The type inference engine is pretty smart. It does more than looking at the
type of the
r-value
during an initialization. It also looks at how the variable is used afterwards
to infer its type. Here's an advanced example of type inference:
fn main() {
// Using local inference, the compiler knows that `elem` has type u8
let elem = 5u8;
// Create an empty vector (a growable array)
let mut vec = Vec::new();
// At this point the compiler doesn't know the exact type of `vec`, it
// just knows that it's a vector of something (`Vec<_>`)
// Insert `elem` in the vector
vec.push(elem);
// Aha! Now the compiler knows that `vec` is a vector of `u8`s (`Vec<u8>`)
// TODO ^ Try commenting out the `vec.push(elem)` line
println!("{:?}", vec);
}
No type annotation of variables was needed, the compiler is happy and so is the
programmer!