Set well known defaults for wasm and arm

As discussed in dalek-cryptography#456 this sets well known defaults for cfg(target_family = "wasm")
and cfg(target_arch = "arm") for 64 bit arithmetric via cfg(curve25519_dalek_bits = "64")

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 +1,30 @@
 //! This selects the curve25519_dalek_bits either by default from target_pointer_width or explicitly set
 
+use platforms::target::{Arch, PointerWidth};
+
+#[allow(non_camel_case_types)]
+enum DalekBits {
+    Dalek32,
+    Dalek64,
+}
+
 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\"");
+    let target_triplet = std::env::var("TARGET").unwrap();
+    let platform = platforms::Platform::find(&target_triplet).unwrap();
+
+    let curve25519_dalek_bits = match platform.target_arch {
+        Arch::Arm => DalekBits::Dalek64,
+        //TODO: Needs tests + benchmarks to back it up across
+        //Arch::Wasm32 => DalekBits::Dalek64,
+        _ => match platform.target_pointer_width {
+            PointerWidth::U64 => DalekBits::Dalek64,
+            PointerWidth::U32 => DalekBits::Dalek32,
+            _ => DalekBits::Dalek32,
+        },
+    };
+
+    match curve25519_dalek_bits {
+        DalekBits::Dalek64 => println!("cargo:rustc-cfg=curve25519_dalek_bits=\"64\""),
+        DalekBits::Dalek32 => println!("cargo:rustc-cfg=curve25519_dalek_bits=\"32\""),
+    }
 }