By skipping the call to `copy` with a zero length. This makes it closer
to `push`. This speeds up rustc (which uses `SmallVec` extensively) by
2% on one benchmark.
Also clarify the panic condition.
Miri does not check all of Stacked Borrows (the prototype aliasing model
for Rust) without -Zmiri-tag-raw-pointers. This enables the check in CI,
and makes a few adjustments to fix places where pointers were
invalidated by construction or use of a mutable reference.
Previously, the test suite only had one trip through the tail-shifting
code in Drain::drop, and that is in the heap state.
In the current implementation, a tail-shifting drain while in the inline
state produces potentially dangerous aliasing which is currently
accepted by default Miri and rejected with -Ztrack-raw-pointers.
Adding this test case ensures that if this ever becomes an actual
problem it will be easy to find.
Release notes:
* The `union` feature is now compatible with stable Rust 1.49 (#248).
* Fixed warnings when compiling with Rust 1.51 nightly (#242, #246).
Nikita Popov