TrustedLen is implemented for Range for every integer primitve T. TrustedLen requires that the upper bound of .size_hint is None if the iterator produces more than usize::MAX elements. <Range as Iterator>.size_hint uses steps_between to produce its result. The implementation of steps_between subtracts the lower from the upper value and casts it to usize. This is implemented undconditionally for u32. On 16bit platforms, the result of the subtraction of two u32 values may not fit inside usize, producing an invalid size_hint.
This means, that e.g. collecting into a Vec using its SpecExtend::spec_extend specialization for TrustedLen allows writing beyond its reserved buffer. One such example input would be (0..(usize::MAX as u32 + 1)).collect::<Vec<_>>().
Reference: #47944 (comment)
/cc @bluss
TrustedLenis implemented for Range for every integer primitve T. TrustedLen requires that the upper bound of.size_hintis None if the iterator produces more thanusize::MAXelements.<Range as Iterator>.size_hintusessteps_betweento produce its result. The implementation ofsteps_betweensubtracts the lower from the upper value and casts it to usize. This is implemented undconditionally for u32. On 16bit platforms, the result of the subtraction of two u32 values may not fit insideusize, producing an invalidsize_hint.This means, that e.g. collecting into a Vec using its
SpecExtend::spec_extendspecialization for TrustedLen allows writing beyond its reserved buffer. One such example input would be(0..(usize::MAX as u32 + 1)).collect::<Vec<_>>().Reference: #47944 (comment)
/cc @bluss