thread 'main' panicked at 'TranslationUnit::parse failed', libcore/option.rs:960:5 - with --target=path/to/target.json

cargo recently gained the ability for --target to point a JSON file, instead of being the name of a target found in RUST_TARGET_PATH.

Using bindgen in a build script when compiling with --target=path/to/target.json produces sadness:

+ [running] ['cargo', 'xbuild', '--target', '/home/travis/build/alex/linux-kernel-module-rust/tests/../x86_64-linux-kernel-module.json']
    Updating registry `https://github.com/rust-lang/crates.io-index`
   Compiling linux-kernel-module v0.1.0 (file:///home/travis/build/alex/linux-kernel-module-rust)
error: failed to run custom build command for `linux-kernel-module v0.1.0 (file:///home/travis/build/alex/linux-kernel-module-rust)`
process didn't exit successfully: `/home/travis/build/alex/linux-kernel-module-rust/tests/printk/../target/debug/build/linux-kernel-module-49ebd67b170fe49c/build-script-build` (exit code: 101)
--- stdout
cargo:rerun-if-changed=src/bindings_helper.h
--- stderr
thread 'main' panicked at 'TranslationUnit::parse failed', libcore/option.rs:960:5
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:511
   5: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:426
   6: rust_begin_unwind
             at libstd/panicking.rs:337
   7: core::panicking::panic_fmt
             at libcore/panicking.rs:92
   8: core::option::expect_failed
             at libcore/option.rs:960
   9: <core::option::Option<T>>::expect
             at /checkout/src/libcore/option.rs:312
  10: bindgen::ir::context::BindgenContext::new
             at /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/bindgen-0.37.0/src/ir/context.rs:559
  11: bindgen::Bindings::generate
             at /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/bindgen-0.37.0/src/lib.rs:1634
  12: bindgen::Builder::generate
             at /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/bindgen-0.37.0/src/lib.rs:1102
  13: build_script_build::main
             at ./build.rs:72
  14: std::rt::lang_start::{{closure}}
             at /checkout/src/libstd/rt.rs:74
  15: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  16: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:105
  17: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  18: std::rt::lang_start
             at /checkout/src/libstd/rt.rs:74
  19: main
  20: __libc_start_main
  21: <unknown>

Full output in CI at https://travis-ci.org/alex/linux-kernel-module-rust/builds/392023111

And the diff showing the change that produces this at https://github.com/alex/linux-kernel-module-rust/pull/50/files

Reproduce the failure with cargo xbuild --target path/to/target.json and the build script at build.rs:72. Start by reading bindgen::ir::context::BindgenContext::new at src/ir/context.rs:559, then trace the Bindings::generate and Builder::generate calls shown in the stack trace. Done means this target form no longer panics during binding generation.