What character encoding do cache strings use?

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I noticed that the documentation for string(SUBSTRING…) says this:

Both <begin> and <length> are counted in bytes, so care must be exercised if <string> could contain multi-byte characters.

This got me thinking: what character encoding do strings in the CMake Language use? I suppose that you could use file(READ…) to create strings with any character encoding, but I’m more thinking about code like this:

cmake_minimum_required(VERSION 3.30.5)
project(example_project)

set(EXAMPLE_VARIABLE "café" CACHE STRING "Example cache variable")

In that example, what character encoding will EXAMPLE_VARIABLE use? Is there any way to ensure that it will use UTF-8?