Provide memory intrinsics via compiler builtins

Use unstable feature to remove rlibc crate dependency.

.cargo/config.toml

@@ -1,5 +1,7 @@
 [unstable]
 build-std = ["core", "compiler_builtins", "alloc"]
+build-std-features = ["compiler-builtins-mem"]
+
 [build]
 # https://internals.rust-lang.org/t/evaluating-pipelined-rustc-compilation/10199/12
 pipelining = true

Cargo.lock

@@ -72,12 +72,6 @@ dependencies = [
  "tock-registers",
 ]
 
-[[package]]
-name = "rlibc"
-version = "1.0.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "fc874b127765f014d792f16763a81245ab80500e2ad921ed4ee9e82481ee08fe"
-
 [[package]]
 name = "snafu"
 version = "0.6.9"
@@ -145,7 +139,6 @@ dependencies = [
  "qemu-exit",
  "r0",
  "register",
- "rlibc",
  "snafu",
  "usize_conversions",
  "ux",

README.md

@@ -30,7 +30,7 @@ Vesper has been influenced by the kernels in L4 family, notably seL4. Fawn and N
 
 ## Build instructions
 
-Use at least rustc nightly 2020-07-15 with cargo nightly of the same or later date. It adds support for `cargo build --build-std` feature.
+Use at least rustc nightly 2020-09-30 with cargo nightly of the same or later date. It adds support for `cargo build --build-std` feature (since 2020-07-15) and support for compiler_builtins memory operations ([since 2020-09-30](https://github.com/rust-lang/rust/pull/77284)).
 
 * Install tools: `cargo install just cargo-make`.
 * Install qemu (at least version 4.1.1): `brew install qemu`.

nucleus/Cargo.toml

@@ -25,7 +25,6 @@ qemu = ["qemu-exit"]
 
 [dependencies]
 r0 = "1.0"
-rlibc = "1.0"
 qemu-exit = { version = "1.0", optional = true }
 cortex-a = "4.0"
 ux = { version = "0.1.3", default-features = false }

nucleus/src/main.rs

@@ -25,8 +25,6 @@
 #[cfg(not(target_arch = "aarch64"))]
 use architecture_not_supported_sorry;
 
-extern crate rlibc; // To enable linking memory intrinsics.
-
 /// Architecture-specific code.
 #[macro_use]
 pub mod arch;