Backtrace initial spans are missing if the topmost span is nonlocal?

[saethlin]

I just noticed that a lot of diagnostics are missing the topmost span, and I'm pretty sure it's related to whether the span is in a local crate.

For example, with raw pointer tagging on smallvec 1.8.0, we currently get:

test tests::drain ... error: Undefined Behavior: trying to reborrow <293683> for SharedReadOnly permission at alloc111484[0x9], but that tag does not exist in the borrow stack for this location
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <293683> was created by a retag at offsets [0x9..0xa]
   --> src/lib.rs:821:23
    |
821 |                 iter: range_slice.iter(),
    |                       ^^^^^^^^^^^^^^^^^^
help: <293683> was later invalidated at offsets [0x0..0x20]
   --> src/lib.rs:822:36
    |
822 |                 vec: NonNull::from(self),
    |                                    ^^^^

But crossbeam-epoch 0.9.8 with raw pointer tagging reports this:

test atomic::tests::array_init ... error: Undefined Behavior: trying to reborrow <untagged> for SharedReadOnly permission at alloc94974[0x0], but that tag does not exist in the borrow stack for this location
    --> src/atomic.rs:264:21
     |
264  |         let array = &*(ptr as *const Array<T>);
     |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^
     |                     |
     |                     trying to reborrow <untagged> for SharedReadOnly permission at alloc94974[0x0], but that tag does not exist in the borrow stack for this location
     |                     this error occurs as part of a reborrow at alloc94974[0x0..0x8]
     |
     = help: this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental
     = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

I feel like we should be able to test for this regression?

[RalfJung]

We always print the topmost span of the backtrace:

miri/src/diagnostics.rs

Line 313 in 1c11fc6

let span = stacktrace.first().map_or(DUMMY_SP, |fi| fi.span);

To me this looks more like stdlib span printing is broken.

[RalfJung]

I see the same thing with ./miri run tests/compile-fail/intrinsics/out_of_bounds_ptr_3.rs. I am pretty sure this used to work.

My guess is that this has to do with the -Zremap-cwd-prefix=rustc_src that @oli-obk recently added to the flags that we build our stdlib with.

[RalfJung]

My guess is that this has to do with the -Zremap-cwd-prefix=rustc_src that @oli-obk recently added to the flags that we build our stdlib with.

Guess confirmed, after removing that flag I get the expected

error: Undefined Behavior: pointer arithmetic failed: alloc1655 has size 4, so pointer to 1 byte starting at offset -1 is out-of-bounds
   --> /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ptr/const_ptr.rs:455:18
    |
455 |         unsafe { intrinsics::offset(self, count) }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ pointer arithmetic failed: alloc1655 has size 4, so pointer to 1 byte starting at offset -1 is out-of-bounds
    |
    = 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 `std::ptr::const_ptr::<impl *const i8>::offset` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ptr/const_ptr.rs:455:18
note: inside `main` at tests/compile-fail/intrinsics/out_of_bounds_ptr_3.rs:5:22
   --> tests/compile-fail/intrinsics/out_of_bounds_ptr_3.rs:5:22
    |
5   |     let x = unsafe { x.offset(-1) };
    |                      ^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace