Using "mem::unitialized()" can lead to "Attempted to instantiate uninhabited type"

[tomaka]

The following code panics:

fn main() {
    enum Void {}
    let sv = smallvec::SmallVec::<[Void; 8]>::new();
}

With:

thread 'main' panicked at 'Attempted to instantiate uninhabited type [main::Void; 8]', /rustc/ceb2512144d1fc26330e85fb9d41c22ba1866259/src/libcore/mem.rs:630:5
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:70
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:58
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   5: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:385
   6: rust_begin_unwind
             at src/libstd/panicking.rs:312
   7: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
   8: core::panicking::panic
             at src/libcore/panicking.rs:49
   9: core::mem::uninitialized
             at /rustc/ceb2512144d1fc26330e85fb9d41c22ba1866259/src/libcore/mem.rs:630
  10: <smallvec::SmallVec<A>>::new
             at /home/pierre/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/smallvec-0.6.8/lib.rs:402
  11: playpen::main
             at src/main.rs:3
  12: std::rt::lang_start::{{closure}}
             at /rustc/ceb2512144d1fc26330e85fb9d41c22ba1866259/src/libstd/rt.rs:64
  13: std::panicking::try::do_call
             at src/libstd/rt.rs:49
             at src/libstd/panicking.rs:297
  14: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:92
  15: std::rt::lang_start_internal
             at src/libstd/panicking.rs:276
             at src/libstd/panic.rs:388
             at src/libstd/rt.rs:48
  16: std::rt::lang_start
             at /rustc/ceb2512144d1fc26330e85fb9d41c22ba1866259/src/libstd/rt.rs:64
  17: main
  18: __libc_start_main
  19: _start

The culprit is here:

rust-smallvec/lib.rs

Line 402 in a775b5f

data: SmallVecData::from_inline(mem::uninitialized()),

"Why would someone create a SmallVec of Void?" you may ask.
In my situation, the type (Void here) is a template parameter of one of my structs.
When the user uses a Void, the SmallVec will of course always remain empty. However creating an empty SmallVec shouldn't itself lead to a panic.

[nox]

Relevant (IIRC): #126

[RalfJung]

This also shows that e.g. smallvec::SmallVec::<[bool; 8]>::new() causes UB -- it creates a bool that is not properly initialized.

Once rust-lang/rust#60445 lands, the ManuallyDrop here should be replaced by MaybeUninit. That should help with these cases.