Fix panic on 'wasm32-unknown-unknown' target.

[alexlapa]

Compiler panic:

Compiler panic occurs when trying to build tracerr on wasm32-unknown-unknown target (cargo build --target=wasm32-unknown-unknown)

thread 'rustc' panicked at 'src/librustc_resolve/macros.rs:715: inconsistent resolution for a macro', src/librustc/util/bug.rs:37:26
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.39.0 (4560ea788 2019-11-04) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type lib

note: some of the compiler flags provided by cargo are hidden

error: could not compile `tracerr`.

To learn more, run the command again with --verbose.

It is already reported rust-lang/rust#64450.

[tyranron]

FCM

Fix panic on 'wasm32-unknown-unknown' target (#1)

See: https://github.com/rust-lang/rust/issues/64450