cargo-miri panic's on no_main binaries

[ppannuto]

While there are many other questions about what miri should do for binaries with #![no_main], the current behavior is to panic. In the spirit of #1001, it would be nice explicitly indicate that miri doesn't support testing freestanding binaries at the moment.

An example backtrace

[-bash] Wed 17 Jun 16:40 [[master $] /Volumes/code/helena-project/tock/boards/hail]
$ cargo miri test
    Checking hail v0.1.0 (/Volumes/code/helena-project/tock/boards/hail)
warning: unused import: `core::panic::PanicInfo`
 --> boards/hail/src/io.rs:2:5
  |
2 | use core::panic::PanicInfo;
  |     ^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default
warning: unused import: cortexm4

--> boards/hail/src/io.rs:4:5

|

4 | use cortexm4;

|     ^^^^^^^^
warning: unused import: kernel::debug

--> boards/hail/src/io.rs:5:5

|

5 | use kernel::debug;

|     ^^^^^^^^^^^^^
warning: unused import: kernel::hil::led

--> boards/hail/src/io.rs:7:5

|

7 | use kernel::hil::led;

|     ^^^^^^^^^^^^^^^^
warning: unused import: crate::CHIP

--> boards/hail/src/io.rs:10:5

|

10 | use crate::CHIP;

|     ^^^^^^^^^^^
warning: unused import: crate::PROCESSES

--> boards/hail/src/io.rs:11:5

|

11 | use crate::PROCESSES;

|     ^^^^^^^^^^^^^^^^
warning: static is never used: WRITER

--> boards/hail/src/io.rs:17:1

|

17 | static mut WRITER: Writer = Writer { initialized: false };

| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

|

= note: #[warn(dead_code)] on by default
thread 'rustc' panicked at 'no main function found!', src/tools/miri/src/bin/miri.rs:35:37

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 1.45.0-nightly (fe10f1a49 2020-06-02) running on x86_64-apple-darwin
note: compiler flags: -C opt-level=z -C embed-bitcode=no -C debuginfo=2 -C debug-assertions=on -C incremental
note: some of the compiler flags provided by cargo are hidden
warning: 7 warnings emitted
error: could not compile hail.
To learn more, run the command again with --verbose.

I've attached a backtrace, but any no_main binary will crash in a similar manner right now.

CC tock/tock#1713

[RalfJung]

Thanks! I agree.

This is an easy bug to fix, well-suited as a first contribution. The panic arises here:

miri/src/bin/miri.rs

Line 35 in 146ee66

let (entry_def_id, _) = tcx.entry_fn(LOCAL_CRATE).expect("no main function found!");

And all it takes is replacing that by a better error message. rustc has some APIs for fatal errors; early_error might be suited here.