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3b2580914b
Add `must_use` attribute to `len_utf8` and `len_utf16`. The `len_utf8` and `len_utf16` methods in `char` should have the `must_use` attribute. The somewhat similar method `<[T]>::len` has had this attribute since #95274. Considering that these two methods would most likely be used to test the size of a buffer (before a call to `encode_utf8` or `encode_utf16`), *not* using their return values could indicate a bug. According to ["When to add `#[must_use]`](https://std-dev-guide.rust-lang.org/policy/must-use.html), this is **not** considered a breaking change (and could be reverted again at a later time).