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

5 files changed, 35 insertions, 4 deletions

diff --git a/src/Cargo.lock b/src/Cargo.lock
index 1f7cf84cedb..42ac9b3c49d 100644
--- a/src/Cargo.lock
+++ b/src/Cargo.lock
@@ -1082,6 +1082,16 @@ version = "0.1.12"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
+name = "nostd"
+version = "0.0.0"
+dependencies = [
+ "alloc 0.0.0",
+ "compiler_builtins 0.0.0",
+ "core 0.0.0",
+ "std_unicode 0.0.0",
+]
+
+[[package]]
 name = "num"
 version = "0.1.42"
 source = "registry+https://github.com/rust-lang/crates.io-index"
diff --git a/src/Cargo.toml b/src/Cargo.toml
index 814c054c51e..babf35d570b 100644
--- a/src/Cargo.toml
+++ b/src/Cargo.toml
@@ -2,6 +2,7 @@
 members = [
   "bootstrap",
   "rustc",
+  "libnostd",
   "libstd",
   "libtest",
   "librustc_trans",
diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index eaf4ab272c6..a93b26ac2ba 100644
--- a/src/bootstrap/compile.rs
+++ b/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();
 
diff --git a/src/libnostd/Cargo.toml b/src/libnostd/Cargo.toml
new file mode 100644
index 00000000000..6919390d3e2
--- /dev/null
+++ b/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" }
\ No newline at end of file
diff --git a/src/libnostd/lib.rs b/src/libnostd/lib.rs
new file mode 100644
index 00000000000..d28afe2838e
--- /dev/null
+++ b/src/libnostd/lib.rs
@@ -0,0 +1,3 @@
+#![feature(staged_api)]
+#![no_std]
+#![unstable(feature = "nostd", issue = "0")]