From 4c91fda4527df153dd8ae92d743fa696a5df3ab6 Mon Sep 17 00:00:00 2001
From: Weihang Lo <me@weihanglo.tw>
Date: Tue, 30 Dec 2025 11:23:37 -0500
Subject: [PATCH] fix(log): add `dependencies` field to `UnitRegistered`

Track unit dependency graph in `LogMessage::UnitRegistered`.

This gives the unit-graph information before running the build,
which will help enable root cause analysis for cascading rebuilds.
---
 src/cargo/ops/cargo_compile/mod.rs    | 9 +++++++++
 src/cargo/ops/cargo_report/timings.rs | 1 +
 src/cargo/util/log_message.rs         | 3 +++
 tests/testsuite/build_analysis.rs     | 8 ++++++++
 4 files changed, 21 insertions(+)

diff --git a/src/cargo/ops/cargo_compile/mod.rs b/src/cargo/ops/cargo_compile/mod.rs
index d9aedc61d52..0640db999ab 100644
--- a/src/cargo/ops/cargo_compile/mod.rs
+++ b/src/cargo/ops/cargo_compile/mod.rs
@@ -535,6 +535,14 @@ pub fn create_bcx<'a, 'gctx>(
 
         for (index, unit) in units.into_iter().enumerate() {
             let index = index as u64;
+            let dependencies = unit_graph
+                .get(unit)
+                .map(|deps| {
+                    deps.iter()
+                        .filter_map(|dep| unit_to_index.get(&dep.unit).copied())
+                        .collect()
+                })
+                .unwrap_or_default();
             logger.log(LogMessage::UnitRegistered {
                 package_id: unit.pkg.package_id().to_spec(),
                 target: (&unit.target).into(),
@@ -547,6 +555,7 @@ pub fn create_bcx<'a, 'gctx>(
                     .map(|s| s.as_str().to_owned())
                     .collect(),
                 requested: root_unit_indexes.contains(&index),
+                dependencies,
             });
         }
         let elapsed = ws.gctx().creation_time().elapsed().as_secs_f64();
diff --git a/src/cargo/ops/cargo_report/timings.rs b/src/cargo/ops/cargo_report/timings.rs
index c12ef48a1a0..5d649183813 100644
--- a/src/cargo/ops/cargo_report/timings.rs
+++ b/src/cargo/ops/cargo_report/timings.rs
@@ -177,6 +177,7 @@ fn prepare_context(log: &Path, run_id: &RunId) -> CargoResult<RenderContext<'sta
                 index,
                 features,
                 requested,
+                dependencies: _,
             } => {
                 if requested {
                     requested_units.insert(index);
diff --git a/src/cargo/util/log_message.rs b/src/cargo/util/log_message.rs
index 77a24b3198b..d742de06026 100644
--- a/src/cargo/util/log_message.rs
+++ b/src/cargo/util/log_message.rs
@@ -86,6 +86,9 @@ pub enum LogMessage {
         /// like via the `-p` flag or the default workspace members.
         #[serde(default, skip_serializing_if = "std::ops::Not::not")]
         requested: bool,
+        /// Unit indices that this unit depends on.
+        #[serde(default, skip_serializing_if = "Vec::is_empty")]
+        dependencies: Vec<u64>,
     },
     /// Emitted when a compilation unit starts.
     UnitStarted {
diff --git a/tests/testsuite/build_analysis.rs b/tests/testsuite/build_analysis.rs
index cbaa68e2ddf..1b5fa064d82 100644
--- a/tests/testsuite/build_analysis.rs
+++ b/tests/testsuite/build_analysis.rs
@@ -606,6 +606,11 @@ fn log_msg_unit_graph() {
     "timestamp": "[..]T[..]Z"
   },
   {
+    "dependencies": [
+      0,
+      1,
+      4
+    ],
     "index": 2,
     "mode": "doc",
     "package_id": "path+[ROOTURL]/foo#0.0.0",
@@ -633,6 +638,9 @@ fn log_msg_unit_graph() {
     "timestamp": "[..]T[..]Z"
   },
   {
+    "dependencies": [
+      3
+    ],
     "index": 4,
     "mode": "run-custom-build",
     "package_id": "path+[ROOTURL]/foo#0.0.0",