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.
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