11740: Insert #[must_use] in generate_is_empty_from_len r=lnicola a=Walther
Inserts `#[must_use]` for the generated `is_empty` methods. I also added the `#[must_use]` for the `len()` methods in the documentation examples and tests for this generator while at it, to make sure they too encourage the use of the annotation.
From discussion in https://github.com/rust-analyzer/rust-analyzer/issues/11736 and https://github.com/rust-analyzer/rust-analyzer/pull/11738
Co-authored-by: Walther <veeti.haapsamo@gmail.com>
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