reference compiler-builtins by git source

[a-nickol]

Fixes #320, but I am not sure, what the previous behavior was!

[RalfJung]

Is it because of the missing source for compiler_builtins in rust-src? I am not sure if it had been there before.

I don't think it has been there before, rust-lang/rust#78209 shows it has been pulled in from crates.io for a long time already.

But then why doesn't it pick up the version from https://crates.io/crates/compiler_builtins? The git reference will likely not match the exact version that this std should be built with.

[RalfJung]

So the question is, in that default case where xargo adds a compiler_builtins dependency itself, which version should it add? Can we just set the version to * to get the latest version from crates.io? That seems less risky than using the latest git version...

[a-nickol]

I will do some more research, but this should be the right path.

[RalfJung]

To be specific, what do you mean by "this"? IMO using a crates.io version is preferable over using the git version.

[a-nickol]

I tested if the version is somehow related to the usage of not usage of the version declaration.

When using version = "*" in the Cargo.toml the version of compiler_builtins is set to 0.1.50.

Without the version and without the git repository the version is set to 0.1.49.

[a-nickol]

Should we read the version of the dependency by parsing the Cargo.lock of the toolchain, e.g .rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/Cargo.lock?

For my build it would be then: compiler_builtins = { version = "0.1.49" }.

Since it is already copied over to the build directory, we would just have to parse it.

[RalfJung]

When using version = "*" in the Cargo.toml the version of compiler_builtins is set to 0.1.50.

Yeah, that is the latest version.

Without the version and without the git repository the version is set to 0.1.49.

Isn't there an error in this case?

When liballoc is being built, we pull in compiler_builtins via that, and the version is given by the Cargo.lock file shipped in rust-src. But I don't think we should get into the business of parsing the lockfile.

Honestly I am not sure what people expect when giving no Xargo.toml; this case is not really well-defined...

[a-nickol]

I made a mistake. I think using version = "*" just works perfect, since we are copying the Cargo.lock from std, the correct version from std is selected! I will change the commit.

[a-nickol]

On my machine the build even passes with the nightly-2019-12-09-toolchain when using the proposed solution.

[RalfJung]

This looks great, thanks :).

bors r+

[bors]

Build succeeded:

• ci