Commits on Nov 11, 2019

1. Add internal RUSTC_INTERNAL_FORCE_PANIC_ABORT env var for `libpanic… …

   …_abort`
   
   Currently, we require `libpanic_abort` to always be build with the
   `abort` panic strategy, regardless of what panic strategy would
   normally be passed to `rustc` (e.g. `unwind` when building `libstd`).
   
   This ensures that when downstream crates build against `libpanic_abort`,
   it is properly detected as the panic runtime for the `abort` strategy.
   
   Previously, this was done by special-casing `libpanic_abort` in
   bootstrap. This meant that building `libpanic_abort` without using
   `x.py` (e.g by using `xargo`) would result in the wrong panic strategy
   being applied, leading to an error when trying to build against it:
   
   ```
   error: the crate `panic_abort` does not have the panic strategy `abort`
   ```
   
   This is a problem for tools like Miri, which require a custom-build
   libstd, and cannot use `x.py`.
   
   To fix this, we add a special environment variable
   `RUSTC_INTERNAL_FORCE_PANIC_ABORT`. This is set in the `build.rs`
   for `libpanic_abort`, and checked by the compiler when determining the
   panic strategy of the current crate. While this is still a hack, it's a
   much better one - the special case is now represented in the
   `libpanic_abort` crate itself, rather than in `bootstrap`.
   
   Ideally, this would be an internal attribute (e.g.
   `#[rustc_panic_abort]`) that we apply to `libpanic_abort`.
   
   Unfortunately, `emscripten` targets require that we be able to determine
   the panic strategy very early on, before we've even started parsing the
   crate:
   
   https://github.com/rust-lang/rust/blob/3fc30d884ae0c988d98452a06737705cfe34806a/src/librustc_codegen_llvm/llvm_util.rs#L77
   
   To avoid invasive changes to emscripten and/or the codewgen backend
   infrastructure, I chose to add a new environment variable.
   
   Note that the hack in bootstrap needs to remain until this changes makes
   its way into the bootstrap compiler, to ensure that we can still build a
   correct `libpanic_abort` with a bootstrap compiler that doesn't know
   about this special environment variable.

   

   Aaron1011 committed Nov 11, 2019
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