Avoid using as to convert u8 to usize.

Enable the clippy lint for this.

Author: briansmith

src/lib.rs

@@ -27,6 +27,7 @@
 #![doc(html_root_url = "https://briansmith.org/rustdoc/")]
 #![cfg_attr(not(feature = "std"), no_std)]
 #![allow(clippy::single_match)]
+#![deny(clippy::as_conversions)]
 
 #[cfg(any(test, feature = "alloc"))]
 #[cfg_attr(test, macro_use)]

src/verify_cert.rs

@@ -232,7 +232,7 @@ fn check_basic_constraints(
             // certificates have pathLenConstraint.
             let path_len_constraint = if !input.at_end() {
                 let value = der::small_nonnegative_integer(input)?;
-                Some(value as usize)
+                Some(usize::from(value))
             } else {
                 None
             };