Using if let expressions to test whether a pattern matches

Using pattern matching to unpack values into multiple variables can be useful, but using pattern matching to make decisions is where this functionality really shines, as in the following example:

let source2 = DemoStruct { id: 35, name: String::from("Another Thing"), probability: 0.42 };
let source3 = DemoStruct { id: 63, name: String::from("Super Thing"), probability: 0.99 };

if let DemoStruct { id: 63, name: y, probability: z } = source2 {
    println!("When id is 63, name is {} and probability is {}", y, z);
}

if let DemoStruct { id: 63, name: y, probability: z } = source3 {
    println!("When id is 63, name is {} and probability is {}", y, z);
}

Here, we've defined two more variables containing DemoStruct values, and then used pattern matching to pull them back apart and assign...