refactor: read JSON fingerprint reading only when needed

src/cargo/core/compiler/fingerprint/mod.rs

@@ -373,6 +373,7 @@ use serde::{Deserialize, Serialize};
 use tracing::{debug, info};
 
 use crate::core::compiler::unit_graph::UnitDep;
+use crate::core::Verbosity;
 use crate::core::Package;
 use crate::util::errors::CargoResult;
 use crate::util::interning::InternedString;
@@ -415,7 +416,9 @@ pub fn prepare_target(cx: &mut Context<'_, '_>, unit: &Unit, force: bool) -> Car
     // information about failed comparisons to aid in debugging.
     let fingerprint = calculate(cx, unit)?;
     let mtime_on_use = cx.bcx.config.cli_unstable().mtime_on_use;
-    let dirty_reason = compare_old_fingerprint(unit, &loc, &*fingerprint, mtime_on_use, force);
+    let verbosity = cx.bcx.config.shell().verbosity();
+    let dirty_reason =
+        compare_old_fingerprint(unit, &loc, &*fingerprint, mtime_on_use, force, verbosity);
 
     let Some(dirty_reason) = dirty_reason else {
         return Ok(Job::new_fresh());
@@ -1741,35 +1744,67 @@ fn compare_old_fingerprint(
     new_fingerprint: &Fingerprint,
     mtime_on_use: bool,
     forced: bool,
+    verbosity: Verbosity,
 ) -> Option<DirtyReason> {
-    let compare = _compare_old_fingerprint(old_hash_path, new_fingerprint, mtime_on_use);
-    log_compare(unit, &compare);
+    if mtime_on_use {
+        // update the mtime so other cleaners know we used it
+        let t = FileTime::from_system_time(SystemTime::now());
+        debug!("mtime-on-use forcing {:?} to {}", old_hash_path, t);
+        paths::set_file_time_no_err(old_hash_path, t);
+    }
+
+    let tracing_enabled = tracing::enabled!(tracing::Level::INFO);
+    let verbose = !matches!(verbosity, Verbosity::Quiet | Verbosity::Normal);
+
+    let compare =
+        _compare_old_fingerprint(old_hash_path, new_fingerprint, tracing_enabled || verbose);
+
+    if tracing_enabled {
+        // Logs the result of fingerprint comparison.
+        // TODO: Obsolete and mostly superseded by [`DirtyReason`]. Could be removed.
+        match compare.as_ref() {
+            Ok(None) => {}
+            Ok(Some(reason)) => {
+                info!(
+                    "fingerprint dirty for {}/{:?}/{:?}",
+                    unit.pkg, unit.mode, unit.target,
+                );
+                info!("    dirty: {reason:?}");
+            }
+            Err(e) => {
+                info!(
+                    "fingerprint error for {}/{:?}/{:?}",
+                    unit.pkg, unit.mode, unit.target,
+                );
+                info!("    err: {e:?}");
+            }
+        }
+    }
+
     match compare {
         Ok(None) if forced => Some(DirtyReason::Forced),
         Ok(reason) => reason,
         Err(_) => Some(DirtyReason::FreshBuild),
     }
 }
 
+/// If detail is not required, just returns a [`DirtyReason::NothingObvious`]
+/// instead of comparing JSON fingerprint files.
 fn _compare_old_fingerprint(
     old_hash_path: &Path,
     new_fingerprint: &Fingerprint,
-    mtime_on_use: bool,
+    detail_required: bool,
 ) -> CargoResult<Option<DirtyReason>> {
     let old_fingerprint_short = paths::read(old_hash_path)?;
 
-    if mtime_on_use {
-        // update the mtime so other cleaners know we used it
-        let t = FileTime::from_system_time(SystemTime::now());
-        debug!("mtime-on-use forcing {:?} to {}", old_hash_path, t);
-        paths::set_file_time_no_err(old_hash_path, t);
-    }
-
     let new_hash = new_fingerprint.hash_u64();
 
     if util::to_hex(new_hash) == old_fingerprint_short && new_fingerprint.fs_status.up_to_date() {
         return Ok(None);
     }
+    if !detail_required {
+        return Ok(Some(DirtyReason::NothingObvious));
+    }
 
     let old_fingerprint_json = paths::read(&old_hash_path.with_extension("json"))?;
     let old_fingerprint: Fingerprint = serde_json::from_str(&old_fingerprint_json)
@@ -1785,29 +1820,6 @@ fn _compare_old_fingerprint(
     Ok(Some(new_fingerprint.compare(&old_fingerprint)))
 }
 
-/// Logs the result of fingerprint comparison.
-///
-/// TODO: Obsolete and mostly superseded by [`DirtyReason`]. Could be removed.
-fn log_compare(unit: &Unit, compare: &CargoResult<Option<DirtyReason>>) {
-    match compare {
-        Ok(None) => {}
-        Ok(Some(reason)) => {
-            info!(
-                "fingerprint dirty for {}/{:?}/{:?}",
-                unit.pkg, unit.mode, unit.target,
-            );
-            info!("    dirty: {reason:?}");
-        }
-        Err(e) => {
-            info!(
-                "fingerprint error for {}/{:?}/{:?}",
-                unit.pkg, unit.mode, unit.target,
-            );
-            info!("    err: {e:?}");
-        }
-    }
-}
-
 /// Parses Cargo's internal [`EncodedDepInfo`] structure that was previously
 /// serialized to disk.
 ///