Ignore upper-bounds on Requires-Python (#4086)

## Summary

This PR modifies our `Requires-Python` handling to treat
`Requires-Python` as a lower bound. There's extensive discussion around
this in #4022 and the references
linked therein. I think it's an experiment worth trying. Even in my own
small projects, I'm running into issues whereby I'm being "forced" to
add a `<4` upper bound to my `Requires-Python` due to these caps.

Separately, we should explore adding a mechanism that's distinct from
`Requires-Python` to enable users to declare a supported range for
locking.

Closes #4022.

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

@@ -1,5 +1,6 @@
 use itertools::Itertools;
 use pubgrub::range::Range;
+use std::ops::Bound;
 
 use pep440_rs::{Operator, PreRelease, Version, VersionSpecifier, VersionSpecifiers};
 
@@ -10,9 +11,8 @@ use crate::ResolveError;
 pub(crate) struct PubGrubSpecifier(Range<Version>);
 
 impl PubGrubSpecifier {
-    /// Returns `true` if the [`PubGrubSpecifier`] is a subset of the other.
-    pub(crate) fn subset_of(&self, other: &Self) -> bool {
-        self.0.subset_of(&other.0)
+    pub(crate) fn iter(&self) -> impl Iterator<Item = (&Bound<Version>, &Bound<Version>)> {
+        self.0.iter()
     }
 }
 

crates/uv-resolver/src/python_requirement.rs

@@ -1,3 +1,5 @@
+use std::collections::Bound;
+
 use pep440_rs::VersionSpecifiers;
 use pep508_rs::StringVersion;
 use uv_interpreter::{Interpreter, PythonVersion};
@@ -76,7 +78,15 @@ impl RequiresPython {
     ///
     /// For example, if the target Python is `>=3.8`, then `>=3.7` would cover it. However, `>=3.9`
     /// would not.
-    pub fn subset_of(&self, requires_python: &VersionSpecifiers) -> bool {
+    ///
+    /// We treat `Requires-Python` as a lower bound. For example, if the requirement expresses
+    /// `>=3.8, <4`, we treat it as `>=3.8`. `Requires-Python` itself was intended to enable
+    /// packages to drop support for older versions of Python without breaking installations on
+    /// those versions, and packages cannot know whether they are compatible with future, unreleased
+    /// versions of Python.
+    ///
+    /// See: <https://packaging.python.org/en/latest/guides/dropping-older-python-versions/>
+    pub fn contains(&self, requires_python: &VersionSpecifiers) -> bool {
         match self {
             RequiresPython::Specifier(specifier) => requires_python.contains(specifier),
             RequiresPython::Specifiers(specifiers) => {
@@ -90,7 +100,40 @@ impl RequiresPython {
                     return false;
                 };
 
-                target.subset_of(&requires_python)
+                // If the dependency has no lower bound, then it supports all versions.
+                let Some((requires_python_lower, _)) = requires_python.iter().next() else {
+                    return true;
+                };
+
+                // If we have no lower bound, then there must be versions we support that the
+                // dependency does not.
+                let Some((target_lower, _)) = target.iter().next() else {
+                    return false;
+                };
+
+                // We want, e.g., `target_lower` to be `>=3.8` and `requires_python_lower` to be
+                // `>=3.7`.
+                //
+                // That is: `requires_python_lower` should be less than or equal to `target_lower`.
+                match (requires_python_lower, target_lower) {
+                    (Bound::Included(requires_python_lower), Bound::Included(target_lower)) => {
+                        requires_python_lower <= target_lower
+                    }
+                    (Bound::Excluded(requires_python_lower), Bound::Included(target_lower)) => {
+                        requires_python_lower < target_lower
+                    }
+                    (Bound::Included(requires_python_lower), Bound::Excluded(target_lower)) => {
+                        requires_python_lower <= target_lower
+                    }
+                    (Bound::Excluded(requires_python_lower), Bound::Excluded(target_lower)) => {
+                        requires_python_lower < target_lower
+                    }
+                    // If the dependency has no lower bound, then it supports all versions.
+                    (Bound::Unbounded, _) => true,
+                    // If we have no lower bound, then there must be versions we support that the
+                    // dependency does not.
+                    (_, Bound::Unbounded) => false,
+                }
             }
         }
     }

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

@@ -730,7 +730,7 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
                 // The version is incompatible due to its Python requirement.
                 if let Some(requires_python) = metadata.requires_python.as_ref() {
                     if let Some(target) = self.python_requirement.target() {
-                        if !target.subset_of(requires_python) {
+                        if !target.contains(requires_python) {
                             return Ok(Some(ResolverVersion::Unavailable(
                                 version.clone(),
                                 UnavailableVersion::IncompatibleDist(IncompatibleDist::Source(

crates/uv-resolver/src/version_map.rs

@@ -467,7 +467,7 @@ impl VersionMapLazy {
         // _installed_ Python version (to build successfully)
         if let Some(requires_python) = requires_python {
             if let Some(target) = self.python_requirement.target() {
-                if !target.subset_of(&requires_python) {
+                if !target.contains(&requires_python) {
                     return SourceDistCompatibility::Incompatible(
                         IncompatibleSource::RequiresPython(
                             requires_python,
@@ -534,7 +534,7 @@ impl VersionMapLazy {
         // Check for a Python version incompatibility
         if let Some(requires_python) = requires_python {
             if let Some(target) = self.python_requirement.target() {
-                if !target.subset_of(&requires_python) {
+                if !target.contains(&requires_python) {
                     return WheelCompatibility::Incompatible(IncompatibleWheel::RequiresPython(
                         requires_python,
                         PythonRequirementKind::Target,