Fix error for custom targets

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))]
@@ -8,13 +10,36 @@ enum DalekBits {
     Dalek64,
 }
 
+// Only when the curve25519_dalek_bits is not explicitly set.
+// Error handling when the bits setting cannot be determined
 #[cfg(all(not(curve25519_dalek_bits = "64"), not(curve25519_dalek_bits = "32")))]
-#[deny(dead_code)]
-fn lotto_curve25519_dalek_bits() -> DalekBits {
+fn determine_curve25519_dalek_bits_error(error_code: i32, trigger_msg: &str) -> ! {
+    eprintln!("Error: {trigger_msg}");
+    eprintln!("Please set cfg(curve25519_dalek_bits) explicitly either as 32 or 64.");
+    std::process::exit(error_code)
+}
+
+// Only when the curve25519_dalek_bits is not explicitly set.
+#[cfg(all(not(curve25519_dalek_bits = "64"), not(curve25519_dalek_bits = "32")))]
+fn determine_curve25519_dalek_bits() -> DalekBits {
     use platforms::target::PointerWidth;
 
-    let target_triplet = std::env::var("TARGET").unwrap();
-    let platform = platforms::Platform::find(&target_triplet).unwrap();
+    // 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(
+            10,
+            "Standard Cargo TARGET environment variable is not set",
+        ),
+    };
+
+    // platforms crate is the source of truth used to determine the platform
+    // Custom target platforms require explicit settings.
+    let platform = match platforms::Platform::find(&target_triplet) {
+        Some(p) => p,
+        None => determine_curve25519_dalek_bits_error(20, "Unrecognised Rust target"),
+    };
 
     #[allow(clippy::match_single_binding)]
     match platform.target_arch {
@@ -26,6 +51,8 @@ fn lotto_curve25519_dalek_bits() -> DalekBits {
         _ => 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,
         },
     }
@@ -39,7 +66,7 @@ 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 = determine_curve25519_dalek_bits();
 
     match curve25519_dalek_bits {
         DalekBits::Dalek64 => println!("cargo:rustc-cfg=curve25519_dalek_bits=\"64\""),