Fix linking issues when using multiple codegen units

[japaric]

To improve build times rustc is now defaulting to building crates using multiple codegen units. The
problem is that using multiple codegen units can sometimes break linking of no-std applications
("undefined reference to rust_begin_unwind").

We are looking for someone who can help us fix the problem in the compiler

Tasks

• Tweak rustc linking process to properly support multiple codegen units. The problem and
  solution is described in Multiple codegen units breaks linking: "undefined reference to rust_begin_unwind" rust-lang/rust#47074 (comment)

The current workaround for this issue is to force the compiler to use a single codegen unit in
Cargo.toml

# workaround rust-lang/rust#47074
[profile.dev]
codegen-units = 1
incremental = false

[thejpster]

I've also seen cargo/xargo emit entirely empty binaries unless this property is set.

[japaric]

@thejpster with the cortex-m-rt linker script or with some other custom linker script? I have patched the cortex-m-rt linker script a few times to make it work with multiple codegen units but I wouldn't be surprised if there are still issues out there ...

[thejpster]

This was with stellaris-launchpad, which had its own linker script dating back to my original C implementation. Would it be useful if we wrote up requirements for linker scripts somewhere, rather than relying on some magic in one example?

[nunolucas]

The empty binaries, on Windows, may be related to this bug under heavy load:
https://randomascii.wordpress.com/2018/02/25/compiler-bug-linker-bug-windows-kernel-bug/

[japaric]

@thejpster all the magic is documented in cortex-m-rt's linker script. All the symbols in that linker script are documented as well.

[thejpster]

That is beautifully done, but I still wonder if it's worth pulling out into something that's more generic and not Cortex-M specific, to help people with other platforms and improve discoverability.

[japaric]

Tasks
Tweak rustc linking process to properly support multiple codegen units. The problem and
solution is described in rust-lang/rust#47074 (comment)

PR rust-lang/rust#49316 should fix this. (merged)

[japaric]

Update: rust-lang/rust#49316 didn't quite fix this. See rust-lang/rust#49316 (comment).

[japaric]

Update: rust-lang/rust#49672 should fix this for real

[japaric]

Fixed in rust-lang/rust#49672. 🎉