Off by one error in str::index_mut (specifically) for RangeToInclusive (specifically)

[ExpHP]

Caught while fleshing out a much beefier test suite for SliceIndex, which I plan to submit as a PR. (this Issue is here just in case that PR never happens).

rust/src/libcore/str/mod.rs

Lines 2107 to 2115 in 1ef1563

	fn index_mut(self, slice: &mut str) -> &mut Self::Output {
	assert!(self.end != usize::max_value(),
	"attempted to index str up to maximum usize");
	if slice.is_char_boundary(self.end) {
	unsafe { self.get_unchecked_mut(slice) }
	} else {
	super::slice_error_fail(slice, 0, self.end + 1)
	}
	}

The unicode character boundary check here should be looking at self.end + 1. Better yet, the method could just delegate to RangeTo instead. (note: it can't delegate to get_mut here without NLL, which seems to be a leading factor as to how this function ended up this way)

[alexcrichton]

Not technically a regression but tagging it as such to ensure that we don't forget about it.