Log most recently modified file for cache-keys (#16338)

For https://github.com/astral-sh/uv/issues/16336. We previously weren't
telling the user which file is responsible for rebuilding.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

Cargo.lock

@@ -5504,6 +5504,7 @@ dependencies = [
  "thiserror 2.0.17",
  "toml",
  "tracing",
+ "uv-fs",
  "walkdir",
 ]
 

crates/uv-cache-info/Cargo.toml

@@ -16,6 +16,8 @@ doctest = false
 workspace = true
 
 [dependencies]
+uv-fs = { workspace = true }
+
 fs-err = { workspace = true }
 globwalk = { workspace = true }
 schemars = { workspace = true, optional = true }

crates/uv-cache-info/src/cache_info.rs

@@ -1,11 +1,12 @@
 use std::borrow::Cow;
-use std::cmp::max;
 use std::collections::BTreeMap;
 use std::path::{Path, PathBuf};
 
 use serde::Deserialize;
 use tracing::{debug, warn};
 
+use uv_fs::Simplified;
+
 use crate::git_info::{Commit, Tags};
 use crate::glob::cluster_globs;
 use crate::timestamp::Timestamp;
@@ -63,7 +64,7 @@ impl CacheInfo {
     pub fn from_directory(directory: &Path) -> Result<Self, CacheInfoError> {
         let mut commit = None;
         let mut tags = None;
-        let mut timestamp = None;
+        let mut last_changed: Option<(PathBuf, Timestamp)> = None;
         let mut directories = BTreeMap::new();
         let mut env = BTreeMap::new();
 
@@ -128,7 +129,12 @@ impl CacheInfo {
                         );
                         continue;
                     }
-                    timestamp = max(timestamp, Some(Timestamp::from_metadata(&metadata)));
+                    let timestamp = Timestamp::from_metadata(&metadata);
+                    if last_changed.as_ref().is_none_or(|(_, prev_timestamp)| {
+                        *prev_timestamp < Timestamp::from_metadata(&metadata)
+                    }) {
+                        last_changed = Some((path, timestamp));
+                    }
                 }
                 CacheKey::Directory { dir } => {
                     // Treat the path as a directory.
@@ -258,14 +264,25 @@ impl CacheInfo {
                         }
                         continue;
                     }
-                    timestamp = max(timestamp, Some(Timestamp::from_metadata(&metadata)));
+                    let timestamp = Timestamp::from_metadata(&metadata);
+                    if last_changed.as_ref().is_none_or(|(_, prev_timestamp)| {
+                        *prev_timestamp < Timestamp::from_metadata(&metadata)
+                    }) {
+                        last_changed = Some((entry.into_path(), timestamp));
+                    }
                 }
             }
         }
 
+        let timestamp = if let Some((path, timestamp)) = last_changed {
             debug!(
-            "Computed cache info: {timestamp:?}, {commit:?}, {tags:?}, {env:?}, {directories:?}"
+                "Computed cache info: {timestamp:?}, {commit:?}, {tags:?}, {env:?}, {directories:?}. Most recently modified: {}",
+                path.user_display()
             );
+            Some(timestamp)
+        } else {
+            None
+        };
 
         Ok(Self {
             timestamp,