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log: differ formatted args evaluation

François Laignel requested to merge fengalin/gstreamer-rs:log-alloc-reduction into main

When the format_args! macro is used, formatted args are evaluated in order to form the argument collection that ends up being formatted when requested to do so.

For the log macro, this means that any argument that is an expression was evaluated even if the log level was below current threshold. This can be examplified by the following code:

    gst::debug!(CAT, "{}", {
        println!("fmt arg executed");
        "A debug log"
    });

This used to print "fmt arg executed" even when the log threshold for CAT was filtering debug out.

Note that this could break existing code that would incorrectly rely on the side effect.

CI passes successfully for gst-plugins-rs.

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