Enable building freestanding sysroot with cargo

[Zoxc]

With these changes you can build a freestanding sysroot without using floating points using a Cargo.toml and copying the deps folder cargo outputs.

[package]
name = "sysroot"
version = "0.1.0"

[lib]
path = "lib.rs"
crate-type = ["rlib"]

[dependencies.core]
path = "../vendor/rust/src/src/libcore"
features = ["disable_float"]

[dependencies]
collections = { path = "../vendor/rust/src/src/libcollections" }

[rust-highfive]

r? @brson

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[hanna-kruppe]

Are you aware of rust-lang/rfcs#1364? Over there, a different approach has been discussed. I'm not sure how much this PR and that issue overlap, because I am not sure what a "freestanding sysroot" is, can you elaborate on what one would use this for and why that's important?

[alexcrichton]

Thanks for the PR! I think that the disable_float feature and its relevant changes may want to be held off for now (as @rkruppe) mentioned, so perhaps those can be separated out for another PR?

[alexcrichton]

Ah also I don't think this PR will work because alloc does indeed link to alloc_system. I think that can move to the standard library, however?

[Zoxc]

alloc links to some unknown crate implementing allocators. std already links to alloc_system. (and also optionally to alloc_jemalloc, not sure how that works)

[alexcrichton]

Sorry, but to clarify, this will not compile. Crates built with Cargo can only depend on other crates listed in Cargo.toml. If alloc_system is listed in src/liballoc/lib.rs, it will not compile.

The business of selecting alloc_system unconditionally in stage0 can be moved to src/libstd/lib.rs, however, where it's probably more appropriately placed anyway.

[Zoxc]

I moved that to libstd

[alexcrichton]

@bors: r+ 8ade080

Thanks!

[bors]

⌛ Testing commit 8ade080 with merge f93a62b...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt