Use a single buffer for hints on resolver errors (#7497)

This changes the structure of the hints generator in the resolver when
encountering solution errors, so that it re-uses a single output buffer
owned by the caller.
It avoids repeated allocations of a temporary buffer within each
recursive function call.

crates/uv-resolver/src/error.rs

@@ -2,6 +2,7 @@ use std::collections::{BTreeMap, BTreeSet};
 use std::fmt::Formatter;
 use std::sync::Arc;
 
+use indexmap::IndexSet;
 use pubgrub::{DefaultStringReporter, DerivationTree, Derived, External, Range, Reporter};
 use rustc_hash::FxHashMap;
 
@@ -248,15 +249,18 @@ impl std::fmt::Display for NoSolutionError {
         write!(f, "{report}")?;
 
         // Include any additional hints.
-        for hint in formatter.hints(
+        let mut additional_hints = IndexSet::default();
+        formatter.generate_hints(
             &self.error,
             &self.selector,
             &self.index_locations,
             &self.unavailable_packages,
             &self.incomplete_packages,
             &self.fork_urls,
             &self.markers,
-        ) {
+            &mut additional_hints,
+        );
+        for hint in additional_hints {
             write!(f, "\n\n{hint}")?;
         }
 

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

@@ -499,7 +499,7 @@ impl PubGrubReportFormatter<'_> {
     ///
     /// The [`PubGrubHints`] help users resolve errors by providing additional context or modifying
     /// their requirements.
-    pub(crate) fn hints(
+    pub(crate) fn generate_hints(
         &self,
         derivation_tree: &ErrorTree,
         selector: &CandidateSelector,
@@ -508,8 +508,8 @@ impl PubGrubReportFormatter<'_> {
         incomplete_packages: &FxHashMap<PackageName, BTreeMap<Version, IncompletePackage>>,
         fork_urls: &ForkUrls,
         markers: &ResolverMarkers,
-    ) -> IndexSet<PubGrubHint> {
-        let mut hints = IndexSet::default();
+        output_hints: &mut IndexSet<PubGrubHint>,
+    ) {
         match derivation_tree {
             DerivationTree::External(
                 External::Custom(package, set, _) | External::NoVersions(package, set),
@@ -518,7 +518,12 @@ impl PubGrubReportFormatter<'_> {
                     // Check for no versions due to pre-release options.
                     if !fork_urls.contains_key(name) {
                         self.prerelease_available_hint(
-                            package, name, set, selector, markers, &mut hints,
+                            package,
+                            name,
+                            set,
+                            selector,
+                            markers,
+                            output_hints,
                         );
                     }
                 }
@@ -532,7 +537,7 @@ impl PubGrubReportFormatter<'_> {
                         index_locations,
                         unavailable_packages,
                         incomplete_packages,
-                        &mut hints,
+                        output_hints,
                     );
                 }
             }
@@ -547,7 +552,7 @@ impl PubGrubReportFormatter<'_> {
                     &**dependency,
                     PubGrubPackageInner::Python(PubGrubPython::Target)
                 ) {
-                    hints.insert(PubGrubHint::RequiresPython {
+                    output_hints.insert(PubGrubHint::RequiresPython {
                         source: self.python_requirement.source(),
                         requires_python: self.python_requirement.target().clone(),
                         package: package.clone(),
@@ -558,27 +563,28 @@ impl PubGrubReportFormatter<'_> {
             }
             DerivationTree::External(External::NotRoot(..)) => {}
             DerivationTree::Derived(derived) => {
-                hints.extend(self.hints(
+                self.generate_hints(
                     &derived.cause1,
                     selector,
                     index_locations,
                     unavailable_packages,
                     incomplete_packages,
                     fork_urls,
                     markers,
-                ));
-                hints.extend(self.hints(
+                    output_hints,
+                );
+                self.generate_hints(
                     &derived.cause2,
                     selector,
                     index_locations,
                     unavailable_packages,
                     incomplete_packages,
                     fork_urls,
                     markers,
-                ));
+                    output_hints,
+                );
             }
-        }
-        hints
+        };
     }
 
     fn index_hints(