What does 'let x = x' do in Rust?

It makes fields immutable again.

fields was previously defined as mutable (let mut fields = …;), to be used with sort_by_key which sorts in-place and requires the target to be mutable. The author has chosen here to explicitly prevent further mutability.

"Downgrading" a mutable binding to immutable is quite common in Rust.

Another common way to do this is to use a block expression:

let fields = {
    let mut fields = …;
    fields.sort_by_key(…);
    fields
};

The statement let var = var; makes var immutable and bound to its current value. fields was declared as mut earlier.