For bootstrapping purposes, this commit does not remove all uses of
the keyword "pure" -- doing so would cause the compiler to no longer
bootstrap due to some syntax extensions ("deriving" in particular).
Instead, it makes the compiler ignore "pure". Post-snapshot, we can
remove "pure" from the language.
There are quite a few (~100) borrow check errors that were essentially
all the result of mutable fields or partial borrows of `@mut`. Per
discussions with Niko I think we want to allow partial borrows of
`@mut` but detect obvious footguns. We should also improve the error
message when `@mut` is erroneously reborrowed.
needed.
Regarding soundness: there was a subtle bug in how it was done before; see the
compile-fail test for an example.
Regarding reborrowing: reborrowing allows mut and const
slices/borrowed-pointers to be used with pure fns that expect immutable data.
r=brson
