Re-land "Make building Rust targets faster in eng builds"

This reverts commit af9fc54, reversing
changes made to f617e18.

Additionally, patch set #2 adds a fix for b/b/325547632.  We should not
pass the `-Z dylib-lto` flag.

Test: m rust
Test: ABTD rialto_test
Bug: https://b.corp.google.com/issues/289094772
Change-Id: I0b7cb62866391968029a23ab2daade28628944ea

Author: chriswailes

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 +257,20 @@ 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, "-C lto=thin")
+		}
+	}
+
 	// Collect rustc flags
 	rustcFlags = append(rustcFlags, flags.GlobalRustFlags...)
 	rustcFlags = append(rustcFlags, flags.RustFlags...)
@@ -278,15 +286,6 @@ 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")) {