clippy: don't use lossless casts

When casting between numeric types, if the conversion is infallible then
we can use `from`. This means that if the types change to make the
conversion lossy, we'll get a compiler error rather than the logic
silently becoming wrong.

Author: apoelstra

Cargo.toml

@@ -57,7 +57,7 @@ assigning_clones = "warn"
 bool_to_int_with_if = "warn"
 borrow_as_ptr = "warn"
 case_sensitive_file_extension_comparisons = "warn"
-cast_lossless = "allow"
+cast_lossless = "warn"
 cast_precision_loss = "warn"
 cast_ptr_alignment = "warn"
 checked_conversions = "warn"

src/num.rs

@@ -299,7 +299,7 @@ impl fmt::Display for U256 {
 
             // Divide by 10, starting at the most significant bytes
             for byte in bytes.iter_mut() {
-                let value = carry * 256 + *byte as u32;
+                let value = carry * 256 + u32::from(*byte);
                 *byte = (value / 10) as u8;
                 carry = value % 10;
 
@@ -334,7 +334,7 @@ impl FromStr for U256 {
 
             // Add to the least significant bytes first
             for byte in bytes.iter_mut().rev() {
-                let value = *byte as u32 * 10 + carry;
+                let value = u32::from(*byte) * 10 + carry;
                 *byte = (value % 256) as u8;
                 carry = value / 256;
             }