add a panic-strategy field to the target specification

[japaric]

Now a target can define its panic strategy in its specification. If a
user doesn't specify a panic strategy via the command line, i.e. '-C
panic', then the compiler will use the panic strategy defined by the
target specification.

Custom targets can pick their panic strategy via the "panic-strategy"
field of their target specification JSON file. If omitted in the
specification, the strategy defaults to "unwind".

closes #36647

---

I checked that compiling an executable for a custom target with "panic-strategy" set to "abort" doesn't need the "eh_personality" lang item and also that standard crates compiled for that custom target didn't contained undefined symbols to _Unwind_Resume. But this needs an actual unit test, any suggestion on how to test this?

Most of the noise in the diff is due to moving PanicStrategy from the rustc to the rustc_back crate.

r? @alexcrichton
cc @phil-opp

[rust-highfive]

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[oli-obk]

travis failed, I think you need some stage1 cfg attributes

Testing librustc stage2 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
   Compiling fmt_macros v0.0.0 (file:///build/src/libfmt_macros)
   Compiling syntax_pos v0.0.0 (file:///build/src/libsyntax_pos)
   Compiling rustc_errors v0.0.0 (file:///build/src/librustc_errors)
   Compiling syntax v0.0.0 (file:///build/src/libsyntax)
   Compiling build_helper v0.1.0 (file:///build/src/build_helper)
   Compiling serialize v0.0.0 (file:///build/src/libserialize)
   Compiling flate v0.0.0 (file:///build/src/libflate)
   Compiling rustc_llvm v0.0.0 (file:///build/src/librustc_llvm)
   Compiling rustc_const_math v0.0.0 (file:///build/src/librustc_const_math)
   Compiling rustc_macro v0.0.0 (file:///build/src/librustc_macro)
   Compiling rustc_data_structures v0.0.0 (file:///build/src/librustc_data_structures)
   Compiling rustc_platform_intrinsics v0.0.0 (file:///build/src/librustc_platform_intrinsics)
   Compiling rustc_bitflags v0.0.0 (file:///build/src/librustc_bitflags)
   Compiling syntax_ext v0.0.0 (file:///build/src/libsyntax_ext)
   Compiling rustc_back v0.0.0 (file:///build/src/librustc_back)
   Compiling arena v0.0.0 (file:///build/src/libarena)
   Compiling log v0.0.0 (file:///build/src/liblog)
   Compiling rustc v0.0.0 (file:///build/src/librustc)
   Compiling rustc_save_analysis v0.0.0 (file:///build/src/librustc_save_analysis)
error[E0308]: mismatched types
    --> src/librustc/session/config.rs:2311:25
     |
2311 |         opts.cg.panic = PanicStrategy::Abort;
     |                         ^^^^^^^^^^^^^^^^^^^^ expected enum `core::option::Option`, found enum `rustc_back::PanicStrategy`
     |
     = note: expected type `core::option::Option<rustc_back::PanicStrategy>`
     = note:    found type `rustc_back::PanicStrategy`

error: aborting due to previous error

[japaric]

@oli-obk Thanks for the heads up. I don't think we need a cfg here because this is a test which can't be run against stage0 (stage1 already has the change to Option).

[alexcrichton]

@bors: r+

Thanks @japaric!

[bors]

📌 Commit 8a46e78 has been approved by alexcrichton

[Ericson2314]

What happens when a Cargo file and target both specify the panic strategy? I'd prefer it if Cargo files specified a set of allowed panic strategies and then the target specification alone picked the one used.

[Also, would be convenient to specify a map of names to targets in a (virtual) workspace root.]

[japaric]

@Ericson2314 Under the current implementation of Cargo, profile.*.panic will be preferred over what the target specifies because Cargo passes -C panic=$PROFILE to its rustc invocations.

[Ericson2314]

Ok, that seems good enough for now