Make building Rust targets faster in eng builds

This CL makes the following changes when building an eng variant to
speed up compilation, possibly at the expense of the performance of the
generated binary:
* Disables LTO
* Enables incremental compilation
* Sets codegen units to 256 to increase parallelism (default set when
  enabling incremental compilation)

These changes reduce the time taken to compile all of Android's Rust
code from 16m20s to 12m10s.

Test: lunch aosp_oriole-eng && m clean && m rust
Bug: https://b.corp.google.com/issues/289094772
Change-Id: I97f1675c67ded69120b0c0e0fb5608aa9314a866

1 files changed, 15 insertions, 15 deletions

diff --git a/rust/builder.go b/rust/builder.go
index c855cfbd2..65995ed2a 100644
--- a/rust/builder.go
+++ b/rust/builder.go
@@ -122,8 +122,6 @@ func init() {
 
 func TransformSrcToBinary(ctx ModuleContext, mainSrc android.Path, deps PathDeps, flags Flags,
 	outputFile android.WritablePath) buildOutput {
-	flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C lto=thin")
-
 	return transformSrctoCrate(ctx, mainSrc, deps, flags, outputFile, "bin")
 }
 
@@ -134,20 +132,16 @@ func TransformSrctoRlib(ctx ModuleContext, mainSrc android.Path, deps PathDeps,
 
 func TransformSrctoDylib(ctx ModuleContext, mainSrc android.Path, deps PathDeps, flags Flags,
 	outputFile android.WritablePath) buildOutput {
-	flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C lto=thin")
-
 	return transformSrctoCrate(ctx, mainSrc, deps, flags, outputFile, "dylib")
 }
 
 func TransformSrctoStatic(ctx ModuleContext, mainSrc android.Path, deps PathDeps, flags Flags,
 	outputFile android.WritablePath) buildOutput {
-	flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C lto=thin")
 	return transformSrctoCrate(ctx, mainSrc, deps, flags, outputFile, "staticlib")
 }
 
 func TransformSrctoShared(ctx ModuleContext, mainSrc android.Path, deps PathDeps, flags Flags,
 	outputFile android.WritablePath) buildOutput {
-	flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C lto=thin")
 	return transformSrctoCrate(ctx, mainSrc, deps, flags, outputFile, "cdylib")
 }
 
@@ -263,6 +257,21 @@ func transformSrctoCrate(ctx ModuleContext, main android.Path, deps PathDeps, fl
 
 	inputs = append(inputs, main)
 
+	if ctx.Config().Eng() {
+		// Per https://doc.rust-lang.org/rustc/codegen-options/index.html#codegen-units
+		// incremental building implies codegen-units=256
+		incrementalPath := android.PathForModuleOut(ctx, "rustc-incremental").String()
+		flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C incremental="+incrementalPath)
+
+	} else {
+		flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C codegen-units=1")
+
+		if !(ctx.RustModule().Rlib() || ctx.RustModule().ProcMacro()) {
+			flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-Z dylib-lto")
+			flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C lto=thin")
+		}
+	}
+
 	// Collect rustc flags
 	rustcFlags = append(rustcFlags, flags.GlobalRustFlags...)
 	rustcFlags = append(rustcFlags, flags.RustFlags...)
@@ -278,15 +287,6 @@ func transformSrctoCrate(ctx ModuleContext, main android.Path, deps PathDeps, fl
 	// Suppress an implicit sysroot
 	rustcFlags = append(rustcFlags, "--sysroot=/dev/null")
 
-	// Enable incremental compilation if requested by user
-	if ctx.Config().IsEnvTrue("SOONG_RUSTC_INCREMENTAL") {
-		incrementalPath := android.PathForOutput(ctx, "rustc").String()
-
-		rustcFlags = append(rustcFlags, "-C incremental="+incrementalPath)
-	} else {
-		rustcFlags = append(rustcFlags, "-C codegen-units=1")
-	}
-
 	// Disallow experimental features
 	modulePath := ctx.ModuleDir()
 	if !(android.IsThirdPartyPath(modulePath) || strings.HasPrefix(modulePath, "prebuilts")) {