Improve usage of `compiler-builtins`

[jamesmunns]

This is regarding the changes made in #49380. This change works, however when the user makes some kind of mistake, especially forgetting to install the new targets (like thumbv7em-none-eabihf), the error messages are not currently helpful.

The current error message when forgetting to install the target looks like this:

cargo build --target thumbv7em-none-eabihf --example blinky
   Compiling vcell v0.1.0
   Compiling bare-metal v0.1.1
   Compiling aligned v0.1.1
   Compiling untagged-option v0.1.1
error[E0463]: can't find crate for `compiler_builtins`

error[E0463]: can't find crate for `compiler_builtins`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error[E0463]: can't find crate for `compiler_builtins`

error: aborting due to previous error

error[E0463]: can't find crate for `compiler_builtins`
For more information about this error, try `rustc --explain E0463`.

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.

compared to the following when you try to compile for a not-installed target:

cargo build --target i686-unknown-linux-gnu
   Compiling foo v0.1.0 (file:///tmp/foo)
error[E0463]: can't find crate for `std`
  |
  = note: the `i686-unknown-linux-gnu` target may not be installed

error: aborting due to previous error

For more information about this error

The following #[no_std] code shows (the first part of) a typical embedded project:

//! Blinks an LED

#![deny(unsafe_code)]
#![deny(warnings)]
#![no_std]

extern crate cortex_m;

using cargo expand, this code expands to:

#![feature(prelude_import)]
#![no_std]
//! Blinks an LED

#![deny(unsafe_code)]
#![deny(warnings)]
#![no_std]
#[prelude_import]
use core::prelude::v1::*;
#[macro_use]
extern crate compiler_builtins;
#[macro_use]
extern crate core;

extern crate cortex_m;

The Embedded WG talked about this, and we think the error messages would be improved if the order of extern crate compiler_builtins; and extern crate core; were swapped.

[jamesmunns]

CC @japaric and @oli-obk

[japaric]

we think the error messages would be improved if the order of extern crate compiler_builtins; and extern crate core; were swapped.

I can confirm that swapping them (checked with a hand written #![no_core] crate) produces the old, more helpful error messages of "note: the thumbv6m-none-eabi target may not be installed". It would also be good to drop the #[macro_use] from extern crate compiler_builtins as it seems like it could inject weird macros into #![no_std] programs.

cc @oli-obk

[jamesmunns]

Also I just noticed that in the expanded version, #![no_std] appears twice.

[oli-obk]

I wonder how that order swapping happens... should be easy enough to fix. The macro use comes from reusing the core import code. I guess we'll have to duplicate some now, but it's not too much

[oli-obk]

Ah.. inserting at position 0 will do that:

https://github.com/oli-obk/rust/blob/679657b863c2a53a3052d8af9defbce48e12db10/src/libsyntax/std_inject.rs#L61

This should be an easy to fix issue. Also needs a ui test

[nagisa]

Furthermore, since extern crate statements can now appear at any position, it should be fine to replace that insert with a push.

[rcoh]

I have a diff for this but I'm having a hard time reproducing the error message in the ui tests. Any pointers @oli-obk ? I assume I need compiler flags but the only error I can seem to get is:

"Error loading target specification: Could not find specification for target \"thumbv7em-none-eabih\""

[japaric]

@rcoh That looks like a typo in the target name; it should be thumbv7em-none-eabihf, with an f at the end.

[rcoh]

Yep, that was it. Much appreciated.

[japaric]

This was fixed in #50006. Thanks @rcoh!