Log versions tried from batch prefetch

This is required for evaluating #3087

Closes #2270 - This is less invasive compared to the other PR, we can revisit number of network/build request tracking later

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

@@ -1,5 +1,6 @@
 use std::cmp::min;
 
+use itertools::Itertools;
 use pubgrub::range::Range;
 use rustc_hash::FxHashMap;
 use tokio::sync::mpsc::Sender;
@@ -173,4 +174,23 @@ impl BatchPrefetcher {
             || (num_tried >= 20 && num_tried - previous_prefetch >= 20);
         (num_tried, do_prefetch)
     }
+
+    /// Log stats about how many versions we tried.
+    ///
+    /// Note that they may be inflated when we count the same version repeatedly during
+    /// backtracking.
+    pub(crate) fn log_tried_versions(&self) {
+        let total_versions: usize = self.tried_versions.values().sum();
+        let mut tried_versions: Vec<_> = self.tried_versions.iter().collect();
+        tried_versions.sort_by(|(p1, c1), (p2, c2)| {
+            c1.cmp(c2)
+                .reverse()
+                .then(p1.to_string().cmp(&p2.to_string()))
+        });
+        let counts = tried_versions
+            .iter()
+            .map(|(package, count)| format!("{package} {count}"))
+            .join(", ");
+        debug!("Tried {total_versions} versions: {counts}");
+    }
 }

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

@@ -14,7 +14,7 @@ use pubgrub::range::Range;
 use pubgrub::solver::{Incompatibility, State};
 use rustc_hash::{FxHashMap, FxHashSet};
 use tokio_stream::wrappers::ReceiverStream;
-use tracing::{debug, info_span, instrument, trace, warn, Instrument};
+use tracing::{debug, enabled, info_span, instrument, trace, warn, Instrument, Level};
 
 use distribution_types::{
     BuiltDist, Dist, DistributionMetadata, IncompatibleDist, IncompatibleSource, IncompatibleWheel,
@@ -323,6 +323,9 @@ impl<
                         priorities.get(package).unwrap_or_default()
                     })
             else {
+                if enabled!(Level::DEBUG) {
+                    prefetcher.log_tried_versions();
+                }
                 let selection = state.partial_solution.extract_solution();
                 return ResolutionGraph::from_state(
                     &selection,
@@ -482,6 +485,9 @@ impl<
                             .iter()
                             .any(|(dependency, _)| dependency == package) =>
                     {
+                        if enabled!(Level::DEBUG) {
+                            prefetcher.log_tried_versions();
+                        }
                         return Err(PubGrubError::SelfDependency {
                             package: package.clone(),
                             version: version.clone(),