prost_build does not work on no_std

[benma]

Hi

I am trying to use prost in a no_std target. Everything works great, except that build.rs, which uses prost_build::compile_protos, does not work as it relies on std:

error[E0463]: can't find crate for `std`
 --> src/vendor/getrandom/src/error_impls.rs:8:1
  |
8 | extern crate std;
  | ^^^^^^^^^^^^^^^^^ can't find crate
  |

(via prost-build -> tempfile -> rand -> getrandom).

The funny thing is that compiling the .proto files does not actually depend on my target, as it's a build-step and could be done on std.

In fact this is how I currently work around it: I can compile the .proto files fine if I compile my project on a standard std target, and then copy the resulting .rs files from the build directory and add it directly to my project.

It would be really good to fix up prost-build to run on no_std also, OR to provide a CLI tool which I can run to manually generate the .rs file outside of build.rs.

[danburkert]

I assume you are using unreleased prost from git? Anyway, this is more-or-less expected. I don't do much no-std work with Rust, but my understanding was that most of the time you cross-compiled to the target platform, as getting rustc and/or cargo to run in the no-std environments was next to impossible. When cross-compiling I don't think it should matter that a build dependency requires std?

[benma]

I assume you are using unreleased prost from git?

Yes exactly.

When cross-compiling I don't think it should matter that a build dependency requires std?

This is what I expected, and yet it seems that build.rs also uses no_std if I cross compile to a no_std target, somehow.

Anyway, in this case I guess I'll close this and just use a handrolled CLI tool that calls prost_build::compile_protos(), and run this outside of Rust's build.rs infrastructure. Thanks.

[benma]

When cross-compiling I don't think it should matter that a build dependency requires std?

This is what I expected, and yet it seems that build.rs also uses no_std if I cross compile to a no_std target, somehow.

For the record, this is a bug and tracked here:

rust-lang/cargo#7915

It is resolved on nightly with an unstable feature. Let's hope it will stabilize soon.

[dzimmanck]

I am also experiencing this issue. I am trying to use prost in an embedded application (ARM M4), and have to build the data structures in a separate script. The issue is still there with nightly. Do I need something special in my Cargo.toml file to tell Cargo it can use std in the build dependencies?