elliptic-curve: remove bits feature (#2417)

I was not able to find any users of it searching GitHub.

This feature is backed by `bitvec` which both has a high number of
transitive dependencies which have been minorly problematic (#2414), but
also my understanding based on Zulip discussions on #t-opsem is that
`bitvec` is not sound under either stacked or tree borrows, which makes
it a bit worrisome for use in cryptographic applications.

We can always add the feature back if it turns out there are users, but
if not it seems like it's not worth the maintenance burden.

Author: tarcieri

.github/workflows/elliptic-curve.yml

@@ -46,7 +46,6 @@ jobs:
       - run: cargo build --target ${{ matrix.target }} --release --no-default-features
       - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features alloc
       - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features arithmetic
-      - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features bits
       - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features critical-section
       - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features dev
       - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features digest
@@ -104,6 +103,6 @@ jobs:
       - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
         with:
-          toolchain: nightly-2026-02-11 # pinned due to rust-lang/miri#4855
+          toolchain: nightly-2026-05-01 # pinned due to ensure determinism
       - run: rustup component add miri && cargo miri setup
       - run: cargo miri test --all-features

Cargo.lock

@@ -36,7 +36,6 @@ pem-rfc7468 = { version = "1", optional = true, features = ["alloc"] }
 pkcs8 = { version = "0.11", optional = true, default-features = false }
 sec1 = { version = "0.8", optional = true, features = ["ctutils", "subtle", "zeroize"] }
 serdect = { version = "0.4", optional = true, default-features = false, features = ["alloc"] }
-tap = { version = "1.0.1", optional = true, default-features = false } # make `bitvec`'s `wyz` work with `minimal-versions`
 
 [dev-dependencies]
 hex-literal = "1"
@@ -62,7 +61,6 @@ std = [
 arithmetic = ["group"]
 basepoint-table = ["arithmetic"]
 critical-section = ["basepoint-table", "once_cell/critical-section"]
-bits = ["dep:tap", "arithmetic", "ff/bits"]
 dev = ["arithmetic", "dep:hex-literal", "pem", "pkcs8"]
 ecdh = ["arithmetic", "digest", "dep:hkdf"]
 getrandom = ["arithmetic", "bigint/getrandom", "common/getrandom"]

elliptic-curve/Cargo.toml

@@ -31,8 +31,6 @@ use pkcs8::AssociatedOid;
 
 #[cfg(feature = "alloc")]
 use alloc::vec::Vec;
-#[cfg(feature = "bits")]
-use ff::PrimeFieldBits;
 
 /// Pseudo-coordinate for fixed-based scalar mult output
 pub const PSEUDO_COORDINATE_FIXED_BASE_MUL: [u8; 32] =
@@ -56,10 +54,6 @@ pub type SecretKey = crate::SecretKey<MockCurve>;
 /// Scalar value type.
 pub type ScalarValue = crate::ScalarValue<MockCurve>;
 
-/// Scalar bits.
-#[cfg(feature = "bits")]
-pub type ScalarBits = crate::scalar::ScalarBits<MockCurve>;
-
 /// Mock elliptic curve type useful for writing tests which require a concrete
 /// curve type.
 ///
@@ -161,23 +155,6 @@ impl PrimeField for Scalar {
     }
 }
 
-#[cfg(feature = "bits")]
-impl PrimeFieldBits for Scalar {
-    #[cfg(target_pointer_width = "32")]
-    type ReprBits = [u32; 8];
-
-    #[cfg(target_pointer_width = "64")]
-    type ReprBits = [u64; 4];
-
-    fn to_le_bits(&self) -> ScalarBits {
-        self.0.as_uint().to_words().into()
-    }
-
-    fn char_le_bits() -> ScalarBits {
-        MockCurve::ORDER.to_words().into()
-    }
-}
-
 impl Generate for Scalar {
     fn try_generate_from_rng<R: TryCryptoRng + ?Sized>(
         rng: &mut R,

elliptic-curve/src/dev/mock_curve.rs

@@ -21,10 +21,6 @@ use crate::CurveArithmetic;
 #[cfg(feature = "arithmetic")]
 pub type Scalar<C> = <C as CurveArithmetic>::Scalar;
 
-/// Bit representation of a scalar field element of a given curve.
-#[cfg(feature = "bits")]
-pub type ScalarBits<C> = ff::FieldBits<<Scalar<C> as ff::PrimeFieldBits>::ReprBits>;
-
 /// Instantiate a scalar from an unsigned integer without checking for overflow.
 pub trait FromUintUnchecked {
     /// Unsigned integer type (i.e. `Curve::Uint`)