Show a dedicated PubGrub hint for --unsafe-best-match (#7645)

## Summary

Closes https://github.com/astral-sh/uv/issues/5510.

crates/uv-resolver/src/error.rs

@@ -6,7 +6,7 @@ use indexmap::IndexSet;
 use pubgrub::{DefaultStringReporter, DerivationTree, Derived, External, Range, Reporter};
 use rustc_hash::FxHashMap;
 
-use distribution_types::{BuiltDist, IndexLocations, InstalledDist, SourceDist};
+use distribution_types::{BuiltDist, IndexLocations, IndexUrl, InstalledDist, SourceDist};
 use pep440_rs::Version;
 use pep508_rs::MarkerTree;
 use tracing::trace;
@@ -122,6 +122,7 @@ pub(crate) type ErrorTree = DerivationTree<PubGrubPackage, Range<Version>, Unava
 pub struct NoSolutionError {
     error: pubgrub::NoSolutionError<UvDependencyProvider>,
     available_versions: FxHashMap<PackageName, BTreeSet<Version>>,
+    available_indexes: FxHashMap<PackageName, BTreeSet<IndexUrl>>,
     selector: CandidateSelector,
     python_requirement: PythonRequirement,
     index_locations: IndexLocations,
@@ -137,6 +138,7 @@ impl NoSolutionError {
     pub(crate) fn new(
         error: pubgrub::NoSolutionError<UvDependencyProvider>,
         available_versions: FxHashMap<PackageName, BTreeSet<Version>>,
+        available_indexes: FxHashMap<PackageName, BTreeSet<IndexUrl>>,
         selector: CandidateSelector,
         python_requirement: PythonRequirement,
         index_locations: IndexLocations,
@@ -149,6 +151,7 @@ impl NoSolutionError {
         Self {
             error,
             available_versions,
+            available_indexes,
             selector,
             python_requirement,
             index_locations,
@@ -254,6 +257,7 @@ impl std::fmt::Display for NoSolutionError {
             &tree,
             &self.selector,
             &self.index_locations,
+            &self.available_indexes,
             &self.unavailable_packages,
             &self.incomplete_packages,
             &self.fork_urls,

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

@@ -8,8 +8,9 @@ use owo_colors::OwoColorize;
 use pubgrub::{DerivationTree, Derived, External, Map, Range, ReportFormatter, Term};
 use rustc_hash::FxHashMap;
 
-use distribution_types::IndexLocations;
+use distribution_types::{IndexLocations, IndexUrl};
 use pep440_rs::Version;
+use uv_configuration::IndexStrategy;
 use uv_normalize::PackageName;
 
 use crate::candidate_selector::CandidateSelector;
@@ -504,6 +505,7 @@ impl PubGrubReportFormatter<'_> {
         derivation_tree: &ErrorTree,
         selector: &CandidateSelector,
         index_locations: &IndexLocations,
+        available_indexes: &FxHashMap<PackageName, BTreeSet<IndexUrl>>,
         unavailable_packages: &FxHashMap<PackageName, UnavailablePackage>,
         incomplete_packages: &FxHashMap<PackageName, BTreeMap<Version, IncompletePackage>>,
         fork_urls: &ForkUrls,
@@ -528,12 +530,14 @@ impl PubGrubReportFormatter<'_> {
                         );
                     }
 
-                    // Check for no versions due to no `--find-links` flat index
+                    // Check for no versions due to no `--find-links` flat index.
                     Self::index_hints(
                         package,
                         name,
                         set,
+                        selector,
                         index_locations,
+                        available_indexes,
                         unavailable_packages,
                         incomplete_packages,
                         output_hints,
@@ -582,6 +586,7 @@ impl PubGrubReportFormatter<'_> {
                     &derived.cause1,
                     selector,
                     index_locations,
+                    available_indexes,
                     unavailable_packages,
                     incomplete_packages,
                     fork_urls,
@@ -593,6 +598,7 @@ impl PubGrubReportFormatter<'_> {
                     &derived.cause2,
                     selector,
                     index_locations,
+                    available_indexes,
                     unavailable_packages,
                     incomplete_packages,
                     fork_urls,
@@ -608,7 +614,9 @@ impl PubGrubReportFormatter<'_> {
         package: &PubGrubPackage,
         name: &PackageName,
         set: &Range<Version>,
+        selector: &CandidateSelector,
         index_locations: &IndexLocations,
+        available_indexes: &FxHashMap<PackageName, BTreeSet<IndexUrl>>,
         unavailable_packages: &FxHashMap<PackageName, UnavailablePackage>,
         incomplete_packages: &FxHashMap<PackageName, BTreeMap<Version, IncompletePackage>>,
         hints: &mut IndexSet<PubGrubHint>,
@@ -686,6 +694,27 @@ impl PubGrubReportFormatter<'_> {
                 }
             }
         }
+
+        // Add hints due to the package being available on an index, but not at the correct version,
+        // with subsequent indexes that were _not_ queried.
+        if matches!(selector.index_strategy(), IndexStrategy::FirstIndex) {
+            if let Some(found_index) = available_indexes.get(name).and_then(BTreeSet::first) {
+                // Determine whether the index is the last-available index. If not, then some
+                // indexes were not queried, and could contain a compatible version.
+                if let Some(next_index) = index_locations
+                    .indexes()
+                    .skip_while(|url| *url != found_index)
+                    .nth(1)
+                {
+                    hints.insert(PubGrubHint::UncheckedIndex {
+                        package: package.clone(),
+                        range: set.clone(),
+                        found_index: found_index.clone(),
+                        next_index: next_index.clone(),
+                    });
+                }
+            }
+        }
     }
 
     fn prerelease_available_hint(
@@ -819,11 +848,25 @@ pub(crate) enum PubGrubHint {
         // excluded from `PartialEq` and `Hash`
         package_requires_python: Range<Version>,
     },
+    /// A non-workspace package depends on a workspace package, which is likely shadowing a
+    /// transitive dependency.
     DependsOnWorkspacePackage {
         package: PubGrubPackage,
         dependency: PubGrubPackage,
         workspace: bool,
     },
+    /// A package was available on an index, but not at the correct version, and at least one
+    /// subsequent index was not queried. As such, a compatible version may be available on an
+    /// one of the remaining indexes.
+    UncheckedIndex {
+        package: PubGrubPackage,
+        // excluded from `PartialEq` and `Hash`
+        range: Range<Version>,
+        // excluded from `PartialEq` and `Hash`
+        found_index: IndexUrl,
+        // excluded from `PartialEq` and `Hash`
+        next_index: IndexUrl,
+    },
 }
 
 /// This private enum mirrors [`PubGrubHint`] but only includes fields that should be
@@ -869,6 +912,9 @@ enum PubGrubHintCore {
         dependency: PubGrubPackage,
         workspace: bool,
     },
+    UncheckedIndex {
+        package: PubGrubPackage,
+    },
 }
 
 impl From<PubGrubHint> for PubGrubHintCore {
@@ -921,6 +967,7 @@ impl From<PubGrubHint> for PubGrubHintCore {
                 dependency,
                 workspace,
             },
+            PubGrubHint::UncheckedIndex { package, .. } => Self::UncheckedIndex { package },
         }
     }
 }
@@ -1140,6 +1187,24 @@ impl std::fmt::Display for PubGrubHint {
                     dependency,
                 )
             }
+            Self::UncheckedIndex {
+                package,
+                range,
+                found_index,
+                next_index,
+            } => {
+                write!(
+                    f,
+                    "{}{} `{}` was found on {}, but not at the requested version ({}). A compatible version may be available on a subsequent index (e.g., {}). By default, uv will only consider versions that are published on the first index that contains a given package, to avoid dependency confusion attacks. If all indexes are equally trusted, use `{}` to consider all versions from all indexes, regardless of the order in which they were defined.",
+                    "hint".bold().cyan(),
+                    ":".bold(),
+                    package,
+                    found_index.cyan(),
+                    PackageRange::compatibility(package, range, None).cyan(),
+                    next_index.cyan(),
+                    "--index-strategy unsafe-best-match".green(),
+                )
+            }
         }
     }
 }

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

@@ -1944,6 +1944,7 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
             }
         }
 
+        let mut available_indexes = FxHashMap::default();
         let mut available_versions = FxHashMap::default();
         for package in err.packages() {
             let Some(name) = package.name() else { continue };
@@ -1956,12 +1957,23 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
             }
             if let Some(response) = self.index.packages().get(name) {
                 if let VersionsResponse::Found(ref version_maps) = *response {
+                    // Track the available versions, across all indexes.
                     for version_map in version_maps {
                         available_versions
                             .entry(name.clone())
                             .or_insert_with(BTreeSet::new)
                             .extend(version_map.versions().cloned());
                     }
+
+                    // Track the indexes in which the package is available.
+                    available_indexes
+                        .entry(name.clone())
+                        .or_insert(BTreeSet::new())
+                        .extend(
+                            version_maps
+                                .iter()
+                                .filter_map(|version_map| version_map.index().cloned()),
+                        );
                 }
             }
         }
@@ -1969,6 +1981,7 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
         ResolveError::NoSolution(NoSolutionError::new(
             err,
             available_versions,
+            available_indexes,
             self.selector.clone(),
             self.python_requirement.clone(),
             index_locations.clone(),

crates/uv-resolver/src/version_map.rs

@@ -140,6 +140,14 @@ impl VersionMap {
         }
     }
 
+    /// Return the index URL where this package came from.
+    pub(crate) fn index(&self) -> Option<&IndexUrl> {
+        match &self.inner {
+            VersionMapInner::Eager(_) => None,
+            VersionMapInner::Lazy(lazy) => Some(&lazy.index),
+        }
+    }
+
     /// Return an iterator over the versions and distributions.
     ///
     /// Note that the value returned in this iterator is a [`VersionMapDist`],