[Experiment] Add `never` as a type alias for `!`#79707

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Julian-Wollersberger wants to merge 4 commits intorust-lang:masterfrom

Julian-Wollersberger:never_primitive

Commits on Dec 2, 2020

Add type alias `never` for `!` and put it in the prelude.
Julian Wollersberger
committed
Change some return types to `never`.
Julian Wollersberger
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Change some tests, to see that `never` works in at least some cases.
Julian Wollersberger
committed

Commits on Dec 4, 2020

Test that existing types called `never` shadow the new `never` from the prelude.
Julian Wollersberger
committed