Use collections with cargo without stdlib - embedded

Question

I am currently trying to setup an embedded Rust project. For that it would be nice if I could use the collections crate (and by extension the alloc crate since it is required by collections). Is there an easy way to achieve this? I currently have the following dependencies in Cargo.toml
[build-dependencies]
gcc = "0.3"
[dependencies]
rust-libcore = "*"
[dependencies.rlibc]
git = "https://github.com/hackndev/rlibc"
branch = "zinc"
And use them as follows:
#![no_std]
#![crate_type="staticlib"]
#![feature(lang_items)]
#![feature(start)]
// This is not found when building with Cargo
extern crate collections;
//#[cfg(target_os = "none")]
extern crate rlibc;
#[start]
pub fn main(_argc: isize, _argv: *const *const u8) -> isize {
// or some call like this
core::collections::Vec::new();
0
}
Is there an easy way to include the collections crate?

Answer 1

One possible solution is to compile it yourself. This requires having a checkout of the Rust source. I don't have a working environment to test this in, so take this suggestion with a pinch of salt. Conceptually, you would do something like this:
cd $RUST_SRC_DIR
rustc --version --verbose | grep commit-hash # Grab the hash
git checkout $RUSTC_HASH
mkdir cross-compiled-libraries
rustc --target=arm-whatever-whatever -O src/libcollections/lib.rs \
--out-dir cross-compiled-libraries
Repeat the last step for whatever libraries you need. A lot of this is taken from the ideas in Embedded Rust Right Now!.
A big concern with this solution is that libcollections requires an allocator. Generally, there is jemalloc or the system allocator. I don't know if either are available on the target you are compiling for...
This doesn't quite get you all the way to something easy to use for Cargo, either. The stuff inside of Rust isn't actually Cargo-ified yet, either. You could create a new Cargo project and add something like this to the Cargo.toml:
[lib]
path = "/path/to/rust/src/libcollections/lib.rs"
Which would then allow you to rely on Cargo more.