Previous Stacked Borrows diagnostics were missing a lot of information
about the state of the interpreter, and it was difficult to add
additional state because it was threaded through all the intervening
function signatures.
This change factors a lot of the arguments which used to be passed
individually to many stacked borrows functions into a single
`DiagnosticCx`, which is built in `Stacks::for_each`, and since it
wraps a handle to `AllocHistory`, we can now handle more nuanced
things like heterogeneous borrow of `!Freeze` types.
Breaking posix_memalign precondition is not UB
The `size==0` test here might be overtesting, but I figured might as well test it and leave a comment saying it is fine to remove it if the implementation changes.
Fixes#2099
add special exception for std_miri_test crate to call std-only functions
These being the unit tests of std, they have their own copy of `std::sys` and `std::thread`, so the existing check says this is not std. The check is correct but we want to allow this so we just hard-code the crate name.
The point of this `frame_in_std` check is to prevent people from directly interacting with shims that aren't really properly implemented, but it doesn't need to be 100% airtight. If someone really wants to call their crate `std_miri_test` in order to access some broken shims... they can keep the pieces.
fix RUSTC_BACKTRACE always being set
I kept wondering why Miri programs, whenever isolation is disabled, behave as if RUSTC_BACKTRACE was set. Finally I realized it's because some early rustc setup code sets that env var, and that is then propagated to the interpreted program.
So fix that by taking a copy of the environment before any rustc setup, and use that copy as the basis for what is provided to the interpreted program.
