clippy: enable lossless-cast lint

There is a huge list of lints I'd like to enable (currently in
rust-elements and a few other crates). As a start, enable the
cast_lossless one, which I think is very important: it complaints if you
use a cast where usize::from() would suffice.

The benefit of the from() syntax is that it will compile iff the cast
cannot truncate. So the reader of the code can tell immediately that
there is no danger of changing the value, and if the lint is on,
conversely the reader can tell that anything that *is* a cast is
something that needs further scrutiny.

Author: apoelstra

Cargo.toml

@@ -58,3 +58,10 @@ match_bool = "allow" # Adds extra indentation and LOC.
 match_same_arms = "allow" # Collapses things that are conceptually unrelated to each other.
 must_use_candidate = "allow" # Useful for audit but many false positives.
 similar_names = "allow" # Too many (subjectively) false positives.
+# Cast-related lints
+cast_lossless = "warn"
+cast_possible_truncation = "allow" # All casts should include a code comment (except test code).
+cast_possible_wrap = "allow" # Same as above re code comment.
+cast_precision_loss = "warn"
+cast_ptr_alignment = "warn"
+cast_sign_loss = "allow" # All casts should include a code comment (except in test code).

src/policy/satisfy.rs

@@ -555,14 +555,14 @@ mod tests {
             let witness = to_witness(&program);
             assert_eq!(2, witness.len());
 
-            assert_eq!(Value::u1(bit as u8), *witness[0]);
+            assert_eq!(Value::u1(u8::from(bit)), *witness[0]);
             let preimage_bytes = witness[1]
                 .iter_padded()
                 .try_collect_bytes()
                 .expect("to bytes");
             let witness_preimage =
                 Preimage32::try_from(preimage_bytes.as_slice()).expect("to array");
-            assert_eq!(preimages[bit as usize], witness_preimage);
+            assert_eq!(preimages[usize::from(bit)], witness_preimage);
 
             execute_successful(program, &env);
         };
@@ -663,7 +663,7 @@ mod tests {
                         ],
                     );
 
-                    match bit0 as u8 + bit1 as u8 + bit2 as u8 {
+                    match u8::from(bit0) + u8::from(bit1) + u8::from(bit2) {
                         3 => assert_branches(&policy, &[bit0, bit1, false]),
                         2 => assert_branches(&policy, &[bit0, bit1, bit2]),
                         _ => assert!(policy.satisfy(&satisfier, &env).is_err()),