EncodeUtf8::as_slice results in suboptimal code

[nagisa]

// We know for sure this method cannot slice out-of-bounds because:
// * 0 ≤ self.pos ≤ 3
// * self.buf.len() = 4
//
// This way the slicing will always succeed, but LLVM is incapable of figuring out both
// these conditions hold, resulting in suboptimal code, especially after inlining,
// containing bound checks and unwinding support code.

[durka]

Where are these comments from? Anyway, sounds like a case for the new panic-safe slicing stuff (#36340).

[nagisa]

Here

[nagisa]

Not applicable anymore, as a different approach is now used.