Prioritize forks based on Python narrowing (#5642)

## Summary

First part of: https://github.com/astral-sh/uv/issues/4926. We should
solve forks that _don't_ expand the world of supported versions (e.g.,
`python_version >= '3.11'` enables us to select new packages, since we
narrow the supported version range).

crates/uv-resolver/src/pubgrub/dependencies.rs

@@ -14,7 +14,7 @@ use uv_normalize::{ExtraName, PackageName};
 use crate::pubgrub::{PubGrubPackage, PubGrubPackageInner};
 use crate::{PubGrubSpecifier, ResolveError};
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Eq, PartialEq)]
 pub(crate) struct PubGrubDependency {
     pub(crate) package: PubGrubPackage,
     pub(crate) version: Range<Version>,

crates/uv-resolver/src/requires_python.rs

@@ -350,6 +350,12 @@ impl<'de> serde::Deserialize<'de> for RequiresPython {
 #[derive(Debug, Clone, Eq, PartialEq, Hash)]
 pub struct RequiresPythonBound(Bound<Version>);
 
+impl Default for RequiresPythonBound {
+    fn default() -> Self {
+        Self(Bound::Unbounded)
+    }
+}
+
 impl RequiresPythonBound {
     pub fn new(bound: Bound<Version>) -> Self {
         Self(match bound {

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

@@ -1,6 +1,7 @@
 //! Given a set of requirements, find a set of compatible packages.
 
 use std::borrow::Cow;
+use std::cmp::Ordering;
 use std::collections::hash_map::Entry;
 use std::collections::{BTreeMap, BTreeSet, VecDeque};
 use std::fmt::{Display, Formatter, Write};
@@ -576,6 +577,7 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
                             state.markers,
                             start.elapsed().as_secs_f32()
                         );
+
                         for new_fork_state in self.forks_to_fork_states(
                             state,
                             &version,
@@ -2766,6 +2768,12 @@ impl Dependencies {
             forks = new_forks;
             diverging_packages.push(name.clone());
         }
+
+        // Prioritize the forks. Prefer solving forks with lower Python bounds, since they're more
+        // likely to produce solutions that work for forks with higher Python bounds (whereas the
+        // inverse is not true).
+        forks.sort();
+
         ForkedDependencies::Forked {
             forks,
             diverging_packages,
@@ -2808,7 +2816,7 @@ enum ForkedDependencies {
 /// have the same name and because the marker expressions are disjoint,
 /// a fork occurs. One fork will contain `a<2` but not `a>=2`, while
 /// the other fork will contain `a>=2` but not `a<2`.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Eq, PartialEq)]
 struct Fork {
     /// The list of dependencies for this fork, guaranteed to be conflict
     /// free. (i.e., There are no two packages with the same name with
@@ -2829,6 +2837,23 @@ struct Fork {
     markers: MarkerTree,
 }
 
+impl Ord for Fork {
+    fn cmp(&self, other: &Self) -> Ordering {
+        // A higher `requires-python` requirement indicates a _lower-priority_ fork. We'd prefer
+        // to solve `<3.7` before solving `>=3.7`, since the resolution produced by the former might
+        // work for the latter, but the inverse is unlikely to be true.
+        let self_bound = requires_python_marker(&self.markers).unwrap_or_default();
+        let other_bound = requires_python_marker(&other.markers).unwrap_or_default();
+        other_bound.cmp(&self_bound)
+    }
+}
+
+impl PartialOrd for Fork {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
 impl Fork {
     /// Add the given dependency to this fork.
     ///

crates/uv/tests/pip_compile.rs

@@ -7867,9 +7867,7 @@ fn universal_no_repeated_unconditional_distributions() -> Result<()> {
     ----- stdout -----
     # This file was autogenerated by uv via the following command:
     #    uv pip compile --cache-dir [CACHE_DIR] requirements.in -p 3.8 --universal
-    alabaster==0.7.13 ; python_version < '3.11'
-        # via sphinx
-    alabaster==0.7.16 ; python_version >= '3.11'
+    alabaster==0.7.13
         # via sphinx
     astroid==3.1.0
         # via pylint
@@ -7915,31 +7913,19 @@ fn universal_no_repeated_unconditional_distributions() -> Result<()> {
         # via sphinx
     snowballstemmer==2.2.0
         # via sphinx
-    sphinx==7.1.2 ; python_version < '3.11'
+    sphinx==7.1.2
         # via -r requirements.in
-    sphinx==7.2.6 ; python_version >= '3.11'
-        # via -r requirements.in
-    sphinxcontrib-applehelp==1.0.4 ; python_version < '3.11'
+    sphinxcontrib-applehelp==1.0.4
         # via sphinx
-    sphinxcontrib-applehelp==1.0.8 ; python_version >= '3.11'
+    sphinxcontrib-devhelp==1.0.2
         # via sphinx
-    sphinxcontrib-devhelp==1.0.2 ; python_version < '3.11'
-        # via sphinx
-    sphinxcontrib-devhelp==1.0.6 ; python_version >= '3.11'
-        # via sphinx
-    sphinxcontrib-htmlhelp==2.0.1 ; python_version < '3.11'
-        # via sphinx
-    sphinxcontrib-htmlhelp==2.0.5 ; python_version >= '3.11'
+    sphinxcontrib-htmlhelp==2.0.1
         # via sphinx
     sphinxcontrib-jsmath==1.0.1
         # via sphinx
-    sphinxcontrib-qthelp==1.0.3 ; python_version < '3.11'
+    sphinxcontrib-qthelp==1.0.3
         # via sphinx
-    sphinxcontrib-qthelp==1.0.7 ; python_version >= '3.11'
-        # via sphinx
-    sphinxcontrib-serializinghtml==1.1.5 ; python_version < '3.11'
-        # via sphinx
-    sphinxcontrib-serializinghtml==1.1.10 ; python_version >= '3.11'
+    sphinxcontrib-serializinghtml==1.1.5
         # via sphinx
     tomli==2.0.1 ; python_version < '3.11'
         # via pylint
@@ -7956,7 +7942,7 @@ fn universal_no_repeated_unconditional_distributions() -> Result<()> {
 
     ----- stderr -----
     warning: The requested Python version 3.8 is not available; 3.12.[X] will be used to build dependencies instead.
-    Resolved 41 packages in [TIME]
+    Resolved 34 packages in [TIME]
     "###
     );