NFC: Polyfill const_option feature.

src/polyfill.rs

@@ -35,7 +35,9 @@ mod leading_zeros_skipped;
 #[cfg(test)]
 mod test;
 
-pub use self::{array_flat_map::ArrayFlatMap, chunks_fixed::*};
+mod unwrap_const;
+
+pub use self::{array_flat_map::ArrayFlatMap, chunks_fixed::*, unwrap_const::unwrap_const};
 
 #[cfg(feature = "alloc")]
 pub use leading_zeros_skipped::LeadingZerosStripped;

src/polyfill/unwrap_const.rs

@@ -0,0 +1,15 @@
+/// Polyfill for `Option::unwrap()` as a const fn; feature `const_option`.
+/// https://github.com/rust-lang/rust/issues/67441.
+/// TODO(MSRV): Replace this with `x.unwrap()`.
+///
+/// `T: Copy` avoids "constant functions cannot evaluate destructors."
+pub const fn unwrap_const<T>(x: Option<T>) -> T
+where
+    T: Copy,
+{
+    if let Some(x) = x {
+        x
+    } else {
+        panic!("unwrap_const on `None`");
+    }
+}

src/rsa/public_exponent.rs

@@ -1,5 +1,5 @@
 use crate::error;
-use crate::polyfill::{ArrayFlatMap, LeadingZerosStripped};
+use crate::polyfill::{unwrap_const, ArrayFlatMap, LeadingZerosStripped};
 use core::num::NonZeroU64;
 
 /// The exponent `e` of an RSA public key.
@@ -10,16 +10,8 @@ impl PublicExponent {
     #[cfg(test)]
     const ALL_CONSTANTS: [Self; 3] = [Self::_3, Self::_65537, Self::MAX];
 
-    // TODO: Use `NonZeroU64::new(...).unwrap()` when `feature(const_panic)` is
-    // stable.
-    pub(super) const _3: Self = Self(match NonZeroU64::new(3) {
-        Some(nz) => nz,
-        None => unreachable!(),
-    });
-    pub(super) const _65537: Self = Self(match NonZeroU64::new(65537) {
-        Some(nz) => nz,
-        None => unreachable!(),
-    });
+    pub(super) const _3: Self = Self(unwrap_const(NonZeroU64::new(3)));
+    pub(super) const _65537: Self = Self(unwrap_const(NonZeroU64::new(65537)));
 
     // This limit was chosen to bound the performance of the simple
     // exponentiation-by-squaring implementation in `elem_exp_vartime`. In
@@ -32,13 +24,7 @@ impl PublicExponent {
     // [1] https://www.imperialviolet.org/2012/03/16/rsae.html
     // [2] https://www.imperialviolet.org/2012/03/17/rsados.html
     // [3] https://msdn.microsoft.com/en-us/library/aa387685(VS.85).aspx
-    //
-    // TODO: Use `NonZeroU64::new(...).unwrap()` when `feature(const_panic)` is
-    // stable.
-    const MAX: Self = Self(match NonZeroU64::new((1u64 << 33) - 1) {
-        Some(nz) => nz,
-        None => unreachable!(),
-    });
+    const MAX: Self = Self(unwrap_const(NonZeroU64::new((1u64 << 33) - 1)));
 
     pub(super) fn from_be_bytes(
         input: untrusted::Input,