A Rust Programming Puzzle

Rust Structure Packing

How Can We Control Structure Layout?

Let’s see what you already know about structures in Rust and how the compiler handles them.

See if you can guess the output from the following program:

▶ Click here to run the program on the Rust Playground.

Result

You may be surprised by the result:

OneByte occupies 1 bytes.
TwoBytes occupies 2 bytes.
ThreeBytes occupies 4 bytes.

The OneByte and TwoBytes structures are occupying 1- and 2-bytes of memory — as you would expect. ThreeBytes has only 3-bytes of data but reports a size of 4 bytes.

Discussion

By default, Rust makes no promises about the layout of structures, other than your variables will be included inside them. In particular, the Rust/LLVM toolchain (the compiler system Rust uses) is free to make two changes to your structures if it determines that they will help performance: