WIP: Started working on a post for reviewing unsafe code

[Michael-F-Bryan]

No description provided.

[dtolnay]

FYI this isn't the main reason. We could do extern crate alloc unconditionally and use alloc::boxed::Box even in std mode, and get the same effect. But that only works on 1.36+. The more verbose setup is necessary in order for anyhow in std mode to support back to 1.34+.

[dtolnay]

The cast from *const to *mut is required for using NonNull, whose constructors take *mut but is intended for use with both *const and *mut. There isn't a separate NonNull type dedicated to *const.

[Michael-F-Bryan]

I recently discovered that NonNull<T> implements From<&T> and From<&mut T>. So maybe the overall object_downcast() function could look like this?

unsafe fn object_downcast<E>(e: NonNull<ErrorImpl<()>>, target: TypeId) -> Option<NonNull<()>> where E: 'static,
{
    if TypeId::of::<E>() == target {
        let unerased = e.cast::<ErrorImpl<E>>().as_ref();
        Some(NonNull::from(&unerased._object))
    } else {
        None
    }
}

[dtolnay]

If you feel up to it, could you look into what else is being flagged by cargo miri test? I see the following even after commenting out all the downcast_mut in the test suite:

error: Miri evaluation error: trying to reborrow for Unique, but parent tag <88627> does not have an appropriate item in the borrow stack
   --> /git/anyhow/src/error.rs:523:20
    |
523 |     let unerased = mem::transmute::<Box<ErrorImpl<()>>, Box<ErrorImpl<E>>>(e);
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ trying to reborrow for Unique, but parent tag <88627> does not have an appropriate item in the borrow stack
    |
    = note: inside call to `anyhow::error::object_drop::<anyhow::error::ContextError<i32, anyhow::Error>>` at /git/anyhow/src/error.rs:504:13
    = note: inside call to `anyhow::error::<impl std::ops::Drop for anyhow::Error>::drop` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/ptr/mod.rs:174:1
note: inside call to `std::intrinsics::drop_in_place::<anyhow::Error> - shim(Some(anyhow::Error))` at tests/test_chain.rs:17:1
   --> tests/test_chain.rs:17:1
    |
17  | }
    | ^
note: inside call to `test_iter` at tests/test_chain.rs:8:1
   --> tests/test_chain.rs:8:1
    |
8   | / fn test_iter() {
9   | |     let e = error();
10  | |     let mut chain = e.chain();
11  | |     assert_eq!("3", chain.next().unwrap().to_string());
...   |
16  | |     assert!(chain.next_back().is_none());
17  | | }
    | |_^
    = note: inside call to closure at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/ops/function.rs:232:5
    = note: inside call to `<[closure@tests/test_chain.rs:8:1: 17:2] as std::ops::FnOnce<()>>::call_once - shim` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/ops/function.rs:232:5
    = note: inside call to `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:515:5
    = note: inside call to `test::__rust_begin_short_backtrace::<fn()>` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:506:30
    = note: inside call to closure at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/ops/function.rs:232:5
    = note: inside call to `<[closure@DefId(15:633 ~ test[6578]::run_test[0]::{{closure}}[2]) 0:fn()] as std::ops::FnOnce<()>>::call_once - shim(vtable)` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/boxed.rs:1015:9
    = note: inside call to `<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panic.rs:318:9
    = note: inside call to `<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>> as std::ops::FnOnce<()>>::call_once` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:305:40
    = note: inside call to `std::panicking::r#try::do_call::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:281:13
    = note: inside call to `std::panicking::r#try::<(), std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>>` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panic.rs:394:14
    = note: inside call to `std::panic::catch_unwind::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:539:18
    = note: inside call to `test::run_test_in_process` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:452:39
    = note: inside call to closure at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:473:13
    = note: inside call to `test::run_test::run_test_inner` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:503:28
    = note: inside call to `test::run_test` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:286:13
    = note: inside call to `test::run_tests::<[closure@DefId(15:232 ~ test[6578]::console[0]::run_tests_console[0]::{{closure}}[2]) 0:&mut test::console::ConsoleTestState, 1:&mut std::boxed::Box<dyn test::formatters::OutputFormatter>]>` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/console.rs:282:5
    = note: inside call to `test::run_tests_console` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:122:15
    = note: inside call to `test::test_main` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:141:5
    = note: inside call to `test::test_main_static`
    = note: inside call to `main` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:67:34
    = note: inside call to closure at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:52:73
    = note: inside call to closure at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/sys_common/backtrace.rs:129:5
    = note: inside call to `std::sys_common::backtrace::__rust_begin_short_backtrace::<[closure@DefId(1:6019 ~ std[d5d1]::rt[0]::lang_start_internal[0]::{{closure}}[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:52:13
    = note: inside call to closure at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:305:40
    = note: inside call to `std::panicking::r#try::do_call::<[closure@DefId(1:6018 ~ std[d5d1]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:281:13
    = note: inside call to `std::panicking::r#try::<i32, [closure@DefId(1:6018 ~ std[d5d1]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panic.rs:394:14
    = note: inside call to `std::panic::catch_unwind::<[closure@DefId(1:6018 ~ std[d5d1]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:51:25
    = note: inside call to `std::rt::lang_start_internal` at /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:67:5
    = note: inside call to `std::rt::lang_start::<()>`