Resolve clippy::assign_op_pattern

    error: manual implementation of an assign operation
       --> src/helpers.rs:673:17
        |
    673 |                 len = len + size1;
        |                 ^^^^^^^^^^^^^^^^^ help: replace it with: `len += size1`
        |
        = note: `-D clippy::assign-op-pattern` implied by `-D clippy::all`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assign_op_pattern

Author: dtolnay

src/helpers.rs

@@ -670,7 +670,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             if byte == 0 {
                 break;
             } else {
-                len = len + size1;
+                len += size1;
             }
         }
 

src/lib.rs

@@ -10,7 +10,6 @@
 #![allow(clippy::cast_lossless)]
 // TODO: Uncategorized. Some of these we'll want to fix, some keep ignored.
 #![allow(
-    clippy::assign_op_pattern,
     clippy::clone_on_copy,
     clippy::collapsible_else_if,
     clippy::collapsible_if,