Use PubGrubPython type in Python incompatibility reporting (#3992)

## Summary

Rather than re-testing compatibility, I think we can just rely on the
types directly.

crates/distribution-types/src/prioritized_distribution.rs

@@ -103,8 +103,8 @@ impl Display for IncompatibleDist {
                     Some(_) => f.write_str("was published after the exclude newer time"),
                     None => f.write_str("has no publish time"),
                 },
-                IncompatibleWheel::RequiresPython(python) => {
-                    write!(f, "requires at python {python}")
+                IncompatibleWheel::RequiresPython(python, _) => {
+                    write!(f, "requires Python {python}")
                 }
             },
             Self::Source(incompatibility) => match incompatibility {
@@ -123,8 +123,8 @@ impl Display for IncompatibleDist {
                     Some(_) => f.write_str("was published after the exclude newer time"),
                     None => f.write_str("has no publish time"),
                 },
-                IncompatibleSource::RequiresPython(python) => {
-                    write!(f, "requires python {python}")
+                IncompatibleSource::RequiresPython(python, _) => {
+                    write!(f, "requires Python {python}")
                 }
             },
             Self::Unavailable => f.write_str("has no available distributions"),
@@ -132,6 +132,16 @@ impl Display for IncompatibleDist {
     }
 }
 
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum PythonRequirementKind {
+    /// The installed version of Python.
+    Installed,
+    /// The target version of Python; that is, the version of Python for which we are resolving
+    /// dependencies. This is typically the same as the installed version, but may be different
+    /// when specifying an alternate Python version for the resolution.
+    Target,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum WheelCompatibility {
     Incompatible(IncompatibleWheel),
@@ -142,7 +152,7 @@ pub enum WheelCompatibility {
 pub enum IncompatibleWheel {
     ExcludeNewer(Option<i64>),
     Tag(IncompatibleTag),
-    RequiresPython(VersionSpecifiers),
+    RequiresPython(VersionSpecifiers, PythonRequirementKind),
     Yanked(Yanked),
     NoBinary,
 }
@@ -156,7 +166,7 @@ pub enum SourceDistCompatibility {
 #[derive(Debug, PartialEq, Eq, Clone)]
 pub enum IncompatibleSource {
     ExcludeNewer(Option<i64>),
-    RequiresPython(VersionSpecifiers),
+    RequiresPython(VersionSpecifiers, PythonRequirementKind),
     Yanked(Yanked),
     NoBuild,
 }
@@ -466,16 +476,16 @@ impl IncompatibleSource {
             Self::ExcludeNewer(timestamp_self) => match other {
                 // Smaller timestamps are closer to the cut-off time
                 Self::ExcludeNewer(timestamp_other) => timestamp_other < timestamp_self,
-                Self::NoBuild | Self::RequiresPython(_) | Self::Yanked(_) => true,
+                Self::NoBuild | Self::RequiresPython(_, _) | Self::Yanked(_) => true,
             },
-            Self::RequiresPython(_) => match other {
+            Self::RequiresPython(_, _) => match other {
                 Self::ExcludeNewer(_) => false,
                 // Version specifiers cannot be reasonably compared
-                Self::RequiresPython(_) => false,
+                Self::RequiresPython(_, _) => false,
                 Self::NoBuild | Self::Yanked(_) => true,
             },
             Self::Yanked(_) => match other {
-                Self::ExcludeNewer(_) | Self::RequiresPython(_) => false,
+                Self::ExcludeNewer(_) | Self::RequiresPython(_, _) => false,
                 // Yanks with a reason are more helpful for errors
                 Self::Yanked(yanked_other) => matches!(yanked_other, Yanked::Reason(_)),
                 Self::NoBuild => true,
@@ -497,21 +507,23 @@ impl IncompatibleWheel {
                         timestamp_other < timestamp_self
                     }
                 },
-                Self::NoBinary | Self::RequiresPython(_) | Self::Tag(_) | Self::Yanked(_) => true,
+                Self::NoBinary | Self::RequiresPython(_, _) | Self::Tag(_) | Self::Yanked(_) => {
+                    true
+                }
             },
             Self::Tag(tag_self) => match other {
                 Self::ExcludeNewer(_) => false,
                 Self::Tag(tag_other) => tag_other > tag_self,
-                Self::NoBinary | Self::RequiresPython(_) | Self::Yanked(_) => true,
+                Self::NoBinary | Self::RequiresPython(_, _) | Self::Yanked(_) => true,
             },
-            Self::RequiresPython(_) => match other {
+            Self::RequiresPython(_, _) => match other {
                 Self::ExcludeNewer(_) | Self::Tag(_) => false,
                 // Version specifiers cannot be reasonably compared
-                Self::RequiresPython(_) => false,
+                Self::RequiresPython(_, _) => false,
                 Self::NoBinary | Self::Yanked(_) => true,
             },
             Self::Yanked(_) => match other {
-                Self::ExcludeNewer(_) | Self::Tag(_) | Self::RequiresPython(_) => false,
+                Self::ExcludeNewer(_) | Self::Tag(_) | Self::RequiresPython(_, _) => false,
                 // Yanks with a reason are more helpful for errors
                 Self::Yanked(yanked_other) => matches!(yanked_other, Yanked::Reason(_)),
                 Self::NoBinary => true,

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

@@ -19,7 +19,7 @@ use crate::candidate_selector::CandidateSelector;
 use crate::python_requirement::PythonRequirement;
 use crate::resolver::{IncompletePackage, UnavailablePackage, UnavailableReason};
 
-use super::{PubGrubPackage, PubGrubPackageInner};
+use super::{PubGrubPackage, PubGrubPackageInner, PubGrubPython};
 
 #[derive(Debug)]
 pub(crate) struct PubGrubReportFormatter<'a> {
@@ -44,44 +44,29 @@ impl ReportFormatter<PubGrubPackage, Range<Version>, UnavailableReason>
                 format!("we are solving dependencies of {package} {version}")
             }
             External::NoVersions(package, set) => {
-                if matches!(&**package, PubGrubPackageInner::Python(_)) {
                 if let Some(python) = self.python_requirement {
-                        if python.target() == python.installed() {
-                            // Simple case, the installed version is the same as the target version
-                            return format!(
-                                "the current {package} version ({}) does not satisfy {}",
-                                python.target(),
-                                PackageRange::compatibility(package, set)
-                            );
-                        }
-                        // Complex case, the target was provided and differs from the installed one
-                        // Determine which Python version requirement was not met
-                        if !set.contains(python.target()) {
+                    if matches!(
+                        &**package,
+                        PubGrubPackageInner::Python(PubGrubPython::Target)
+                    ) {
                         return format!(
                             "the requested {package} version ({}) does not satisfy {}",
                             python.target(),
                             PackageRange::compatibility(package, set)
                         );
                     }
-                        // TODO(zanieb): Explain to the user why the installed version is relevant
-                        //               when they provided a target version; probably via a "hint"
-                        debug_assert!(
-                            !set.contains(python.installed()),
-                            "There should not be an incompatibility where the range is satisfied by both Python requirements"
-                        );
+                    if matches!(
+                        &**package,
+                        PubGrubPackageInner::Python(PubGrubPython::Installed)
+                    ) {
                         return format!(
                             "the current {package} version ({}) does not satisfy {}",
                             python.installed(),
                             PackageRange::compatibility(package, set)
                         );
                     }
-                    // We should always have the required Python versions, if we don't we'll fall back
-                    // to a less helpful message in production
-                    debug_assert!(
-                        false,
-                        "Error reporting should always be provided with Python versions"
-                    );
                 }
+
                 let set = self.simplify_set(set, package);
 
                 if set.as_ref() == &Range::full() {

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

@@ -20,7 +20,8 @@ use tracing::{debug, enabled, instrument, trace, warn, Level};
 
 use distribution_types::{
     BuiltDist, Dist, DistributionMetadata, IncompatibleDist, IncompatibleSource, IncompatibleWheel,
-    InstalledDist, RemoteSource, ResolvedDist, ResolvedDistRef, SourceDist, VersionOrUrlRef,
+    InstalledDist, PythonRequirementKind, RemoteSource, ResolvedDist, ResolvedDistRef, SourceDist,
+    VersionOrUrlRef,
 };
 pub(crate) use locals::Locals;
 use pep440_rs::{Version, MIN_VERSION};
@@ -406,9 +407,11 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
                         if let UnavailableVersion::IncompatibleDist(
                             IncompatibleDist::Source(IncompatibleSource::RequiresPython(
                                 requires_python,
+                                kind,
                             ))
                             | IncompatibleDist::Wheel(IncompatibleWheel::RequiresPython(
                                 requires_python,
+                                kind,
                             )),
                         ) = reason
                         {
@@ -420,18 +423,25 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
                                 })?;
 
                             let package = &state.next;
-                            for kind in [PubGrubPython::Installed, PubGrubPython::Target] {
-                                state.pubgrub.add_incompatibility(
-                                    Incompatibility::from_dependency(
+                            state
+                                .pubgrub
+                                .add_incompatibility(Incompatibility::from_dependency(
                                     package.clone(),
                                     Range::singleton(version.clone()),
                                     (
-                                            PubGrubPackage::from(PubGrubPackageInner::Python(kind)),
+                                        PubGrubPackage::from(PubGrubPackageInner::Python(
+                                            match kind {
+                                                PythonRequirementKind::Installed => {
+                                                    PubGrubPython::Installed
+                                                }
+                                                PythonRequirementKind::Target => {
+                                                    PubGrubPython::Target
+                                                }
+                                            },
+                                        )),
                                         python_version.clone(),
                                     ),
-                                    ),
-                                );
-                            }
+                                ));
                             state
                                 .pubgrub
                                 .partial_solution
@@ -722,12 +732,29 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
 
                 // The version is incompatible due to its Python requirement.
                 if let Some(requires_python) = metadata.requires_python.as_ref() {
+                    let installed = self.python_requirement.installed();
                     let target = self.python_requirement.target();
+                    if target != installed {
                         if !requires_python.contains(target) {
                             return Ok(Some(ResolverVersion::Unavailable(
                                 version.clone(),
                                 UnavailableVersion::IncompatibleDist(IncompatibleDist::Source(
-                                IncompatibleSource::RequiresPython(requires_python.clone()),
+                                    IncompatibleSource::RequiresPython(
+                                        requires_python.clone(),
+                                        PythonRequirementKind::Target,
+                                    ),
+                                )),
+                            )));
+                        }
+                    }
+                    if !requires_python.contains(installed) {
+                        return Ok(Some(ResolverVersion::Unavailable(
+                            version.clone(),
+                            UnavailableVersion::IncompatibleDist(IncompatibleDist::Source(
+                                IncompatibleSource::RequiresPython(
+                                    requires_python.clone(),
+                                    PythonRequirementKind::Installed,
+                                ),
                             )),
                         )));
                     }

crates/uv-resolver/src/version_map.rs

@@ -8,7 +8,8 @@ use tracing::instrument;
 use distribution_filename::{DistFilename, WheelFilename};
 use distribution_types::{
     HashComparison, IncompatibleSource, IncompatibleWheel, IndexUrl, PrioritizedDist,
-    RegistryBuiltWheel, RegistrySourceDist, SourceDistCompatibility, WheelCompatibility,
+    PythonRequirementKind, RegistryBuiltWheel, RegistrySourceDist, SourceDistCompatibility,
+    WheelCompatibility,
 };
 use pep440_rs::{Version, VersionSpecifiers};
 use platform_tags::{TagCompatibility, Tags};
@@ -465,11 +466,20 @@ impl VersionMapLazy {
         // Source distributions must meet both the _target_ Python version and the
         // _installed_ Python version (to build successfully)
         if let Some(requires_python) = requires_python {
-            if !requires_python.contains(self.python_requirement.target())
-                || !requires_python.contains(self.python_requirement.installed())
-            {
+            if self.python_requirement.target() != self.python_requirement.installed() {
+                if !requires_python.contains(self.python_requirement.target()) {
+                    return SourceDistCompatibility::Incompatible(
+                        IncompatibleSource::RequiresPython(
+                            requires_python,
+                            PythonRequirementKind::Target,
+                        ),
+                    );
+                }
+            }
+            if !requires_python.contains(self.python_requirement.installed()) {
                 return SourceDistCompatibility::Incompatible(IncompatibleSource::RequiresPython(
                     requires_python,
+                    PythonRequirementKind::Installed,
                 ));
             }
         }
@@ -526,6 +536,7 @@ impl VersionMapLazy {
             if !requires_python.contains(self.python_requirement.target()) {
                 return WheelCompatibility::Incompatible(IncompatibleWheel::RequiresPython(
                     requires_python,
+                    PythonRequirementKind::Target,
                 ));
             }
         }