Miri incorrectly rejects pointer-as-bytes access with `cttz`

[myrrlyn]

Expected behavior: Miri allows me to interpret a pointer as a usize, so that I may call cttz on it to see how aligned it is.

Actual behavior: it does not.

Output of rustup show:

active toolchain
----------------
nightly-x86_64-apple-darwin (directory override for '/Users/myrrlyn/Projects/Rust/miri_tests')
rustc 1.39.0-nightly (e44fdf979 2019-08-21)

Rust minimal test case:

pub union Pointer<T> {
    u: usize,
    r: *const T,
}

#[test]
fn cttz() { unsafe {
    let elt = 0u64;
    assert!(Pointer { r: &elt }.u.trailing_zeros() >= 3);
} }

Miri output:

   Compiling miri_tests v0.1.0 (/Users/myrrlyn/Projects/Rust/miri_tests)

running 1 test
error[E0080]: Miri evaluation error: a raw memory access tried to access part of a pointer value as raw bytes
    --> /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/num/mod.rs:2437:17
     |
2437 |                 intrinsics::cttz(self) as u32
     |                 ^^^^^^^^^^^^^^^^^^^^^^ Miri evaluation error: a raw memory access tried to access part of a pointer value as raw bytes
     |
note: inside call to `core::num::<impl usize>::trailing_zeros` at src/lib.rs:9:13
    --> src/lib.rs:9:13
     |
9    |     assert!(Pointer { r: &elt }.u.trailing_zeros() >= 3);
     |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside call to `cttz` at src/lib.rs:7:1
    --> src/lib.rs:7:1
     |
7    | / fn cttz() { unsafe {
8    | |     let elt = 0u64;
9    | |     assert!(Pointer { r: &elt }.u.trailing_zeros() >= 3);
10   | | } }
     | |___^
     = note: inside call to closure at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/ops/function.rs:235:5
     = note: inside call to `<[closure@src/lib.rs:7:1: 10:4] as std::ops::FnOnce<()>>::call_once - shim` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/ops/function.rs:235:5
     = note: inside call to `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libtest/lib.rs:1462:5
     = note: inside call to `test::__rust_begin_short_backtrace::<fn()>` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libtest/lib.rs:1453:30
     = note: inside call to closure at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/ops/function.rs:235:5
     = note: inside call to `<[closure@DefId(12:366 ~ test[51aa]::run_test[0]::{{closure}}[3]) 0:fn()] as std::ops::FnOnce<()>>::call_once - shim(vtable)` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/liballoc/boxed.rs:922:9
     = note: inside call to `<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/panic.rs:315:9
     = note: inside call to `<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>> as std::ops::FnOnce<()>>::call_once` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/panicking.rs:296:40
     = note: inside call to `std::panicking::try::do_call::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/panicking.rs:292:5
     = note: inside call to `std::panicking::try::<(), std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>>` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/panic.rs:394:9
     = note: inside call to `std::panic::catch_unwind::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libtest/lib.rs:1408:26
     = note: inside call to closure at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libtest/lib.rs:1430:13
     = note: inside call to `test::run_test::run_test_inner` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libtest/lib.rs:1449:28
     = note: inside call to `test::run_test` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libtest/lib.rs:1102:13
     = note: inside call to `test::run_tests::<[closure@DefId(12:315 ~ test[51aa]::run_tests_console[0]::{{closure}}[2]) 0:&mut test::ConsoleTestState, 1:&mut std::boxed::Box<dyn test::formatters::OutputFormatter>]>` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libtest/lib.rs:951:5
     = note: inside call to `test::run_tests_console` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libtest/lib.rs:294:15
     = note: inside call to `test::test_main` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libtest/lib.rs:328:5
     = note: inside call to `test::test_main_static`
     = note: inside call to `main` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/rt.rs:64:34
     = note: inside call to closure at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/rt.rs:52:53
     = note: inside call to closure at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/panicking.rs:296:40
     = note: inside call to `std::panicking::try::do_call::<[closure@DefId(1:5982 ~ std[87a5]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/panicking.rs:292:5
     = note: inside call to `std::panicking::try::<i32, [closure@DefId(1:5982 ~ std[87a5]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/panic.rs:394:9
     = note: inside call to `std::panic::catch_unwind::<[closure@DefId(1:5982 ~ std[87a5]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/rt.rs:52:25
     = note: inside call to `std::rt::lang_start_internal` at /Users/myrrlyn/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/rt.rs:64:5
     = note: inside call to `std::rt::lang_start::<()>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0080`.
test cttz ... 
error: Could not compile `miri_tests`.

To learn more, run the command again with --verbose.

[RalfJung]

Whether such code is legal in LLVM is still up for debate (transmuting, instead of casting, between pointers and integers is a subtle topic). But until that is resolved, Miri should probably accept such code, I agree. It already does in other circumstances.

This is a missing force_bits somewhere.

[oli-obk]

I have a rustc ranch that fixes it, indeed just a force_bits

[RalfJung]

For the fix to be shipped via rustup, we need to solve rust-lang/rust#63843 first.