elliptic-curve: add PointWithBasepointTable trait (#2376)

Adds a trait with an associated `BASEPOINT_TABLE` constant which
provides a place to access a particular curve group's `BasepointTable`
from a generic context.

Author: tarcieri

elliptic-curve/src/point.rs

@@ -8,7 +8,7 @@ mod non_identity;
 pub use {self::non_identity::NonIdentity, lookup_table::LookupTable};
 
 #[cfg(feature = "basepoint-table")]
-pub use self::basepoint_table::BasepointTable;
+pub use self::basepoint_table::{BasepointTable, PointWithBasepointTable};
 
 use crate::{Curve, FieldBytes};
 use subtle::{Choice, CtOption};

elliptic-curve/src/point/basepoint_table.rs

@@ -6,7 +6,7 @@
 #[cfg(not(any(feature = "critical-section", feature = "std")))]
 compile_error!("`basepoint-table` feature requires either `critical-section` or `std`");
 
-use crate::point::LookupTable;
+use super::LookupTable;
 use group::Group;
 use subtle::ConditionallySelectable;
 use {core::ops::Deref, ff::PrimeField};
@@ -16,6 +16,12 @@ use once_cell::sync::Lazy as LazyLock;
 #[cfg(all(feature = "std", not(feature = "critical-section")))]
 use std::sync::LazyLock;
 
+/// Associate a precomputed `BASEPOINT_TABLE` constant with a curve point.
+pub trait PointWithBasepointTable<const WINDOW_SIZE: usize>: Group {
+    /// Basepoint table for this curve.
+    const BASEPOINT_TABLE: &'static BasepointTable<Self, WINDOW_SIZE>;
+}
+
 /// Precomputed lookup table of multiples of a base point, a.k.a. generator.
 ///
 /// This type leverages lazy computation, and requires one of the following crate features to be