feat(build-analysis): emit rebuild reason log entry

Author: weihanglo

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

@@ -397,6 +397,7 @@ use crate::core::compiler::unit_graph::UnitDep;
 use crate::util;
 use crate::util::errors::CargoResult;
 use crate::util::interning::InternedString;
+use crate::util::log_message::LogMessage;
 use crate::util::{StableHasher, internal, path_args};
 use crate::{CARGO_ENV, GlobalContext};
 
@@ -448,6 +449,18 @@ pub fn prepare_target(
         return Ok(Job::new_fresh());
     };
 
+    if let Some(logger) = bcx.logger {
+        // Dont log FreshBuild as it is noisy.
+        if !dirty_reason.is_fresh_build() {
+            logger.log(LogMessage::Rebuild {
+                package_id: unit.pkg.package_id().to_spec(),
+                target: unit.target.clone(),
+                mode: unit.mode,
+                cause: dirty_reason.clone(),
+            });
+        }
+    }
+
     // We're going to rebuild, so ensure the source of the crate passes all
     // verification checks before we build it.
     //

src/cargo/util/log_message.rs

@@ -10,6 +10,7 @@ use serde::Serialize;
 use crate::core::Target;
 use crate::core::compiler::CompilationSection;
 use crate::core::compiler::CompileMode;
+use crate::core::compiler::fingerprint::DirtyReason;
 
 /// A log message.
 ///
@@ -39,6 +40,13 @@ pub enum LogMessage {
         #[serde(skip_serializing_if = "Vec::is_empty")]
         sections: Vec<(String, CompilationSection)>,
     },
+    /// Emitted when a unit needs to be rebuilt.
+    Rebuild {
+        package_id: PackageIdSpec,
+        target: Target,
+        mode: CompileMode,
+        cause: DirtyReason,
+    },
 }
 
 impl LogMessage {

tests/testsuite/build_analysis.rs

@@ -227,6 +227,23 @@ fn log_rebuild_reason_file_changed() {
     "...": "{...}",
     "reason": "build-started"
   },
+  {
+    "cause": {
+      "dirty_reason": "fs-status-outdated",
+      "fs_status": "stale-item",
+      "reference": "[ROOT]/foo/target/debug/.fingerprint/foo-[HASH]/dep-lib-foo",
+      "reference_mtime": "{...}",
+      "stale": "[ROOT]/foo/src/lib.rs",
+      "stale_item": "changed-file",
+      "stale_mtime": "{...}"
+    },
+    "mode": "check",
+    "package_id": "path+[ROOTURL]/foo#0.0.0",
+    "reason": "rebuild",
+    "run_id": "[..]T[..]Z-[..]",
+    "target": "{...}",
+    "timestamp": "[..]T[..]Z"
+  },
   {
     "...": "{...}",
     "reason": "timing-info"