openharmony v4.1 编译错误Assignment had no effect

You set the variable "IPC_CORE_ROOT" here and it was unused before it went out of scope.

[OHOS INFO] root_out_dir=//out/hispark_pegasus/hispark_pegasus_mini_system
[OHOS INFO] root_build_dir=//out/hispark_pegasus/hispark_pegasus_mini_system
[OHOS INFO] root_gen_dir=//out/hispark_pegasus/hispark_pegasus_mini_system/gen
[OHOS INFO] current_toolchain=//build/lite/toolchain:riscv32-unknown-elf
[OHOS INFO] host_toolchain=//build/toolchain/linux:clang_x64
[OHOS INFO] ohos_build_compiler: gcc
[OHOS INFO] ohos_kernel_type: liteos_m
[OHOS INFO] ERROR at //foundation/communication/ipc/BUILD.gn:19:17: Assignment had no effect.
[OHOS INFO] IPC_CORE_ROOT = "$SUBSYSTEM_DIR/ipc/native"
[OHOS INFO]                 ^--------------------------
[OHOS INFO] You set the variable "IPC_CORE_ROOT" here and it was unused before it went
[OHOS INFO] out of scope.
[OHOS INFO] See //build/ohos/ohos_part.gni:25:16: which caused the file to be included.
[OHOS INFO]     _deps += [ get_label_info(module_label, "label_with_toolchain") ]
[OHOS INFO]                ^--------------------------------------------------

[OHOS ERROR] ERROR at //foundation/communication/ipc/BUILD.gn:19:17: Assignment had no effect.

[OHOS ERROR] IPC_CORE_ROOT = "$SUBSYSTEM_DIR/ipc/native"

[OHOS ERROR]                 ^--------------------------

[OHOS ERROR] You set the variable "IPC_CORE_ROOT" here and it was unused before it went

[OHOS ERROR] out of scope.

[OHOS ERROR] See //build/ohos/ohos_part.gni:25:16: which caused the file to be included.

[OHOS ERROR]     _deps += [ get_label_info(module_label, "label_with_toolchain") ]

[OHOS ERROR]                ^--------------------------------------------------

"IPC_CORE_ROOT"变量在离开作用域时没有被用上，一直不理解gn语法，后来各种试，然后再参考手册中找到个方法。

解决方法：

在gn-reference找到：

not_needed: Mark variables from scope as not needed.

not_needed(variable_list_or_star, variable_to_ignore_list = [])
not_needed(from_scope, variable_list_or_star, variable_to_ignore_list = [])

Mark the variables in the current or given scope as not needed, which means you will not get an error about unused variables for these. The variable_to_ignore_list allows excluding variables from "all matches" if variable_list_or_star is "*".

Example

not_needed("*", [ "config" ])
not_needed([ "data_deps", "deps" ])
not_needed(invoker, "*", [ "config" ])
not_needed(invoker, [ "data_deps", "deps" ])

没有被使用的变量必须使用not_needed函数标记出来，否则会报错。

故在foundation/communication/ipc/BUILD.gn中没有使用到的分支，标记下：

SUBSYSTEM_DIR = "//foundation/communication/ipc"
IPC_CORE_ROOT = "$SUBSYSTEM_DIR/ipc/native"

group("ipc_components") {
  if (os_level == "standard") {
    deps = [
      "$IPC_CORE_ROOT/src/core:ipc_common",
      "$SUBSYSTEM_DIR/interfaces/innerkits/ipc_core:ipc_core",
      "$SUBSYSTEM_DIR/interfaces/innerkits/ipc_single:ipc_single",
    ]
    if (!build_ohos_sdk) {
      deps += [
        "$SUBSYSTEM_DIR/interfaces/innerkits/libdbinder:libdbinder",
        "$SUBSYSTEM_DIR/interfaces/innerkits/rust:rust_ipc_component",
      ]
    }
  } else {
    # 这个分支没有使用到变量IPC_CORE_ROOT，故用下边函数标记下
    not_needed([IPC_CORE_ROOT])
    deps = [ "$SUBSYSTEM_DIR/interfaces/innerkits/c:rpc" ]
  }
}

再次执行编译命令不再报错。