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Rollup merge of #80670 - the8472:fix-zip-trusted-random-access-composition, r=m-ou-se

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TrustedRandomAaccess specialization composes incorrectly for nested iter::Zips

I found this while working on improvements for TRA.

After partially consuming a Zip adapter and then wrapping it into another Zip where the adapters use their `TrustedRandomAccess` specializations leads to the outer adapter returning elements which should have already been consumed.

If the optimizer gets tripped up by the addition this might affect performance for chained `zip()` iterators even when the inner one is not partially advanced but it would require more extensive fixes to `TrustedRandomAccess` to communicate those offsets earlier.

Included test fails on nightly, [playground link](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=24fa1edf8a104ff31f5a24830593b01f)
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Mara Bos 2021-01-16 17:29:51 +00:00 • committed by GitHub
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library/core/tests/lib.rs
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@ -75,6 +75,7 @@
#![feature(const_option)]
#![feature(integer_atomics)]
#![feature(slice_group_by)]
#![feature(trusted_random_access)]
#![deny(unsafe_op_in_unsafe_fn)]
extern crate test;