create a nostd crate

the goal is to build, in a single Cargo invocation, several no-std crates that we want to put in the
rust-std component of no-std targets. The nostd crate builds these crates:

- core
- compiler-builtin (with the "c" and "mem" features enabled)
- alloc
- std_unicode

src/Cargo.toml

@@ -2,6 +2,7 @@
 members = [
   "bootstrap",
   "rustc",
+  "libnostd",
   "libstd",
   "libtest",
   "librustc_trans",

src/bootstrap/compile.rs

@@ -145,10 +145,10 @@ pub fn std_cargo(build: &Builder,
     }
 
     if build.no_std(target) == Some(true) {
-        // for no-std targets we only compile core and compiler-builtins
-        cargo.arg("--features").arg("c mem")
-            .arg("--manifest-path")
-            .arg(build.src.join("src/rustc/compiler_builtins_shim/Cargo.toml"));
+        // for no-std targets we compile a minimal nostd crate that only depends on crates that work
+        // without an OS
+        cargo.arg("--manifest-path")
+            .arg(build.src.join("src/libnostd/Cargo.toml"));
     } else {
         let mut features = build.std_features();
 

src/libnostd/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+authors = ["The Rust Project Developers"]
+name = "nostd"
+version = "0.0.0"
+license = "MIT/Apache-2.0"
+repository = "https://github.com/rust-lang/rust.git"
+description = "(not) The Rust Standard Library"
+
+[lib]
+name = "nostd"
+path = "lib.rs"
+
+[dependencies]
+alloc = { path = "../liballoc" }
+compiler_builtins = { path = "../rustc/compiler_builtins_shim", features = ["c", "mem"] }
+core = { path = "../libcore" }
+std_unicode = { path = "../libstd_unicode" }

src/libnostd/lib.rs

@@ -0,0 +1,3 @@
+#![feature(staged_api)]
+#![no_std]
+#![unstable(feature = "nostd", issue = "0")]