When to be explicit with type annotation?

Question

I read Rust programming language , but I could not find a clear answer to what is best for type annotation, in particular with primitive types.

For example, letting the compiler determine the type:

let v = true;

Or using explicit type annotation:

let v: bool = true;

In general, I like to be explicit, but I'm not sure if this is against best practices or a preferred style. Will compilation time be improved due to the fact that the compiler does not have to deduce the type?

Answer 1

against best practices or preferred style

Yes, I would say that the prevailing style is to allow the conclusion of the type as much as possible.

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