Fix MIRI error in inout_buf.rs

[nico-abram]

Running salsa20's tests under MIRI with the flags $ENV:MIRIFLAGS="-Zmiri-symbolic-alignment-check -Zmiri-check-number-validity -Zmiri-tag-raw-pointers -Zmiri-disable-isolation -Zmiri-ignore-leaks -Zmiri-backtrace=full" results in the following:

miri output

running 1 test
error: Undefined Behavior: type validation failed at .value: encountered uninitialized bytes, but expected initialized plain (non-pointer) bytes
   --> D:\dev\stream-ciphers\salsa20\utils\inout\src\inout_buf.rs:20:59
    |
20  |     fn from(buf: &'a mut [T]) -> Self {let x: u8 = unsafe{core::mem::uninitialized()};
    |                                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at .value: encountered uninitialized bytes, but expected initialized plain (non-pointer) bytes
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

    = note: inside `<cipher::inout::InOutBuf<u8> as std::convert::From<&mut [u8]>>::from` at D:\dev\stream-ciphers\salsa20\utils\inout\src\inout_buf.rs:20:59
    = note: inside `<&mut [u8] as std::convert::Into<cipher::inout::InOutBuf<u8>>>::into` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\convert\mod.rs:550:9
    = note: inside `<cipher::StreamCipherCoreWrapper<salsa20::SalsaCore<cipher::typenum::UInt<cipher::typenum::UInt<cipher::typenum::UInt<cipher::typenum::UInt<cipher::typenum::UTerm, cipher::typenum::B1>, cipher::typenum::B0>, cipher::typenum::B1>, cipher::typenum::B0>>> as cipher::StreamCipher>::try_apply_keystream` at D:\.cargo\registry\src\github.com-1ecc6299db9ec823\cipher-0.4.3\src\stream.rs:94:40
    = note: inside `<cipher::StreamCipherCoreWrapper<salsa20::SalsaCore<cipher::typenum::UInt<cipher::typenum::UInt<cipher::typenum::UInt<cipher::typenum::UInt<cipher::typenum::UTerm, cipher::typenum::B1>, cipher::typenum::B0>, cipher::typenum::B1>, cipher::typenum::B0>>> as cipher::StreamCipher>::apply_keystream` at D:\.cargo\registry\src\github.com-1ecc6299db9ec823\cipher-0.4.3\src\stream.rs:120:9
note: inside `salsa20_key0_iv1` at salsa20\tests\mod.rs:107:5
   --> salsa20\tests\mod.rs:107:5
    |
107 |     cipher.apply_keystream(&mut buf);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside closure at salsa20\tests\mod.rs:103:1
   --> salsa20\tests\mod.rs:103:1
    |
102 |   #[test]
    |   ------- in this procedural macro expansion
103 | / fn salsa20_key0_iv1() {
104 | |     let mut cipher = Salsa20::new(&KEY0.into(), &IV1.into());
105 | |     let mut buf = [0; 64];
106 | |
...   |
111 | |     }
112 | | }
    | |_^
    = note: inside `<[closure@salsa20\tests\mod.rs:103:1: 112:2] as std::ops::FnOnce<()>>::call_once - shim` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:227:5
    = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:227:5
    = note: inside `test::__rust_begin_short_backtrace::<fn()>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\test\src\lib.rs:575:5
    = note: inside closure at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\test\src\lib.rs:566:30
    = note: inside `<[closure@test::run_test::{closure#1}] as std::ops::FnOnce<()>>::call_once - shim(vtable)` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:227:5
    = note: inside `<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc\src\boxed.rs:1861:9
    = note: inside `<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>> as std::ops::FnOnce<()>>::call_once` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\panic\unwind_safe.rs:271:9
    = note: inside `std::panicking::r#try::do_call::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\panicking.rs:492:40
    = note: inside `std::panicking::r#try::<(), std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\panicking.rs:456:19
    = note: inside `std::panic::catch_unwind::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\panic.rs:137:14
    = note: inside `test::run_test_in_process` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\test\src\lib.rs:598:18
    = note: inside closure at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\test\src\lib.rs:492:39
    = note: inside `test::run_test::run_test_inner` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\test\src\lib.rs:530:13
    = note: inside `test::run_test` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\test\src\lib.rs:562:28
    = note: inside `test::run_tests::<[closure@test::run_tests_console::{closure#2}]>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\test\src\lib.rs:305:17
    = note: inside `test::run_tests_console` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\test\src\console.rs:301:5
    = note: inside `test::test_main` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\test\src\lib.rs:116:15
    = note: inside `test::test_main_static` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\test\src\lib.rs:135:5
    = note: inside `main`
    = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:227:5
    = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<fn(), ()>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\sys_common\backtrace.rs:122:18
    = note: inside closure at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\rt.rs:145:18
    = note: inside `std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:259:13
    = note: inside `std::panicking::r#try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\panicking.rs:492:40
    = note: inside `std::panicking::r#try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\panicking.rs:456:19
    = note: inside `std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\panic.rs:137:14
    = note: inside closure at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\rt.rs:128:48
    = note: inside `std::panicking::r#try::do_call::<[closure@std::rt::lang_start_internal::{closure#2}], isize>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\panicking.rs:492:40
    = note: inside `std::panicking::r#try::<isize, [closure@std::rt::lang_start_internal::{closure#2}]>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\panicking.rs:456:19
    = note: inside `std::panic::catch_unwind::<[closure@std::rt::lang_start_internal::{closure#2}], isize>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\panic.rs:137:14
    = note: inside `std::rt::lang_start_internal` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\rt.rs:128:20
    = note: inside `std::rt::lang_start::<()>` at D:\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\rt.rs:144:17
    = note: this error originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

test salsa20_key0_iv1 ... error: test failed, to rerun pass '--test mod'

This commit fixes that specific issue. As explained in the commit message, calling as_mut_ptr creates a unique reference, and as_ptr a shared reference. These 2 conflict, and one invalidates the other. Both ptrs need to be reborrows of or be basically the same pointer.

Found by https://miri.saethlin.dev/ub?crate=salsa20&version=0.10.2 (By @saethlin )

See also rust-lang/unsafe-code-guidelines#133

[newpavlov]

Thank you! I will cut release after checking that code does not contain any other similar mistakes.