diff --git a/ui/build/ninja.go b/ui/build/ninja.go
index 1d4285f..e2a568f 100644
--- a/ui/build/ninja.go
+++ b/ui/build/ninja.go
@@ -36,10 +36,16 @@
 	ninjaWeightListFileName = ".ninja_weight_list"
 )
 
+// Runs ninja with the arguments from the command line, as found in
+// config.NinjaArgs().
+func runNinjaForBuild(ctx Context, config Config) {
+	runNinja(ctx, config, config.NinjaArgs())
+}
+
 // Constructs and runs the Ninja command line with a restricted set of
 // environment variables. It's important to restrict the environment Ninja runs
 // for hermeticity reasons, and to avoid spurious rebuilds.
-func runNinjaForBuild(ctx Context, config Config) {
+func runNinja(ctx Context, config Config, ninjaArgs []string) {
 	ctx.BeginTrace(metrics.PrimaryNinja, "ninja")
 	defer ctx.EndTrace()
 
@@ -88,7 +94,7 @@
 			"-w", "missingdepfile=err",
 		}
 	}
-	args = append(args, config.NinjaArgs()...)
+	args = append(args, ninjaArgs...)
 
 	var parallel int
 	if config.UseRemoteBuild() {
@@ -244,6 +250,8 @@
 			"RUST_LOG",
 
 			// SOONG_USE_PARTIAL_COMPILE only determines which half of the rule we execute.
+			// When it transitions true => false, we build phony target "partialcompileclean",
+			// which removes all files that could have been created while it was true.
 			"SOONG_USE_PARTIAL_COMPILE",
 
 			// Directory for ExecutionMetrics