking

crates/uv-resolver/src/marker.rs

@@ -1,6 +1,5 @@
 #![allow(clippy::enum_glob_use)]
 
-use std::cmp::Ordering;
 use std::collections::HashMap;
 use std::ops::Bound::{self, *};
 use std::ops::RangeBounds;
@@ -13,7 +12,7 @@ use pep508_rs::{
 
 use crate::pubgrub::PubGrubSpecifier;
 use crate::RequiresPythonBound;
-use pubgrub::range::{Range as PubGrubRange, Range};
+use pubgrub::range::Range as PubGrubRange;
 
 /// Returns `true` if there is no environment in which both marker trees can both apply, i.e.
 /// the expression `first and second` is always false.
@@ -92,27 +91,27 @@ pub(crate) fn requires_python_marker(tree: &MarkerTree) -> Option<RequiresPython
             let specifier = PubGrubSpecifier::try_from(specifier).ok()?;
 
             // Convert to PubGrub range and perform a union.
-            let range = Range::from(specifier);
+            let range = PubGrubRange::from(specifier);
             let (lower, _) = range.iter().next()?;
 
             // Extract the lower bound.
             Some(RequiresPythonBound::new(lower.clone()))
         }
         MarkerTree::And(trees) => {
-            // Take the minimum of any nested expressions.
-            trees.iter().filter_map(requires_python_marker).min()
+            // Take the maximum of any nested expressions.
+            trees.iter().filter_map(requires_python_marker).max()
         }
         MarkerTree::Or(trees) => {
-            // If all subtrees have a minimum, take the maximum.
-            let mut version = None;
-            for tree in trees.iter() {
-                let next = requires_python_marker(tree)?;
-                version = match version {
-                    Some(version) => Some(std::cmp::max(version, next)),
-                    None => Some(next),
+            // If all subtrees have a bound, take the minimum.
+            let mut min_version = None;
+            for tree in trees {
+                let version = requires_python_marker(tree)?;
+                min_version = match min_version {
+                    Some(min_version) => Some(std::cmp::min(min_version, version)),
+                    None => Some(version),
                 };
             }
-            version
+            min_version
         }
         MarkerTree::Expression(_) => None,
     }

crates/uv-resolver/src/python_requirement.rs

@@ -49,8 +49,9 @@ impl PythonRequirement {
         }
     }
 
-    /// Narrow the [`PythonRequirement`] to the given version.
-    pub fn narrow(&self, target: RequiresPythonBound) -> Option<Self> {
+    /// Narrow the [`PythonRequirement`] to the given version, if it's stricter (i.e., greater)
+    /// than the current `Requires-Python` minimum.
+    pub fn narrow(&self, target: &RequiresPythonBound) -> Option<Self> {
         let Some(PythonTarget::RequiresPython(requires_python)) = self.target.as_ref() else {
             return None;
         };

crates/uv-resolver/src/requires_python.rs

@@ -1,5 +1,6 @@
 use std::cmp::Ordering;
 use std::collections::Bound;
+use std::ops::Deref;
 
 use itertools::Itertools;
 use pubgrub::range::Range;
@@ -29,16 +30,6 @@ pub struct RequiresPython {
 }
 
 impl RequiresPython {
-    /// Narrow the [`RequiresPython`] to the given version, if it's stricter than the current
-    /// target.
-    pub fn narrow(&self, target: RequiresPythonBound) -> Option<Self> {
-        let target = VersionSpecifiers::from(VersionSpecifier::from_lower_bound(&target.0)?);
-        Self::union(std::iter::once(&target))
-            .ok()
-            .flatten()
-            .filter(|next| next.bound > self.bound)
-    }
-
     /// Returns a [`RequiresPython`] to express `>=` equality with the given version.
     pub fn greater_than_equal_version(version: Version) -> Self {
         Self {
@@ -89,6 +80,16 @@ impl RequiresPython {
         Ok(Some(Self { specifiers, bound }))
     }
 
+    /// Narrow the [`RequiresPython`] to the given version, if it's stricter (i.e., greater) than
+    /// the current target.
+    pub fn narrow(&self, target: &RequiresPythonBound) -> Option<Self> {
+        let target = VersionSpecifiers::from(VersionSpecifier::from_lower_bound(target)?);
+        Self::union(std::iter::once(&target))
+            .ok()
+            .flatten()
+            .filter(|next| next.bound > self.bound)
+    }
+
     /// Returns `true` if the `Requires-Python` is compatible with the given version.
     pub fn contains(&self, version: &Version) -> bool {
         self.specifiers.contains(version)
@@ -153,7 +154,7 @@ impl RequiresPython {
         // Alternatively, we could vary the semantics depending on whether or not the user included
         // a pre-release in their specifier, enforcing pre-release compatibility only if the user
         // explicitly requested it.
-        match (target, &self.bound.0) {
+        match (target, self.bound.as_ref()) {
             (Bound::Included(target_lower), Bound::Included(requires_python_lower)) => {
                 target_lower.release() <= requires_python_lower.release()
             }
@@ -179,9 +180,9 @@ impl RequiresPython {
         &self.specifiers
     }
 
-    /// Returns the lower [`Bound`] for the `Requires-Python` specifier.
-    pub fn bound(&self) -> &Bound<Version> {
-        &self.bound.0
+    /// Returns `true` if the `Requires-Python` specifier is unbounded.
+    pub fn is_unbounded(&self) -> bool {
+        self.bound.as_ref() == Bound::Unbounded
     }
 
     /// Returns this `Requires-Python` specifier as an equivalent marker
@@ -197,16 +198,14 @@ impl RequiresPython {
     /// returns a marker tree that evaluates to `true` for all possible marker
     /// environments.
     pub fn to_marker_tree(&self) -> MarkerTree {
-        let (op, version) = match self.bound.0 {
+        let (op, version) = match self.bound.as_ref() {
             // If we see this anywhere, then it implies the marker
             // tree we would generate would always evaluate to
             // `true` because every possible Python version would
             // satisfy it.
             Bound::Unbounded => return MarkerTree::And(vec![]),
-            Bound::Excluded(ref version) => {
-                (Operator::GreaterThan, version.clone().without_local())
-            }
-            Bound::Included(ref version) => {
+            Bound::Excluded(version) => (Operator::GreaterThan, version.clone().without_local()),
+            Bound::Included(version) => {
                 (Operator::GreaterThanEqual, version.clone().without_local())
             }
         };
@@ -281,6 +280,14 @@ impl RequiresPythonBound {
     }
 }
 
+impl Deref for RequiresPythonBound {
+    type Target = Bound<Version>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
 impl PartialOrd for RequiresPythonBound {
     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
         Some(self.cmp(other))
@@ -289,7 +296,7 @@ impl PartialOrd for RequiresPythonBound {
 
 impl Ord for RequiresPythonBound {
     fn cmp(&self, other: &Self) -> Ordering {
-        match (&self.0, &other.0) {
+        match (self.as_ref(), other.as_ref()) {
             (Bound::Included(a), Bound::Included(b)) => a.cmp(b),
             (Bound::Included(_), Bound::Excluded(_)) => Ordering::Less,
             (Bound::Excluded(_), Bound::Included(_)) => Ordering::Greater,

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

@@ -25,7 +25,7 @@ use tracing::{debug, enabled, instrument, trace, warn, Level};
 use distribution_types::{
     BuiltDist, CompatibleDist, Dist, DistributionMetadata, IncompatibleDist, IncompatibleSource,
     IncompatibleWheel, InstalledDist, PythonRequirementKind, RemoteSource, ResolvedDist,
-    ResolvedDistRef, SourceDist, VersionOrUrlRef, WheelCompatibility,
+    ResolvedDistRef, SourceDist, VersionOrUrlRef,
 };
 pub(crate) use locals::Locals;
 use pep440_rs::{Version, MIN_VERSION};
@@ -412,27 +412,6 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
 
                 prefetcher.version_tried(state.next.clone());
 
-                // Narrow the Python requirement, if possible.
-                let (python_requirement, requires_python) = if let Some(python_requirement) = state
-                    .next
-                    .marker()
-                    .and_then(requires_python_marker)
-                    .and_then(|marker| state.python_requirement.narrow(marker))
-                {
-                    let requires_python = if state.requires_python.is_some() {
-                        python_requirement.to_marker_tree()
-                    } else {
-                        None
-                    };
-                    debug!("requires-python: {:?}", requires_python);
-                    (Cow::Owned(python_requirement), Cow::Owned(requires_python))
-                } else {
-                    (
-                        Cow::Borrowed(&state.python_requirement),
-                        Cow::Borrowed(&state.requires_python),
-                    )
-                };
-
                 let term_intersection = state
                     .pubgrub
                     .partial_solution
@@ -451,7 +430,7 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
                     &mut state.pins,
                     &preferences,
                     &state.fork_urls,
-                    &python_requirement,
+                    &state.python_requirement,
                     visited,
                     &request_sink,
                 )?;
@@ -530,7 +509,7 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
                         &version,
                         &state.fork_urls,
                         &state.markers,
-                        requires_python.as_ref().as_ref(),
+                        state.requires_python.as_ref(),
                     )?;
                     match forked_deps {
                         ForkedDependencies::Unavailable(reason) => {
@@ -576,7 +555,7 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
                                     .map(ToString::to_string)
                                     .join(", ")
                             );
-                            // assert!(forks.len() >= 2);
+                            assert!(forks.len() >= 2);
                             // This is a somewhat tortured technique to ensure
                             // that our resolver state is only cloned as much
                             // as it needs to be. We basically move the state
@@ -600,13 +579,12 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
                                 if let Some(python_version) =
                                     requires_python_marker(&forked_state.markers)
                                 {
-                                    debug!("Found `requires-python` bound for fork: {python_version:?}");
-
                                     if let Some(python_requirement) =
-                                        forked_state.python_requirement.narrow(python_version)
+                                        forked_state.python_requirement.narrow(&python_version)
                                     {
-                                        debug!("Narrowed `requires-python` bound to: {python_requirement:?}");
-
+                                        if let Some(target) = python_requirement.target() {
+                                            debug!("Narrowed `requires-python` bound to: {target}");
+                                        }
                                         forked_state.requires_python =
                                             if forked_state.requires_python.is_some() {
                                                 python_requirement.to_marker_tree()
@@ -1862,8 +1840,20 @@ struct ForkState {
     /// that the marker expression that provoked the fork is true), then that
     /// dependency is completely ignored.
     markers: MarkerTree,
-    /// The Python requirement for this state.
+    /// The Python requirement for this fork. Defaults to the Python requirement for
+    /// the resolution, but may be narrowed if a `python_version` marker is present
+    /// in a given fork.
+    ///
+    /// For example, in:
+    /// ```text
+    /// numpy >=1.26 ; python_version >= "3.9"
+    /// numpy <1.26 ; python_version < "3.9"
+    /// ```
+    ///
+    /// The top fork has a narrower Python compatibility range, and thus can find a
+    /// solution that omits Python 3.8 support.
     python_requirement: PythonRequirement,
+    /// The [`MarkerTree`] corresponding to the [`PythonRequirement`].
     requires_python: Option<MarkerTree>,
 }
 
@@ -2434,7 +2424,7 @@ impl Dependencies {
                     continue;
                 }
             };
-            // assert!(fork_groups.forks.len() >= 2, "expected definitive fork");
+            assert!(fork_groups.forks.len() >= 2, "expected definitive fork");
             let mut new_forks: Vec<Fork> = vec![];
             for group in fork_groups.forks {
                 let mut new_forks_for_group = forks.clone();
@@ -2612,18 +2602,7 @@ impl<'a> PossibleForks<'a> {
         let PossibleForks::PossiblyForking(ref fork_groups) = *self else {
             return false;
         };
-        if fork_groups.forks.len() > 1 {
-            return true;
-        }
-        if fork_groups.forks.iter().any(|fork_groups| {
-            fork_groups
-                .packages
-                .iter()
-                .any(|(index, markers)| requires_python_marker(markers).is_some())
-        }) {
-            return true;
-        };
-        false
+        fork_groups.forks.len() > 1
     }
 
     /// Consumes this possible set of forks and converts a "possibly forking"
@@ -2636,14 +2615,7 @@ impl<'a> PossibleForks<'a> {
         let PossibleForks::PossiblyForking(ref fork_groups) = self else {
             return self;
         };
-        if fork_groups.forks.len() == 1
-            && !fork_groups.forks.iter().any(|fork_groups| {
-                fork_groups
-                    .packages
-                    .iter()
-                    .any(|(index, markers)| requires_python_marker(markers).is_some())
-            })
-        {
+        if fork_groups.forks.len() == 1 {
             self.make_no_forks_possible();
             return self;
         }

crates/uv/src/commands/project/lock.rs

@@ -1,5 +1,3 @@
-use std::collections::Bound;
-
 use anstream::eprint;
 
 use distribution_types::UnresolvedRequirementSpecification;
@@ -127,7 +125,7 @@ pub(super) async fn do_lock(
     let requires_python = find_requires_python(workspace)?;
 
     let requires_python = if let Some(requires_python) = requires_python {
-        if matches!(requires_python.bound(), Bound::Unbounded) {
+        if requires_python.is_unbounded() {
             let default =
                 RequiresPython::greater_than_equal_version(interpreter.python_minor_version());
             warn_user!("The workspace `requires-python` field does not contain a lower bound: `{requires_python}`. Set a lower bound to indicate the minimum compatible Python version (e.g., `{default}`).");

crates/uv/tests/pip_compile.rs

@@ -6564,6 +6564,41 @@ fn universal_multi_version() -> Result<()> {
     Ok(())
 }
 
+/// Perform a universal resolution that requires narrowing the supported Python range in one of the
+/// fork branches.
+#[test]
+fn universal_requires_python() -> Result<()> {
+    let context = TestContext::new("3.12");
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str(indoc::indoc! {r"
+        numpy >=1.26 ; python_version >= '3.9'
+        numpy <1.26 ; python_version < '3.9'
+    "})?;
+
+    uv_snapshot!(context.filters(), windows_filters=false, context.pip_compile()
+            .arg("requirements.in")
+            .arg("-p")
+            .arg("3.8")
+            .arg("--universal"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] requirements.in -p 3.8 --universal
+    numpy==1.24.4 ; python_version < '3.9'
+        # via -r requirements.in
+    numpy==1.26.4 ; python_version >= '3.9'
+        # via -r requirements.in
+
+    ----- stderr -----
+    warning: The requested Python version 3.8 is not available; 3.12.[X] will be used to build dependencies instead.
+    Resolved 2 packages in [TIME]
+    "###
+    );
+
+    Ok(())
+}
+
 /// Resolve a package from a `requirements.in` file, with a `constraints.txt` file pinning one of
 /// its transitive dependencies to a specific version.
 #[test]