Enforce clippy::cast_possible_truncation.

For now, just put `#[allow(...)]` directives in the places where the
conversions are done. We'll follow up in the future with the correct
replacement for `as` for each case, as several PRs.

src/aead/aes.rs

@@ -49,6 +49,7 @@ fn set_encrypt_key(
     key: &mut AES_KEY,
 ) -> Result<(), error::Unspecified> {
     // Unusually, in this case zero means success and non-zero means failure.
+    #[allow(clippy::cast_possible_truncation)]
     if 0 == unsafe { f(bytes.as_ptr(), key_bits.as_usize_bits() as c::uint, key) } {
         Ok(())
     } else {
@@ -111,6 +112,7 @@ fn ctr32_encrypt_blocks_(
     assert_eq!(in_out_len % BLOCK_LEN, 0);
 
     let blocks = in_out_len / BLOCK_LEN;
+    #[allow(clippy::cast_possible_truncation)]
     let blocks_u32 = blocks as u32;
     assert_eq!(blocks, polyfill::usize_from_u32(blocks_u32));
 

src/aead/chacha.rs

@@ -272,6 +272,7 @@ mod tests {
             // behavior of ChaCha20 implementation changes dependent on the
             // length of the input.
             for len in 0..=input.len() {
+                #[allow(clippy::cast_possible_truncation)]
                 chacha20_test_case_inner(
                     &key,
                     &nonce,

src/aead/gcm/gcm_nohw.rs

@@ -27,6 +27,7 @@ use crate::polyfill::ArraySplitMap;
 
 #[cfg(target_pointer_width = "64")]
 fn gcm_mul64_nohw(a: u64, b: u64) -> (u64, u64) {
+    #[allow(clippy::cast_possible_truncation)]
     #[inline(always)]
     fn lo(a: u128) -> u64 {
         a as u64

src/ec/suite_b/ecdsa/signing.rs

@@ -14,6 +14,8 @@
 
 //! ECDSA Signatures using the P-256 and P-384 curves.
 
+#![allow(clippy::cast_possible_truncation)] // XXX
+
 use super::digest_scalar::digest_scalar;
 use crate::{
     arithmetic::montgomery::*,

src/ec/suite_b/ops/p256.rs

@@ -248,6 +248,7 @@ fn p256_scalar_inv_to_mont(a: &Scalar<Unencoded>) -> Scalar<R> {
     //    1011110011100110111110101010110110100111000101111001111010000100
     //    1111001110111001110010101100001011111100011000110010010101001111
 
+    #[allow(clippy::cast_possible_truncation)]
     static REMAINING_WINDOWS: [(u8, u8); 26] = [
         (6, B_101111 as u8),
         (2 + 3, B_111 as u8),

src/ec/suite_b/ops/p384.rs

@@ -220,6 +220,7 @@ fn p384_scalar_inv_to_mont(a: &Scalar<Unencoded>) -> Scalar<R> {
     //    0101100000011010000011011011001001001000101100001010011101111010
     //    1110110011101100000110010110101011001100110001010010100101110001
 
+    #[allow(clippy::cast_possible_truncation)]
     static REMAINING_WINDOWS: [(u8, u8); 39] = [
         (2, B_11 as u8),
         (3 + 3, B_111 as u8),

src/io/der_writer.rs

@@ -39,6 +39,7 @@ pub(crate) fn write_all(tag: Tag, write_value: &dyn Fn(&mut dyn Accumulator)) ->
     output.into()
 }
 
+#[allow(clippy::cast_possible_truncation)]
 fn write_tlv<F>(output: &mut dyn Accumulator, tag: Tag, write_value: F)
 where
     F: Fn(&mut dyn Accumulator),

src/lib.rs

@@ -60,6 +60,7 @@
 #![warn(
     clippy::unnecessary_cast,
     clippy::cast_lossless,
+    clippy::cast_possible_truncation,
     clippy::cast_possible_wrap,
     clippy::cast_precision_loss,
     clippy::cast_sign_loss