add crate_root property to rust modules

The crate_root property will be used to specify the entry point for a
rustc compilation. This will allow the srcs property to be used to
collect all src inputs to rustc rather than just the entry point.

Bug: 286077158
Test: m libnum_traits
Change-Id: I1a167182305dcc11cc927d562ceed622153111d3

rust/compiler.go

@@ -73,6 +73,15 @@ type BaseCompilerProperties struct {
 	// If no source file is defined, a single generated source module can be defined to be used as the main source.
 	Srcs []string `android:"path,arch_variant"`
 
+	// Entry point that is passed to rustc to begin the compilation. E.g. main.rs or lib.rs.
+	// When this property is set,
+	//    * sandboxing is enabled for this module, and
+	//    * the srcs attribute is interpreted as a list of all source files potentially
+	//          used in compilation, including the entrypoint, and
+	//    * compile_data can be used to add additional files used in compilation that
+	//          not directly used as source files.
+	Crate_root *string `android:"path,arch_variant"`
+
 	// name of the lint set that should be used to validate this module.
 	//
 	// Possible values are "default" (for using a sensible set of lints
@@ -511,6 +520,8 @@ func srcPathFromModuleSrcs(ctx ModuleContext, srcs []string) (android.Path, andr
 		ctx.PropertyErrorf("srcs", "only a single generated source module can be defined without a main source file.")
 	}
 
+	// TODO: b/297264540 - once all modules are sandboxed, we need to select the proper
+	// entry point file from Srcs rather than taking the first one
 	paths := android.PathsForModuleSrc(ctx, srcs)
 	return paths[srcIndex], paths[1:]
 }