Add support for our temporary target (#68)

For the current phase of work we are requiring that input LLVM IR be
built to target the `aarch64-unknown-none-softfloat` target. This means
that it can make no assumptions about either the host operating system
(as there is none), nor the target ABI (meaning that we fall back to
platform codegen ABI convention, and hence that `rustc` will not do
anything funky here).

We add support in the nix derivation (`flake.nix`) so that this target
is automatically made available in the sandbox. It may be necessary to
add it at the system level as well, depending on how your development
environment is set up.

crates/compiler/src/compile.rs

@@ -49,3 +49,6 @@
 //! translation—the benefits of not having to manually perform this additional
 //! work far outweighs that downside. If we _do_ need any additional control, we
 //! can always modify this process at a later date.
+
+#[cfg(test)]
+mod test {}

crates/rust-test-input/src/lib.rs

@@ -1,14 +1,6 @@
+#![no_std]
+
+/// Adds two numbers together.
 pub fn add(left: u64, right: u64) -> u64 {
     left + right
 }
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
-    }
-}

docs/Getting LLVM IR Output.md

@@ -8,9 +8,9 @@ this project.
 ## Rust
 
 `rustc` will emit LLVM IR when passed the `--emit=llvm-ir` flag, and LLVM bytecode when passed the
-`--emit=llvm-bc`. This will output `.ll` files into the `target` directory corresponding to your
-compiled file. For more information, see the
-[`rustc` developer guide](https://rustc-dev-guide.rust-lang.org/backend/debugging.html).
+`--emit=llvm-bc`. This will output `.ll` (or `.bc`) files into the `target` directory corresponding
+to your compiled file (usually in `build-type/deps/crate-name-hash.ll`). For more information, see
+the [`rustc` developer guide](https://rustc-dev-guide.rust-lang.org/backend/debugging.html).
 
 - This can be passed to the correct compiler when using cargo by calling
   `cargo rustc -- --emit=llvm-ir`.

flake.nix

@@ -18,15 +18,22 @@
     flake-utils.lib.eachDefaultSystem (system:
       let
         # We grab our expected rust version from the Cargo.toml.
-        rustVersion = (builtins.fromTOML (builtins.readFile ./Cargo.toml)).workspace.package.rust-version;
+        rustVersion = (lib.importTOML ./Cargo.toml).workspace.package.rust-version;
 
         # Then we set up our libraries for building this thing.
         pkgs = nixpkgs.legacyPackages.${system};
         inherit (pkgs) lib;
         fenixLib = fenix.packages.${system};
+        toolchainHash = "sha256-VZZnlyP69+Y3crrLHQyJirqlHrTtGTsyiSnZB8jEvVo=";
         fenixStable = fenixLib.fromToolchainName {
             name = rustVersion;
-            sha256 = "sha256-VZZnlyP69+Y3crrLHQyJirqlHrTtGTsyiSnZB8jEvVo=";
+            sha256 = toolchainHash;
+        };
+
+        # A target of the same version for our temporary "source" ABI
+        fenixAarch64 = fenixLib.targets.aarch64-unknown-none-softfloat.toolchainOf {
+          channel = rustVersion;
+          sha256 = toolchainHash;
         };
 
         # As we want nightly Rustfmt, we have to build a custom toolchain.
@@ -40,6 +47,7 @@
             "rust-std"
             "rustc"
           ])
+          fenixAarch64.rust-std
         ];
 
         # The crane library configures the Rust toolchain, along with the components we expect it