cargo clean panics on a project with cbindgen

[asomers]

The combination of cargo-kcov and cbindgen triggers a panic in Cargo. I've reproduced it on both FreeBSD 11.3 and Debian 10.3, with a few different versions of cargo nightly. To reproduce, create a blank crate. Fill Cargo.toml with this:

[package]
name = "cargo-kcov-crash"
version = "0.1.0"

[build-dependencies]
cbindgen = "0.14.0"

and create a blank build.rs like this:

fn main() { }

Then invoke cargo like this:

$ env RUST_BACKTRACE=1 cargo +nightly kcov -v
       Clean file:///usr/home/somers/src/rust/cargo-kcov-crash#0.1.0

error: cargo subcommand failure
note: cargo clean exited with code exit code: 101
thread 'main' panicked at 'assertion failed: self.host', src/tools/cargo/src/cargo/core/profiles.rs:940:13
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1069
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1504
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:218
  10: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:511
  11: std::panicking::begin_panic
  12: cargo::core::compiler::unit_dependencies::compute_deps
  13: cargo::core::compiler::unit_dependencies::deps_of
  14: cargo::core::compiler::unit_dependencies::deps_of_roots
  15: cargo::core::compiler::unit_dependencies::build_unit_dependencies
  16: cargo::ops::cargo_clean::clean
  17: cargo::commands::clean::exec
  18: cargo::cli::main
  19: cargo::main
  20: std::rt::lang_start::{{closure}}
  21: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:52
  22: std::panicking::try::do_call
             at src/libstd/panicking.rs:331
  23: std::panicking::try
             at src/libstd/panicking.rs:274
  24: std::panic::catch_unwind
             at src/libstd/panic.rs:394
  25: std::rt::lang_start_internal
             at src/libstd/rt.rs:51
  26: main
  27: _start
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.