rust: Add option to use sccache

Using sccache makes recompilation of rustc and Rust packages faster.

This also makes the rust package visible in menuconfig, in order for the
sccache options to be accessible.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>
(cherry picked from commit b4ec01739865770e3d0432683406844f7fd8e707)

lang/rust/Config.in

+menu "Configuration options (for developers)"
+
+config RUST_SCCACHE
+	bool "Use sccache"
+	help
+	  Shared compilation cache; see https://github.com/mozilla/sccache
+
+config RUST_SCCACHE_DIR
+	string "Set sccache directory" if RUST_SCCACHE
+	default ""
+	help
+	  Store sccache in this directory.
+	  If not set, uses './.sccache'
+
+endmenu

lang/rust/Makefile

@@ -30,7 +30,6 @@ define Package/rust
   TITLE:=Rust Programming Language Compiler
   URL:=https://www.rust-lang.org/
   DEPENDS:=$(RUST_ARCH_DEPENDS)
-  BUILDONLY:=1
 endef
 
 define Package/rust/description
@@ -39,6 +38,10 @@ define Package/rust/description
   guarantee memory safety by using a borrow checker to validate references.
 endef
 
+define Package/rust/config
+  source "$(SOURCE)/Config.in"
+endef
+
 # Rust-lang has an uninstall script
 RUST_UNINSTALL:=$(STAGING_DIR)/host/lib/rustlib/uninstall.sh
 
@@ -80,6 +83,7 @@ define Host/Uninstall
 endef
 
 define Host/Compile
+	$(RUST_SCCACHE_VARS) \
 	CARGO_HOME=$(CARGO_HOME) \
 	OPENWRT_RUSTC_BOOTSTRAP_CACHE=$(DL_DIR)/rustc \
 	$(PYTHON) $(HOST_BUILD_DIR)/x.py \

lang/rust/rust-values.mk

@@ -62,12 +62,23 @@ endif
 # Support only a subset for now.
 RUST_ARCH_DEPENDS:=@(aarch64||arm||i386||i686||mips||mipsel||mips64||mips64el||mipsel||powerpc64||riscv64||x86_64)
 
+ifneq ($(CONFIG_RUST_SCCACHE),)
+  RUST_SCCACHE_DIR:=$(if $(call qstrip,$(CONFIG_RUST_SCCACHE_DIR)),$(call qstrip,$(CONFIG_RUST_SCCACHE_DIR)),$(TOPDIR)/.sccache)
+
+  RUST_SCCACHE_VARS:= \
+	CARGO_INCREMENTAL=0 \
+	RUSTC_WRAPPER=sccache \
+	SCCACHE_DIR=$(RUST_SCCACHE_DIR)
+endif
+
 CARGO_HOST_CONFIG_VARS= \
+	$(RUST_SCCACHE_VARS) \
 	CARGO_HOME=$(CARGO_HOME)
 
 CARGO_HOST_PROFILE:=release
 
 CARGO_PKG_CONFIG_VARS= \
+	$(RUST_SCCACHE_VARS) \
 	CARGO_BUILD_TARGET=$(RUSTC_TARGET_ARCH) \
 	CARGO_HOME=$(CARGO_HOME) \
 	CARGO_PROFILE_RELEASE_OPT_LEVEL=s \