Fixes curve25519_dalek_bits defaults for cross and wasm

build.rs was using cfg(target) but it has to evaluate this from env TARGET
as build.rs cfg(target) in build context is the builder host and not the target.

This change fixes curve25519_dalek_bits lottery to determine the correct
automatic curve25119_dalek_bits with the help of platforms crate.

As discussed in dalek-cryptography#456 this also prepares for well known defaults for wasm and
arm serial backend via cfg(curve25519_dalek_bits = "64")

If the wasm32 or armv7 are going to be u64 serial by default these will be
followed up on later.

.github/workflows/rust.yml

@@ -48,6 +48,37 @@ jobs:
         RUSTFLAGS: '--cfg curve25519_dalek_backend="simd" -C target_feature=+avx512ifma'
       run: cargo build --target x86_64-unknown-linux-gnu
 
+  cross:
+    strategy:
+      matrix:
+        include:
+          # ARM32
+          - target: armv7-unknown-linux-gnueabihf
+            rust: stable
+
+          # ARM64
+          - target: aarch64-unknown-linux-gnu
+            rust: stable
+
+          # PPC32
+          - target: powerpc-unknown-linux-gnu
+            rust: stable
+
+          # WASM32
+          - target: wasm32-unknown-unknown
+            rust: stable
+
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - run: ${{ matrix.deps }}
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ matrix.rust }}
+          targets: ${{ matrix.target }}
+      - run: cargo install cross
+      - run: cross test --release --target ${{ matrix.target }}
+
   nightly:
     name: Test nightly compiler
     runs-on: ubuntu-latest

CHANGELOG.md

@@ -22,6 +22,7 @@ major series.
 
 #### Other changes
 
+* Set well known defaults for wasm and arm
 * Update Maintenance Policies for SemVer
 * Migrate documentation to docs.rs hosted
 * Fix backend documentation generation

Cargo.toml

@@ -38,6 +38,9 @@ hex = "0.4.2"
 rand = "0.8"
 rand_core = { version = "0.6", default-features = false, features = ["getrandom"] }
 
+[build-dependencies]
+platforms = "3.0.2"
+
 [[bench]]
 name = "dalek_benchmarks"
 harness = false

build.rs

@@ -1,15 +1,45 @@
 //! This selects the curve25519_dalek_bits either by default from target_pointer_width or explicitly set
 
+#[allow(non_camel_case_types)]
+enum DalekBits {
+    Dalek32,
+    Dalek64,
+}
+
+#[cfg(all(not(curve25519_dalek_bits = "64"), not(curve25519_dalek_bits = "32")))]
+#[deny(dead_code)]
+fn lotto_curve25519_dalek_bits() -> DalekBits {
+    use platforms::target::PointerWidth;
+
+    let target_triplet = std::env::var("TARGET").unwrap();
+    let platform = platforms::Platform::find(&target_triplet).unwrap();
+
+    match platform.target_arch {
+        //Issues: 449 and 456
+        //TODO(Arm): Needs tests + benchmarks to back this up
+        //platforms::target::Arch::Arm => DalekBits::Dalek64,
+        //TODO(Wasm32): Needs tests + benchmarks to back this up
+        //platforms::target::Arch::Wasm32 => DalekBits::Dalek64,
+        _ => match platform.target_pointer_width {
+            PointerWidth::U64 => DalekBits::Dalek64,
+            PointerWidth::U32 => DalekBits::Dalek32,
+            _ => DalekBits::Dalek32,
+        },
+    }
+}
+
 fn main() {
-    #[cfg(any(
-        all(not(target_pointer_width = "64"), not(curve25519_dalek_bits = "64")),
-        curve25519_dalek_bits = "32"
-    ))]
-    println!("cargo:rustc-cfg=curve25519_dalek_bits=\"32\"");
-
-    #[cfg(any(
-        all(target_pointer_width = "64", not(curve25519_dalek_bits = "32")),
-        curve25519_dalek_bits = "64"
-    ))]
-    println!("cargo:rustc-cfg=curve25519_dalek_bits=\"64\"");
+    #[cfg(curve25519_dalek_bits = "32")]
+    let curve25519_dalek_bits = DalekBits::Dalek32;
+
+    #[cfg(curve25519_dalek_bits = "64")]
+    let curve25519_dalek_bits = DalekBits::Dalek64;
+
+    #[cfg(all(not(curve25519_dalek_bits = "64"), not(curve25519_dalek_bits = "32")))]
+    let curve25519_dalek_bits = lotto_curve25519_dalek_bits();
+
+    match curve25519_dalek_bits {
+        DalekBits::Dalek64 => println!("cargo:rustc-cfg=curve25519_dalek_bits=\"64\""),
+        DalekBits::Dalek32 => println!("cargo:rustc-cfg=curve25519_dalek_bits=\"32\""),
+    }
 }