k256: use std::sync::LazyLock instead of once_cell (#1135)

When building with the `precomputed-tables` and `std` features, uses the
built-in implementation of `LazyLock` in lieu of `once_cell::sync::Lazy`
thus avoiding the extra crate dependency.

The `critical-section` feature is retained and uses
`once_cell::sync::Lazy`.

When `precomputed-tables`, `critical-section`, and `std` are all
enabled, uses `once_cell/critical-section` and enables `once_cell/std`,
which should use a `std::sync::Mutex` for synchronization.

Author: tarcieri

k256/Cargo.toml

@@ -58,7 +58,7 @@ hash2curve = ["arithmetic", "elliptic-curve/hash2curve"]
 jwk = ["elliptic-curve/jwk"]
 pem = ["ecdsa-core/pem", "elliptic-curve/pem", "pkcs8"]
 pkcs8 = ["ecdsa-core/pkcs8", "elliptic-curve/pkcs8"]
-precomputed-tables = ["arithmetic", "once_cell"]
+precomputed-tables = ["arithmetic"]
 schnorr = ["arithmetic", "sha256", "signature"]
 serde = ["ecdsa-core/serde", "elliptic-curve/serde", "serdect"]
 sha256 = ["digest", "sha2"]

k256/src/arithmetic/mul.rs

@@ -52,8 +52,13 @@ use elliptic_curve::{
     subtle::{Choice, ConditionallySelectable, ConstantTimeEq},
 };
 
-#[cfg(feature = "precomputed-tables")]
-use once_cell::sync::Lazy;
+#[cfg(all(feature = "precomputed-tables", feature = "critical-section"))]
+use once_cell::sync::Lazy as LazyLock;
+#[cfg(all(
+    feature = "precomputed-tables",
+    all(feature = "std", not(feature = "critical-section"))
+))]
+use std::sync::LazyLock;
 
 /// Lookup table containing precomputed values `[p, 2p, 3p, ..., 8p]`
 #[derive(Copy, Clone, Default)]
@@ -363,7 +368,7 @@ fn lincomb(
 
 /// Lazily computed basepoint table.
 #[cfg(feature = "precomputed-tables")]
-static GEN_LOOKUP_TABLE: Lazy<[LookupTable; 33]> = Lazy::new(precompute_gen_lookup_table);
+static GEN_LOOKUP_TABLE: LazyLock<[LookupTable; 33]> = LazyLock::new(precompute_gen_lookup_table);
 
 #[cfg(feature = "precomputed-tables")]
 fn precompute_gen_lookup_table() -> [LookupTable; 33] {

k256/src/lib.rs

@@ -32,6 +32,9 @@
 #[macro_use]
 extern crate alloc;
 
+#[cfg(feature = "std")]
+extern crate std;
+
 #[cfg(feature = "arithmetic")]
 mod arithmetic;