Fix error for custom targets (#510)

Author: pinkforest

build.rs

@@ -1,5 +1,7 @@
 //! This selects the curve25519_dalek_bits either by default from target_pointer_width or explicitly set
 
+#![deny(clippy::unwrap_used, dead_code)]
+
 #[allow(non_camel_case_types)]
 enum DalekBits {
     #[cfg_attr(curve25519_dalek_bits = "64", allow(dead_code))]
@@ -8,29 +10,6 @@ enum DalekBits {
     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();
-
-    #[allow(clippy::match_single_binding)]
-    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(curve25519_dalek_bits = "32")]
     let curve25519_dalek_bits = DalekBits::Dalek32;
@@ -39,10 +18,64 @@ fn main() {
     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();
+    let curve25519_dalek_bits = deterministic::determine_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\""),
     }
 }
+
+// Deterministic cfg(curve25519_dalek_bits) when this is not explicitly set.
+#[cfg(all(not(curve25519_dalek_bits = "64"), not(curve25519_dalek_bits = "32")))]
+mod deterministic {
+
+    use super::*;
+
+    // Standard Cargo TARGET environment variable of triplet is required
+    static ERR_MSG_NO_TARGET: &str = "Standard Cargo TARGET environment variable is not set.";
+
+    // Custom Non-Rust standard target platforms require explicit settings.
+    static ERR_MSG_NO_PLATFORM: &str = "Unknown Rust target platform.";
+
+    // Error handling when the bits setting cannot be determined
+    fn determine_curve25519_dalek_bits_error(cause: &str) -> ! {
+        eprintln!("Error: {cause}");
+        eprintln!("Please set cfg(curve25519_dalek_bits) explicitly either as 32 or 64.");
+        std::process::exit(1)
+    }
+
+    // Determine the curve25519_dalek_bits based on Rust standard TARGET triplet
+    pub(super) fn determine_curve25519_dalek_bits() -> DalekBits {
+        use platforms::target::PointerWidth;
+
+        // TARGET environment is supplied by Cargo
+        // https://doc.rust-lang.org/cargo/reference/environment-variables.html
+        let target_triplet = match std::env::var("TARGET") {
+            Ok(t) => t,
+            Err(_) => determine_curve25519_dalek_bits_error(ERR_MSG_NO_TARGET),
+        };
+
+        // platforms crate is the source of truth used to determine the platform
+        let platform = match platforms::Platform::find(&target_triplet) {
+            Some(p) => p,
+            None => determine_curve25519_dalek_bits_error(ERR_MSG_NO_PLATFORM),
+        };
+
+        #[allow(clippy::match_single_binding)]
+        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,
+                // Intended default solely for non-32/64 target pointer widths
+                // Otherwise known target platforms only.
+                _ => DalekBits::Dalek32,
+            },
+        }
+    }
+}