Fix simd_backend + --no-default-features

...and one other `alloc` cleanup.

`simd_backend` requires `alloc` and so this combination previously
caused a compile error.

Also one additional test that works with an `alloc`-only profile wasn't
enabled, causing a warning in this case.

Finally, changes CI to test this configuration.

Cargo.toml

@@ -63,4 +63,4 @@ alloc = ["zeroize/alloc"]
 # fiat-crypto backend with formally-verified field arithmetic
 fiat_backend = ["fiat-crypto"]
 # The SIMD backend uses parallel formulas, using either AVX2 or AVX512-IFMA.
-simd_backend = ["nightly", "packed_simd"]
+simd_backend = ["alloc", "nightly", "packed_simd"]

src/lib.rs

@@ -27,7 +27,8 @@
 // External dependencies:
 //------------------------------------------------------------------------
 
-#[cfg(all(feature = "alloc", not(feature = "std")))]
+#[cfg(feature = "alloc")]
+#[allow(unused_imports)]
 #[macro_use]
 extern crate alloc;
 

src/montgomery.rs

@@ -470,9 +470,9 @@ mod test {
     ];
 
     #[test]
-    #[cfg(feature = "std")] // Vec
+    #[cfg(feature = "alloc")] // Vec
     fn montgomery_elligator_correct() {
-        let bytes: std::vec::Vec<u8> = (0u8..32u8).collect();
+        let bytes: alloc::vec::Vec<u8> = (0u8..32u8).collect();
         let bits_in: [u8; 32] = (&bytes[..]).try_into().expect("Range invariant broken");
 
         let fe = FieldElement::from_bytes(&bits_in);