Skip the usize to u64 conversion

Author: tiif

src/shims/unix/fd.rs

@@ -653,19 +653,14 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         dest: &MPlaceTy<'tcx>,
     ) -> InterpResult<'tcx> {
         let this = self.eval_context_mut();
-        // `File::read` never returns a value larger than `count`, so this cannot fail.
-        match result.map(|c| i64::try_from(c).unwrap()) {
-            // try to pass this the write_ptr inside write
-            // Pass the pointer inside the write function.
+        match result {
             Ok(read_bytes) => {
                 // If reading to `bytes` did not fail, we write those bytes to the buffer.
                 // Crucially, if fewer than `bytes.len()` bytes were read, only write
                 // that much into the output buffer!
-                this.write_bytes_ptr(
-                    buf,
-                    bytes[..usize::try_from(read_bytes).unwrap()].iter().copied(),
-                )?;
-                this.write_int(read_bytes, dest)?;
+                this.write_bytes_ptr(buf, bytes[..read_bytes].iter().copied())?;
+                // The actual read size is always lesser than `count` so this cannot fail.
+                this.write_int(u64::try_from(read_bytes).unwrap(), dest)?;
                 return Ok(());
             }
             Err(e) => {