Fix --no-default-features test warnings; consolidate CI jobs (#461)

Previously `cargo test --no-default-features` would succeed but with
warnings. This commit fixes all of those warnings and tests
`--no-default-features` in CI to ensure that in perpetuity.

.github/workflows/rust.yml

@@ -8,6 +8,7 @@ on:
 
 env:
   CARGO_TERM_COLOR: always
+  RUSTFLAGS: '-D warnings'
 
 jobs:
   test:
@@ -26,8 +27,10 @@ jobs:
       - uses: dtolnay/rust-toolchain@stable
       - run: rustup target add ${{ matrix.target }}
       - run: ${{ matrix.deps }}
+      - run: cargo test --target ${{ matrix.target }} --no-default-features
       - run: cargo test --target ${{ matrix.target }}
       - run: cargo test --target ${{ matrix.target }} --features fiat_backend
+      - run: cargo test --target ${{ matrix.target }} --features serde
 
   build-simd:
     name: Build simd backend (nightly)
@@ -43,24 +46,6 @@ jobs:
         RUSTFLAGS: "-C target_feature=+avx512ifma"
       run: cargo build --target x86_64-unknown-linux-gnu --features simd_backend
 
-  test-defaults-serde:
-    name: Test default feature selection and serde
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v3
-    - uses: dtolnay/rust-toolchain@stable
-    - run: cargo test --features "serde"
-
-  test-alloc-u32:
-    name: Test no_std+alloc with u32 backend
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v3
-    - uses: dtolnay/rust-toolchain@stable
-    - run: rustup target add i686-unknown-linux-gnu
-    - run: sudo apt update && sudo apt install gcc-multilib
-    - run: cargo test --lib --no-default-features --features alloc --target i686-unknown-linux-gnu
-
   nightly:
     name: Test nightly compiler
     runs-on: ubuntu-latest

src/montgomery.rs

@@ -482,6 +482,7 @@ mod test {
         assert_eq!(result, expected.to_montgomery())
     }
 
+    #[cfg(feature = "alloc")]
     const ELLIGATOR_CORRECT_OUTPUT: [u8; 32] = [
         0x5f, 0x35, 0x20, 0x00, 0x1c, 0x6c, 0x99, 0x36, 0xa3, 0x12, 0x06, 0xaf, 0xe7, 0xc7, 0xac,
         0x22, 0x4e, 0x88, 0x61, 0x61, 0x9b, 0xf9, 0x88, 0x72, 0x44, 0x49, 0x15, 0x89, 0x9d, 0x95,

src/ristretto.rs

@@ -179,6 +179,9 @@ use digest::Digest;
 use crate::constants;
 use crate::field::FieldElement;
 
+#[cfg(feature = "alloc")]
+use cfg_if::cfg_if;
+
 use subtle::Choice;
 use subtle::ConditionallyNegatable;
 use subtle::ConditionallySelectable;
@@ -196,16 +199,17 @@ use crate::traits::Identity;
 #[cfg(feature = "alloc")]
 use crate::traits::{MultiscalarMul, VartimeMultiscalarMul, VartimePrecomputedMultiscalarMul};
 
-#[cfg(not(all(
-    feature = "simd_backend",
-    any(target_feature = "avx2", target_feature = "avx512ifma")
-)))]
-use crate::backend::serial::scalar_mul;
-#[cfg(all(
-    feature = "simd_backend",
-    any(target_feature = "avx2", target_feature = "avx512ifma")
-))]
-use crate::backend::vector::scalar_mul;
+#[cfg(feature = "alloc")]
+cfg_if! {
+    if #[cfg(all(
+        feature = "simd_backend",
+        any(target_feature = "avx2", target_feature = "avx512ifma")
+    ))] {
+        use crate::backend::vector::scalar_mul;
+    } else {
+        use crate::backend::serial::scalar_mul;
+    }
+}
 
 // ------------------------------------------------------------------------
 // Compressed points

src/scalar.rs

@@ -1025,6 +1025,7 @@ impl Scalar {
 
     /// Returns a size hint indicating how many entries of the return
     /// value of `to_radix_2w` are nonzero.
+    #[cfg(any(feature = "alloc", test))]
     pub(crate) fn to_radix_2w_size_hint(w: usize) -> usize {
         debug_assert!(w >= 4);
         debug_assert!(w <= 8);